In this section you will find answers to some of the more commonly asked technical questions.
When using lead, the Abutment Ventilation System simplifies the installation of the lead flashing at a top edge by eliminating the need to dress lead over the roll of profiled tiles. One width of lead sheet suits all pitches and profiles of tiles. The tray supports the lead across the gap between the tiles and the wall. The lead sheet should be measured and folded prior to being installed. Trying to dress the fold onto the support tray will result in support tray deformation. The lead should be lapped at adjacent sections by 150mm. Two layers of Code 4 Lead can be fitted in the channel on the leading edge. Merely butt jointing the lead will result in leaks.
When using a lead-alternative flashing the manufacturer of the flashing should be consulted for their instructions. In the case of Redland Rapid Flashing the flashing material should be fitted into the channel on the leading edge first before dressing and chasing into the wall. An alternative method is to use two widths of Rapid Flashing – one fitted into the leading edge of the tray; the other dressed and chased into the wall. In this method the two widths are then joined and sealed together using the self-fusing properties of Rapid Flashing to complete the cover flashing and make it weathertight.
Liner trays are designed to replace timber rafters typically in industrial and commercial buildings. Tiling battens are normally fixed to 50 x 50mm counterbattens which are screwed to the crowns of the liner trays. Insulation is installed in the base of the trays. To create a space for a secret gutter at a side abutment an additional 50 x 50mm batten must be fixed to the side of the final counterbatten. This batten supports the ends of the tiling battens whilst the final counterbatten supports the base of the secret gutter.
A raking abutment is one that is not perpendicular to the eaves/ridge line. The detail will be different depending upon whether the eaves length is shorter or longer than the ridge length.
Where the eaves length is shorter, the ridge or top edge abutment will be longer than the eaves which means that water running down the roof slope will run into the abutment. This is very similar to an inclined valley where the water drains into the valley. Raking abutments of this type should be designed as an open lead valley with one side dressed up the abutment under a step and cover flashing and the other dressed under the tiling with a tilting fillet and welt. The width of the open section of the valley is dependent on the plan area of roof discharging into it, normally between 50mm and 125mm. The true pitch of the valley should not be below 11º, this being dictated by the lap of the lead sheet. Where the eaves length is longer, the ridge or top edge abutment will be shorter than the eaves. This means that water running down the roof slope will run away from the abutment as it does on a hip. This type of raking abutment should be designed with a lead cover flashing. If the tiles are flat then there should also be a secret gutter.
Preformed abutment flashing units are only suitable for standard abutments and are not suitable for either situation. Where insulation is also partially filling the rafters leaving an air gap between the top of the rafters and the insulation, one ventilation tile per rafter space may also be required depending on the below ceiling construction to let ventilation in or out at raking abutments.
The RedVent EavesVent tray fits over the rafter below the tiling batten. The front of the tray sits on top of the fascia board. At low pitches with tiles with small batten gauges such as Cambrian Slates, there is a negative fall on the tray (and hence underlay) between the bottom of the first batten and the top of the fascia board.
For this reason the minimum rafter pitch for Cambrian Slates when used with RedVent EavesVent is 25º. Where rigid sarking and counterbattens are used this negative fall will be even greater as the tray sits below the sarking board. The thicker the counterbattens the greater the negative fall. Consequently it is recommended to contact Redland Technical Solutions for advice on the minimum pitch of Cambrian Slates with RedVent EavesVent when using rigid sarking.
The same issues exist with plain tiles due to their small gauge. As with Cambrian Slates it is recommended to contact Technical Solutions for advice on minimum pitch.
The recommendation for filling in the corrugations of a profiled tile is to stop birds and rodents from nesting in the batten cavity. It has been found that the smallest bird or rodent that nests in house roofs has a body of over 16mm diameter. Some tiles have a corrugation low enough to leave a gap of less than 16mm so do not need any form of filler. However, where this is not the case an eaves filler is needed and it is important to use a good one. The most common product used to prevent bird and rodent access is a good quality eaves comb. The practice of filling the corrugation of the tile at the eaves with mortar is traditional in some parts of the country. However, unlike an eaves comb, this does not allow the eaves tile to be clipped to resist wind uplift if required in the manufacturer’s fixing specification. Moisture can also get trapped behind the fascia board which leads to rotting of the underlay as moisture is prevented from discharging into the eaves gutter.
If the fascia board is fixed too low the eaves course of tiles will lie at a steeper angle than the rest of the tiles above. Contact with the next course is along the top edge only making the weather bar arrangement in the headlap ineffective. If the fascia board is fixed too high the eaves course of tiles will lie at a shallower angle than the rest of the tiles above and may be below the minimum pitch for the tile. Additionally contact with the next course is along the leading edge only greatly reducing the effectiveness of the weather bar arrangement. In both cases the tiles in the first and second courses will not lie in the same plane. The tile clips for many tiles will also not fit unless all tiles lie in the same plane.
