Tile Laying and Fixing

Is it more effective to clip or nail 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.


What is the correct sand grade for mortar bedding ridge and hip tiles?

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.


What size counterbatten should be used with rigid sarking?

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.


Can interlocking tiles be laid on a curved roof?

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.


What is the double battening method of fixing for Stonewold II?

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.


How much should an eaves tile overhang into a gutter?

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.


Which Redland tiles should be laid straight and which broken bond?

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.


What is the purpose of roofing underlay?

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 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.

What is the permissible deflection in a tiling batten? 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.