To satisfy Part L of the 2013 Building Regulations will require mineral wool insulation for most domestic dwellings to be more than 300mm thick when laid between and over the ceiling joists. The lower the roof pitch and the wider the soffit the longer an interrupter tray must be to ensure that an adequate eaves ventilation path is maintained between the roofing underlay and the insulation. With an insulation thickness of 300mm a tray, up to one metre in length may be necessary. Some trays such as those supplied with the RedVent EavesVent system sit on the fascia board and therefore the distance from the fascia to the wallplate must be added to the inclined insulation distance. This may be as much as two metres depending on the soffit overhang. It is important to choose a system that can be extended to any length. For rafter pitches below 30º extension trays will be required. The standard tray supplied with RedVent EavesVent extends 450mm from the front of the fascia board. Each extension tray adds 380mm to this dimension. Alternatively from the Rapid + Range, Rapid EavesVent Rafter Roll extends 800mm per single roll width.
With a 30º rafter pitch, no soffit and 200mm thick insulation, the tray must be at least 600mm long. It is essential to measure the inclined distance from the fascia board to the top of the insulation either on site or from drawings. When reroofing, it is advisable to fix the trays above the existing insulation to allow for any future installation of additional insulation.
According to BS 5250, Code of Practice for Control of Condensation in Buildings, unless you have both a well-sealed ceiling and a continuous vapour control layer below the thermal insulation positioned at rafter level then some form of ventilation should be provided to the air space between the insulation and roofing underlay. Where insulation partially fills the rafter space leaving an air gap between the insulation and top of the rafters, individual rafter spaces must be ventilated as each rafter space acts as an individual duct. With open rafters, as there are no fascia or soffit boards there is no simple means of introducing low-level ventilation. However, there are two possible approaches. In one method the RedVent 25 Over-Fascia Vent should be positioned between the rafters at the wall line venting directly into the 50mm void above the insulation just below the underlay. This is best achieved by leaving 50mm clear space between the structural wall and the top of the rafters. The vent should be cut to length to fit between the rafters and nailed to the top of timber trimmers also cut to fit between the rafters. Each of these assemblies can then be slid up the face of the structural wall between the rafters until the vent comes into contact with the underlay, 5mm below the top of the rafter. The trimmer should be fixed in this position to the wall or the rafters. Providing the trimmers do not come below the bottom of the rafters, this arrangement will give a neat detail at the junction of the wall and the roof. Alternatively, the use of ventilation tiles can be considered. These should be spaced so as to give the requisite amount of low level ventilation.
Dry Hip Support Trays are designed to span underneath the center of one hip tile to the next to ensure that the hip tiles are kept as straight and at the same angle as possible and the joints between hip tiles are kept as far from the joints in the support trays as possible. To assist the roofing contractor a 'V' shape marking showing where to cut the first unit at the eaves is provided. The remainder will then fall into place automatically.
Hip irons are normally made from 6mm thick galvanised steel, 25mm wide and 300mm long bent into the shape of a hook with a scrolled end. They are screwed to the top of the hip rafter at the eaves. The curled end should stand upright beyond the fascia board to line up with the eaves overhang of the roof tiles.
Hip irons are traditionally used with mortar bedded Third Round, Plain Angle or Universal Angle Hip tiles. Their function is to prevent the lowest hip tile sliding off the roof particularly whilst the mortar is wet or if the mortar has failed due to hairline cracks. However, with the modern requirement that all hip tiles are mechanically fixed the use of hip irons can be argued nowadays to be more for aesthetic rather than structural reasons. Hip Irons are not used with Bonnet Hips, Arris Hips, mitred hips or Dry Hip. With the latter a Block-End Hip tile is screwed directly to the hip rafter.
A plan angle is the angle of intersection between the eaves of one slope and the eaves of the adjacent slope when viewed on plan. The most common plan angle is 90º since most buildings have square corners. However some buildings have walls that do not meet at 90º such as where a wall follows the line of a boundary, which is not parallel with the orientation of the building or bay windows that are formed in a series of facets to produce a panoramic view. A pyramid roof is one that has hips meeting at a point. A pyramid with four equal sides has a plan angle of 90º. With six equal sides the plan angle is 120º and with eight equal sides the plan angle is 135º.
The pitches we quote in our literature and on our website always refer to the pitch of the rafter, and not the pitch of the tile. When laid, tiles sit at a slightly reduced angle as they have to overlap with the ones below. This can be as much as 5-8°, and is higher for double lapped products such as plain tiles. We recommend that when using pitch to describe the angle of the roof, you always refer to the rafter pitch.
Efflorescence or "lime bloom" is a natural phenomenon resulting from the reaction between cement and water which produces calcium hydroxide 'lime'. In certain conditions the lime will move through the concrete to the surface of the tile where it reacts with carbon dioxide in the atmosphere to produce a deposit of calcium carbonate crystals causing white patches. Although not attractive it is not harmful and the effect is temporary. Wind and rain will gradually remove the deposits leaving the true colour of the tile. This process can take between 12 to 36 months depending on weather conditions but once the efflorescence disappears it will not come back. The removal process can be hastened by treating the tiles with a dilute (5%) solution of hydrochloric acid. However great care must be taken with access to the roof and the safe application of the acid treatment.
The European Standards for concrete tiles and fittings came into effect in August 1995 replacing the old British Standards. The Standards BS EN 490:1994 and BS EN 491:1994 for concrete roofing tiles and fittings were quite different to the old British Standards they replaced. There is a popular misconception that European Standards are not as stringent as existing British Standards. In the case of concrete roof tiles this is certainly not true. Where the old British Standard was prescriptive laying down the correct dimensions for a tile, the BS EN standard focuses more on the product's performance. The notion that there is no point having a tile that looks good if it leaks or cracks is fundamental to the new European Standard. British concrete tiles now have to meet strict performance criteria in terms of strength, impermeability to rainwater, and freeze-thaw resistance if they are to comply with the European standard. The European product and test standards for concrete tiles and fittings are revised approximately every five years with the current versions being BS EN 490: 2011 and BS EN 491: 2011 respectively.
Flat interlocking slates such as Cambrian Slate, Richmond 10, Saxon 10, Landmark 10 Slate, MockBond Richmond 10, Stonewold II, Mini Stonewold and MockBond Mini Stonewold require their minimum roof pitches to be increased when the rafter length exceeds 10m. Wind tunnel testing has shown that water flows can be so great on very large roofs that the side interlocks of flat tiles can become overloaded. It is recommended to contact Redland Technical Solutions for further advice if this applies on your project.
Concrete interlocking tiles complying with the British Standard have since the mid 1950s been required to have a distinguishing mark to identify their origin. Compliance with the code is referred to on the underside of the tile. A Registered Design Number usually starting "RegDes" followed by a six or seven figure number is also normally included. This can be checked with us, the Design Registry Department of the Patents Office or the Science Reference Library in Holborn, London.
BMI Redland tiles include either our name or the product name on the underside. There will however be no reference to its colour or surface finish as the moulds used are for all colours of the same profile. The original colour of the tile can be obtained by looking at the headlap area of the tile. If the tile is through coloured the colour on the underside of the tile should be checked as this will not have been affected by weathering. However new tiles will take years to weather down to the colour of the existing roof surface. If only a few tiles are required it may be advisable to use salvage tiles that have already weathered on a roof to match more closely. For a large roof area it may be better to strip an elevation and re-tile in new tiles to ensure a consistent colour match and use the salvaged tiles on another part of the building. Pre-1970s roofs may use Imperial tile sizes that are not compatible with their metric equivalents but this should be made apparent by a change in the Registered Design Number of the tile.
Plain and profiled refers to the appearance of the tile. Plain tiles are small flat tiles while profiled tiles as their name suggests have a shape or contour. A pantile is an example of a profiled tile. However these are only two of the three possible categories of roof tile, the third being natural slate or slate appearance tiles. Interlocking is a description of the way some tiles fit together. Plain tiles are made to the traditional design dating back to the Middle Ages with no added extras apart from nail holes, a slight camber and nib on the back where the tile sits on the batten.
Interlocking tiles are generally much larger - hence they can be laid much more quickly - and have a grooved 'interlock' down either side. The interlock allows the tile to be simply connected to the adjoining tiles forming a weathertight joint. There is no difference between the quality of the tiles. However the smaller size of the plain tiles, along with their lack of an interlock, makes them a better choice for roofs with intricate detailing and for curved roofs or those with turrets. Interlocking tiles are larger than plain tiles and so better for simple roof shapes and lower pitches. Plain tiles in general should never be used on pitches below 35°, albeit certain specific proprietary plain tiles may be used to 30° rafter pitch when other precautions are taken such as specific underlays used below the tiles. Consult Redland Technical Solutions for more information.
Under severe wind driven rain conditions, a tile laid below the minimum recommended rafter pitch will allow rain to be blown over the headlap or through the side laps of the tiles. Once this occurs the underlay is the only thing preventing water from reaching the rest of the building. If water is allowed to regularly flow down the underlay it will begin to decay where the flow is greatest, near the eaves, and allow water into the building. How long this construction will last depends on the type of underlay, the number of layers and the quantity of water flowing down the underlay, but could be anything between five and thirty years.
For a boarded roof, once the underlay has failed, it will cause the boarding below to rot. The only solution is to strip the tiles, battens, underlay and boarding and replace it with the same labour cost as a total reroof. When the tiles are laid below the minimum recommended rafter pitch, the normal manufacturer’s tile guarantee will also be invalidated, so unless the underlay or a functional weatherproof sub-roof system below the tiles has a guarantee the roof is unlikely to be covered by any guarantee.
Durability is defined as the ability to last, maintain its primary function and resist wear and tear. With roof tiles this means the individual roofing product will withstand deterioration caused by frost, acid rain, ultra-violet light and heat for a specified period of time. However durability relates only to the composition of the tile or other roof covering material.
In order to keep the rain out of a building under all weather conditions the roof system as a whole must be weathertight. This means that the arrangement of tiles and roofing underlay as a system must be able to resist the ingress of wind driven and deluge rainfall. This system must also resist the suction effect of high winds that would lift the tile and allow rain in through the headlap.
However weathertightness should not be confused with watertightness which means that if the roof were turned into a receptacle it would hold water which clearly a tiled roof would not. A roof is not hermetically sealed for good reason since allowing some air movement through roofs is a good thing as it helps to allow the building to breathe. Consequently, small quantities of water can penetrate through the junctions of tiles under certain driving rain conditions and this normally is not a problem so long as the roofing underlay is installed properly. Other situations also give rise to water presence under the tiles e.g. when the dewpoint corresponds with the atmospheric conditions of the batten cavity, temporary condensation may form. Also, wind driven snow can be blown in through the smallest of joints and will eventually thaw leaving water in the batten cavity, which will run down the underlay and into the gutter.
The correct selection of Angle Ridge Tile for a roof is made by calculation. Add the rafter pitches each side of the ridge and then deduct from 180º. Add to this figure twice the difference between rafter pitch and tile pitch. The angle of the ridge tile should be the same or slightly smaller than the answer, e.g. on a 30º pitched roof using Mini Stonewold tiles the angle ridge tile should be 180º - (30º + 30º) + (5º + 5º) = 130º. In this case a 120º angle ridge tile should be used being the nearest size smaller than 130º.
During the firing process clay ridge tiles can become slightly larger at one end than the other. On a mortar bedded ridge this is unimportant as it is compensated for by the mortar. However with dry fixed ridge the ridge tile sits in a profile filler unit, which has no adjustment. The variation in size from one end of the ridge to the other shows as a step in the ridge line. The plate that holds the ridge tiles may not lie perfectly flat. The tolerances permitted in BS EN 1304: 2013, clay roofing tiles and fittings, are such that this may occur. The solution is to 'top and tail' each ridge tile, i.e. fix large ends together and small ends together.
Gas Flue Ridge Terminals are licensed with British Gas for use with appliances with a maximum output rating of 60kW. The Gas Flue Ridge Terminal must also be connected to an 'R' type adaptor - available from metal flue liner specialists such as SFL Flues and Chimneys (www.sflchimneys.com) - which converts the circular flue to the rectangular outlet of the ridge terminal. The adaptor, available in 100mm and 125mm diameter, is bolted to the underside of the ridge terminal with a flue gasket fitted between them.
Whilst Gas Flue Ridge Terminals are suitable for most boilers, gas wall heaters and back boilers, they are not compatible with coal effect gas fires. These generally require a larger flue diameter than 125mm. This is because the temperature of the flue gases is lower than for other heating appliances and they need assistance from the stack effect to draw them out. It is always advisable to consult British Gas with any queries regarding the compatibility of gas-related products.
All mortar bedded ridges must be mechanically fixed to comply with BS 5534: 2014: Slating and tiling for pitched roofs and vertical cladding - Code of practice by using wood screws and clamping plates to fix universal angle or half round ridge tiles to a ridge board or ridge batten. The wood screws and clamping plates are available in packs of six. Specify product code 9566 for half round ridge tile plates, 9567 for universal angle ridge tile plates and 9584 for wood screws.
It is essential that the ridge board or ridge batten is positioned to provide a minimum of 15mm penetration of the wood screw. For trussed rafter roofs where no ridge board is present additionally specify product code 9447 for packs of six batten fixing straps.
It is not possible to place Ridge Vent Terminals immediately next to each other when using Dry or DryVent Ridge as it is essential that the ridge batten spans at least two rafters to ensure its stability. The closest the Ridge Vent Terminals should be placed together is every other rafter spacing. At 450mm rafter centres it may be possible to space the terminals at 900mm centres without the need to cut ridge tiles. At 600mm rafter centres, 150mm pieces of ridge tile must be cut and fixed between the terminals.
As part of the testing procedure for a soil vent pipe from the main drain to the point of termination, an air test is required to ensure there are no leaks in the pipes that may result in noxious smells seeping into the building. An airtight pipe bung is placed into the top of the soil pipe and air pumped into the pipe to create a positive pressure. The pressure is monitored over a short period of time for a return to normal. If it does so quickly it fails the test.
With RedLine Vent Tiles and Ridge Vent Terminals it is not possible to install a pipe bung from the roof. The flexible pipe must be disconnected from the vent and pipe bung installed in the spigot. The flexible pipe is then reconnected. Following completion of the test the pipe bung is removed and flexible pipe reconnected.
There are no restrictions when using our high capacity 8.8k ThruVent Tile or Hi-Flow ThruVent (or Ridge Vent Terminals) for soil ventilation. However, when using our 4.5k ThruVent its use for soil ventilation is limited to single and two storey housing. BS EN 12056-2: 2000, which covers sanitary pipework, requires soil vent pipes, except for single occupancy houses up to two storeys, to maintain the diameter of the pipework above the highest connection to the outside air. As 4.5k ThruVent Tiles have a spigot diameter of 75mm they are not suitable for connecting to a 100mm diameter soil pipe in these instances.
BS 5534: 2014: Slating and tiling for pitched roofs and vertical cladding - Code of Practice specifically discourages the design of discharging rainwater from one roof onto another. Collecting water from a large roof area into an inclined valley, which then discharges back onto the roof, can also cause problems. In both cases the concentration of water could be greater than the tiles are designed to withstand resulting in water being forced through the side laps or interlocks of the tiles.
Clips are far more efficient at holding tiles onto the roof than nails and provide much greater fixing resistance. Nailing tiles onto battens has been common practice for many years and if done with nails that will not rust or decay such as aluminium alloy or stainless steel, will normally ensure the tiles remain attached to the batten especially on steep roofs. Galvanised nails are not appropriate as they can be scratched by the aggregate in the concrete and will then rust.
However in areas that are affected by high winds nailing alone may not be enough. In very high winds the wind suction may be more than the weight of the tile can cope with and it will lift even when nailed at the head of the tile. Tiles can perform a 'Mexican Wave' as high wind pressure waves pass over making a lot of noise in the process as they clatter against each other. When this happens the tile will either drop back onto the lower course of tiles causing impact damage or in severe conditions, could wrench the head off the nail. Even worse the nail could be pulled out of the batten and the tile fly off the roof causing possibly injury to anyone passing by or damage to property. In addition water can penetrate the roof through a broken or missing tile. The new calculation method and general tile security recommendations given in BS 5534: 2014, Slating and tiling for pitched roofs and vertical cladding - Code of Practice should be used to determine the correct fixing specification for tiles on each particular building. This will show whether nailing is adequate or if clipping is necessary or even nailing and clipping. Redland can provide fixing specifications free of charge for all Redland tiles via its FixMaster service or its new online tool FixMaster Online.
BS 5534: 2014, Slating and tiling for pitched roofs and vertical cladding - Code of Practice stipulates that roofing mortar should be 1:3 cement: sand ratio. The sand selected should either be a blended soft sand and sharp sand mix, with the sharp sand making up no less than one third of the sand content, or a course building sand with a particle size distribution in line with a blended mix of fine building sand and sharp sand. Alternatively a proprietary roofing mortar can be mixed and used in accordance with the manufacturers’ instructions.
The answer to this question depends upon whether the counterbattens and rigid sarking can be considered structural or not. If the counterbattens are independently fixed to the structural timbers below sufficiently so as to resist the wind uplift forces acting upon the roof then they can be considered structural. Timber sarking similarly adequately fixed to the structural rafters below can be considered structural. However, some forms of rigid sarking are not structural such as thermal insulation. For structural counterbattens the size of counterbattens, in the simplest case, should be thick enough to fully accommodate the required nail penetration for the tiling batten fixings so as to resist the wind uplift forces acting upon the batten nails. Redland’s free of charge FixMaster service can determine the smallest structural counterbatten size required from the anticipated wind uplift for any of our tiles, at any pitch and any rafter centres in any location. Commonly the minimum structural counterbatten depth is 38mm because the minimum length batten nail is 65mm long
Alternatively, if the counterbattens are not structural, and they are not being used to provide an air gap for roof ventilation purposes, they can be reduced in thickness down to as thin as 12mm – the smallest sufficient to allow adequate drainage between the tiling battens and rigid sarking. In such cases where the counterbatten is thinner than the required batten nail penetration to resist wind uplift the tiling battens should not be fixed just into the counterbatten. Instead the tiling batten nail should be long enough to pass through the tiling batten, counterbatten and any insulation/sarking board and into the rafters by at least the minimum nail penetration required. This could mean a very long nail is required especially if the rigid sarking is insulation. Often in these situations “headless” fixings such as helical nails are commonly used since they have high pull-out resistance and are available in very long lengths. If in any doubt the golden rule is to contact Redland Technical Solutions for advice.
With one or two exceptions, interlocking tiles are not designed to be laid on a curved roof whether the curve is horizontal (cone), vertical (bellcast), or diagonal (hyperbolic paraboloid). Interlocking tiles must be laid to an exact module because they have an interlock on the left and right hand edge that, unlike the headlap, is not designed to be variable. Thus laying such tiles over even the slightest curve results in gapping which reduces their ability to resist wind driven rain.
There are examples of flat interlocking tiles being laid to a very shallow curve where the coursing can sometimes drift and in some cases may line up. This is not particularly attractive and when all the interlock shunt is utilised becomes impossible to lay the tiles. Profiled tiles such as 50 Double Roman are restricted by the need for each corrugation to sit into the one below. On a cone shaped roof the radius diminishes as it rises requiring the spacing of the corrugations to close. This is impossible to achieve with a fixed corrugation spacing. When they are laid on a bellcast, the weatherbars in the headlap area are not in close contact so their performance is reduced. With hyperbolic paraboloids, the twisting effect on the tile increases the gapping on all four edges again reducing performance against wind driven rain.
BS 5534: 2014, Slating and tiling for pitched roofs and vertical cladding - Code of Practice and BS 8000: Part 6: Workmanship on building sites – Code of Practice for slating and tiling of roofs and walls recommends against using interlocking tiles on a curved roof. Anyone choosing to do so must therefore take full liability for its construction and design. If tiles must be laid to any form of curve, then the use of a double lap product such as plain tiles should be specified. Even then a lot of careful work has to be undertaken to ensure that the tiles are cut to fit to reduce gapping, that minimum side laps and pitches are maintained and that water run-off is down the tile, not across it.
There are occasions - in severely exposed locations - when it is necessary to 'double batten' Stonewold II slates. This involves fixing a 50x38mm secondary batten above the head of the tile. Into this batten is nailed a 9204 double battening tile clip. Contact Redland Technical Solutions for a copy of the relevant detail.
With most interlocking tiles it is possible to set out the spacing of the battens to ensure a whole number of tile courses in any rafter length except for very short distances between fixed points. Tiles should not overhang gutters by more than 50mm measured horizontally from the fascia board. Overhangs greater than this will result in rainwater running off the tiles overshooting the gutter. In addition, cleaning the gutter becomes extremely difficult. If the overhang is less than 50mm the rain is likely to be blown onto the underlay and accelerate its decay. In addition, the eaves course is likely to sprocket as the weather bars at the tail of the tile will lift the tile off the fascia board.
It is not possible to lay Fenland Pantiles broken bond or Cambrian Slates, Richmond 10, Saxon 10, or Landmark 10 Slate in straight bond. Regent, Grovebury, 50 Double Roman, Renown, Landmark Double Pantile and Redland 49 tiles must be laid straight bond. Mini Stonewold and Stonewold II slates should only be laid broken bond. The broken bond appearance more closely resembles natural slate and for flat interlocking tiles performs better in driving rain conditions than straight bond as the interlocks can flood if laid in straight lines. DuoPlain, MockBond Mini Stonewold slates and MockBond Richmond 10 should be laid three-quarter bond.
The primary function of a roofing underlay in a pitched roof is to counter the pressure that can build up inside the roof space caused by the suction effect of the wind. Its weatherproofing ability is a secondary function. Tiles are sucked rather than blown off roofs so the choice of underlay and correct underlay installation is crucial to ensure tiles remain on the roof. BS 5534: 2014, Slating and tiling for pitched roofs and vertical cladding - Code of Practice has introduced a new requirement that roofing underlays have sufficient wind uplift resistance for the roof they are used on. This requirement is assessed by a test that measures how much an underlay stretches or “balloons” when subjected to wind pressure. If an underlay stretches too much such that it touches the back of the tiles then the tiles can be removed from the roof as the underlay does not shield the tiles adequately from the pressure that builds up in the roof. Furthermore, when installing the underlay adjacent layers normally should be overlapped with at least one batten fixed over the horizontal lap to prevent wind building up and bursting open the lap forcing tiles off the roof. Alternatively, certain proprietary underlays such as BMI Redland’s Spirtech 400 2S have double integrated glue strips that when glued together at the horizontal laps not only ensure very high wind uplift resistance but also eliminate the need for fixing battens over the lap. Roofing underlay also keeps the inside of the roof dry while tiling takes place. When tiling is complete it provides a secondary barrier to wind driven rain, snow and condensation that can form in the space between the underlay and the tiles.
There are three types of load on a batten. There is short-term dead load, e.g. a person walking on the roof, long-term dead load, i.e. the finished weight of the roof covering and wind load. Dead loads are downward forces whereas wind loads produce upward forces. Dead load is resisted by the strength and stiffness of the timber batten. The weaker the timber or the greater the span between supports the more the timber batten will deflect. The deflection should not exceed the effective span divided by 100. For 600mm rafter centres and 35mm wide supports the effective span is 582.5mm. The maximum deflection should therefore not exceed 5.825mm. For rafter centres greater than 600mm the maximum deflection should not exceed the effective span divided by 125.
Under normal production conditions colour variation may occur in both concrete and clay roof tiles and fittings. For the best visual effect on the roof, tiles should be selected randomly from a minimum of three different pallets. Tiles of the same colour and profile, which are manufactured at different plants, must not be mixed on the same roof area.
Special attention must be given to unequal pitches discharging into a Cambrian valley. The pressure of water running down the steeper slope can drive in under the shallower side of the valley. To counteract this an additional raking batten must be fixed on the shallow side of the valley to act as a barrier to this greater water flow.
GRP valleys should be recessed into the fascia board at the eaves to allow the bottom end to discharge into the gutter without the corrugations of the valley raising the tiles in the eaves course which causes tile gapping and hence the possibility of rain ingress. Cambrian Slates require the eaves course to be clipped using verge clips. This is not possible unless the top of the valley is flush with the top of the fascia board. Where a valley discharges onto another roof as with a dormer, the GRP valley should finish at the head of the tile course onto which it discharges and a lead saddle installed under the valley and over the tiles.
Many roofing contractors do not mortar bed the cut tiles at the sides of a plain tile open lead valley. Whilst not against the British Standard, we have always recommended that plain tile lead valleys be mortar bedded. BS 8000-6: 2013, Code of practice for slating and tiling of roofs and walls – Workmanship on building sites, states where bedding is used that “The tiles should be bedded only and not pointed between the tiles to allow the flow of water between tiles and into the valley.” This means that mortar should only be placed between the lead valley and the underside of the lowest tile and not between tiles. The reason for this is the need for water to drain freely between overlapping tiles. As plain tiles are typically 13mm thick, gaps of up to this figure can occur through which wind, rain and even large insects can gain access to the batten cavity. The mortar bedding prevents this as well as acting as the first of three lines of defence, the others being the tilting fillet and the lead welt. Without mortar bedding the risk of the valley leaking is much higher.
A plan angle is the angle of intersection between the eaves of one slope and the eaves of the adjacent slope when viewed on plan. The most common plan angle is 90º since most buildings have square corners. However some buildings have walls that do not meet at 90º such as where a wall follows the line of a boundary which is not parallel with the orientation of the building or bay windows that are formed in a series of facets to produce a panoramic view.
It depends. The recommended method and amount of ventilation of the roof void depends on two critical factors: the degree of sealing or airtightness of the ceiling below the roof void and the type of roofing underlay used - whether water vapour permeable or not. If a ceiling contains a loft hatch without an effective seal, unsealed pipe or service or other penetrations then high level ventilation of the roof void should never be installed without adequate low level ventilation of the same void regardless of the type of underlay. In the absence of this low level ventilation warm, moist air from the living space can be drawn up into the roof void through the holes and gaps in the ceiling to replace air sucked through the ridge thereby raising the risk of harmful condensation occurring in the roof void. However, with careful design, specification and workmanship well sealed ceilings can be constructed to help meet the need for more energy efficient dwellings and other buildings. These better sealed ceilings allow the use of vapour permeable underlays (VPU's) for felting the roof void. With such construction - more prevalent in modern new housing - high level ventilation of the roof void only is sufficient to ensure adequate control of condensation risk even during the drying out stage of newly constructed buildings. Furthermore with a well sealed ceiling external air is encouraged into the roof void at low level through the tiles and laps in the underlay, in preference to moist air from the living space, thereby creating an effective method of ventilation without the need for an eaves ventilation product. From January 1st 2011, the NHBC require the use of high level ventilation when using a VPU in a cold pitched roof for all properties covered under its Buildmark Warranty.
BS 5655 requires the effective ventilation area at high level to be equivalent to 1% of the plan area of a lift shaft, e.g. if a lift shaft is 1.6 m x 2.4 m the ventilation area must be equivalent to 38,400 mm². This could be achieved by using a number of different product combinations.
Using DryVent Ridge, which provides 10,000 mm²/m and two ridge ventilation terminals, which provide 8,400 mm2 each, would be sufficient if the ridge ran along the 2.4 m length of the lift shaft. Alternatively nine 4.5k ThruVent tiles with a vent area of 4,500 mm² each could be installed or any other multiple combination equivalent to 38,400 mm².
However although this may work in theory, in practice it may be difficult to achieve in the roof area available especially with rafters at 600 mm centres. An alternative would be to form a truncated dormer with a louvre grille in the vertical face with the required ventilation area. This arrangement works well at the intersection of a ridge with a hip. The final choice will very much depend upon the exact situation directly above the lift shaft.
Using the roof space is an excellent way of creating additional space within the original building envelope. However careful attention must be paid to the ventilation and insulation requirements in this type of construction. Unlike in a house where the loft space is not used, in this type of construction the room-in-the-roof temperature is approximately the same as the rooms below. Depending on the precise position of the insulation in relation to the roofing underlay there may or may not be a cold roof void sandwiched in between which needs to be appropriately ventilated. For constructions where the insulation is either on top of rafters and/or fully filling rafters a vapour permeable underlay can be supported by and placed directly in contact with the insulation so long as the ceiling below is both well sealed and contains a suitable vapour control layer (VCL). If these criteria are met then for most common roof coverings, except less air permeable fibre cement and metal roof coverings, no further roof space ventilation is required. However for constructions where the insulation is partially filling and/or fixed below rafters or where the insulation is on top of and/or fully filling rafters but the ceiling below cannot be well sealed or does not contain a VCL, then the roof space between the underlay and the insulation needs to be ventilated or in the latter case a gap created (with counterbattens) and ventilated. Where a sloped ceiling and insulation deviates from the line of the rafters a gap will be created between the insulation and the underlay which needs to be ventilated e.g. a mini-loft above a room-in-the-roof with a horizontal ceiling. In all of these constructions, the precise ventilation requirements depend upon a range of factors including the ceiling quality, the roofing underlay used, the roof covering, and other aspects of the construction such as whether timber sarking is present or not. It is always advised to contact BMI Redland Technical Solutions to discuss the specifics of your project.
Approved Document C of the Building Regulations and BS 5250: Code of practice for control of condensation in buildings state that at 15º pitch or less the level of eaves ventilation should be 25,000 mm²/m regardless of whether the insulation is laid between the ceiling joists or the rafters. With Regent tiled roofs at 15º rafter pitch and less any ventilation system that only provides 10,000 mm²/m such as the RedVent Eaves Ventilation System should not be used. RedVent 25 Over-Fascia Vent should be used. At all other pitches the choice of eaves ventilation product depends on the location of the insulation in the roof structure.
Condensation in a roof space does not necessarily mean roof ventilation systems are not performing. Other reasons may include central heating expansion tanks in the loft having no lid, bathroom extract fans blowing steam and water vapour directly into the loft, soil vent pipes venting into lofts due to missing one way valves or poorly fitting loft hatches in bathrooms and kitchens letting high volumes of water vapour vent into the loft. Problems such as these need to be eliminated first before any attempt to investigate cold bridging or deficiency in the roof ventilation system are made.
With Delta tiles there is a limited range of roof space ventilation products available. At the eaves, all soffit and fascia vents are compatible. However at high level, DryVent Ridge, DryVent Monoridge and the Top Edge Abutment Ventilation System are not suitable due to the height and sharpness of the Delta profile. There are also no vent tiles suitable in the Redland range. NB. Please note that Delta tiles are no longer manufactured by Redland.
Ambi-Dry Verge units can be used with Stonewold I slates. Additional battens are fixed to a minimum of two supports 2 mm below the tiling battens to which the Ambi-Dry Verge Units are fixed. This is an ideal solution in situations where the original mortar bedded verge has failed and a new dry verge is required.
BS 8000: Part 6: 2013: Code of practice for slating and tiling of roofs and walls, Workmanship on building sites, states 'A 100mm wide bed of fully compressed mortar width should be laid onto the undercloak. Tiles should be bedded solidly and edges finished neatly’. The width of the undercloak is normally 150mm of which 50mm laps under the batten ends. Once the tiles are bedded into the mortar it will spread by a further 25mm therefore a 75mm width of mortar should be applied which will spread to 100mm when bedded/compressed.
BS 5534: 2014, Code of practice for slating and tiling for pitched roofs and vertical cladding, states that a verge overhang, when unsupported, can be between 38 and 50mm for double-lapped plain tiles, fibre-cement slates and natural slates and between 30 and 60mm for single-lapped interlocking tiles. On a verge formed with a gable ladder, 35mm wide rafters, 19mm bargeboard and 38mm overhang, the distance from the verge edge to the ends of the tiling battens should be 100mm such that the 100mm wide compressed mortar bed just touches the batten ends, which is acceptable with treated battens.
If the overhang is increased to 50mm, the distance from the verge edge to the ends of the battens becomes 112mm giving a gap of 12mm between the compressed mortar bed and ends of the battens which avoids contact between mortar and batten. It is for this reason that it is better that the overhang is 50mm on gable ladder verges.
Winchester cutting of vertical plain tiles is a traditional means of finishing tiles against a verge. However the process of cutting two tile-and-a-halves to fit the verge results in only a single nail fixing at roof pitches below 40º. This can result in cut tiles rattling in the wind or tiles rotating out of position. It is for this reason that BS 8000: Part 6: 2013: Code of practice for slating and tiling of roofs and walls, Workmanship on building sites, states a minimum pitch for Winchester cutting of 40º. For pitches below 40º a continuous lead flashing should be dressed under the verge and over the cut tiles by 150mm. This will cover part of the detail but act as a continuous verge clip. Alternatively a soldier course or double soldier course of tiles under the verge can be used.
Award-winning roof returns school to former glory
Find out why pitched roofing awards winner St Josephs school was such a challenging project.
BMI helps Children in Need keep young people safe
When homeless and vulnerable young people in Blackburn need help, they turn to Nightsafe, a local charity which offers them a caring and supportive environment to keep them safe from harm at critical times in their lives.
Keeping the college on message
The roofing department at Leeds College of Building is now a NFRC centre of excellence as well as being the largest college of its kind in the UK. Chris Messenger, lecturer in roof slating and tiling, takes us through a typical day at the beginning of the teaching year.