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Sandtoft Roof Tiles - Frequently Asked Questions

Click on the drop down links below to read a frequently asked question:

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If I have eaves ventilation, do I need ridge ventilation?

When using a ‘high resistance’, or vapour impermeable roof underlay, and the roof pitch is 35 degrees and above, or when the building is 10 metres wide or more, then BS 5250 recommends the use of ridge ventilation combined with eaves ventilation. This actually provides a much more efficient ventilation system because air passing over the ridge will draw fresh air into the eaves. When there is just eaves ventilation there needs to be enough wind to force air in through one side of the building and out through the other.

How can I achieve a 'continuous 5mm gap' at the ridge using ridge ventilators?

Although the ideal situation is a continuous gap, a suitable alternative method is to fix tile or ridge vents at intervals to give the equivalent of the required gap. For example, a 5mm wide gap is actually 5,000 square mm per metre. If a vent has a 10,000 square mm hole then by placing them at 2 metre centres this will be the equivalent of 5,000 square mm per metre.

My building does not have a fascia or a soffit, therefore I cannot use an over fascia vent or a soffit vent. So how do I ventilate the roofspace?

Over fascia ventilator strips can be used in a variety of different situations. As its name implies, the ventilator is designed to fix on top of the fascia board, tilting the tiles to their correct plane. However, where there is no fascia the ventilator can be fixed, for example, directly onto corbelled masonry. Over fascia ventilators can sometimes be used at junctions between sloping and flat roofs and also at monopitch ridge junctions. They can even be used in internal box gutter situations, provided that there is no risk water leakage through the ventilator if the box gutter was to flood in heavy rain conditions. Contact Sandtoft Technical Support for sectional drawings and further information.

What are the advantages of using a dry ridge system?

Dry fix systems offer a number of advantages over the traditional method of mortar bedding. These include:

  • Allows all-weather laying and fixing
  • Essentially maintenance-free
  • Overcomes problems associated with mortar such as cracking and loss of ridges in high winds
  • Provides full mechanical fixing of all components to resist highest wind speeds ever likely to occur
  • Provides continuous ventilation at the ridge - this is particularly useful in many modern buildings where the use of insulation between or above the rafter precludes the use of traditional ridge vents
  • Provides a consistent attractive finish

What is tile 'chatter' and how can I eliminate it?

'Chatter' is the sound of the tails of the tiles or slates being lifted and then dropped by the wind forces. The problem is often highlighted in roof designs where there is living accommodation in the roof space. The sound created by the movement of the tiles/slates can be amplified by forms of roof construction where the ceiling is fixed directly to the rafters. For single lapped tiles the use of tile clips will help to minimise the affect of chatter. Remember that single lapped tiles only overlap each other by, for example, 75mm, leaving the remaining surface exposed. Therefore clipping, particularly on steep roof pitches or in exposed locations, is important. Movement can also occur in double lapped tiles and slates. Care should be taken when fixing centre-nailed slates to ensure that the nails have been driven home sufficiently. On steep roof pitches or in exposed locations slate hooks can be used to secure the tails of the slates.

The roof pitch on my project is too low for the tiles that I must use. Can I still use the chosen tiles?

The minimum recommended roof pitch for a tile or slate is the roof pitch at which the manufacturer is confident that the tile/slate will perform effectively. At low roof pitches the main concern is to prevent water from entering through the tiling. Sandtoft's minimum pitch recommendations are based on the assumption that a particular tile or slate may be used on a large building in the most exposed location anywhere in the UK. However, the vast majority of enquiries received by Sandtoft Technical Support regarding low roof pitches are concerned with small, single-storey extensions to dwellings. On projects such as these, provision can usually be made to ensure that any water that may enter through the tiling is taken away to the gutter by the underlay. Although Sandtoft does not have any recommendations on the design of a totally waterproof sub-roof, we offer the following suggestions:

  1. Fix rigid sarking (eg waterproof plywood boarding) over rafters
  2. Lay roofing felt over rigid sarking
  3. Fix counterbattens over rigid sarking. Use length of nail that will give adequate penetration into the rafter, assuming that the rigid sarking has no structural value
  4. Lay roofing felt over counterbattens, allowing it to sag between counterbattens (this enables water to drain into the troughs, away from nail holes)
  5. Fix tiling battens onto counterbattens

Type 1F underlay is considered unsuitable in this situation. Therefore a more durable roofing felt should be used, such as BS 747 Type 5U, which is heavier and has a more durable polyester base. Vapour permeable underlay could also be considered.

The ridge tiles on my new house have come off in recent high gales. Why has this happened? Is the mortar too weak?

The correct mix of mortar (ie 1:3 cement:aggregate) is usually more than adequate to resist the highest winds ever likely to occur in the UK. However, where there is movement within a roof structure, particularly where the roof passes over masonry walls, the mortar bed can crack, leaving the ridge tile vulnerable to wind uplift. In new buildings in particular, where timbers etc are drying out and shrinking, differential movement can be a problem. For this reason, the British Standard for slating and tiling, BS 5534, recommends that where differential movement can take place, such as at gables and abutments, ridges should be mechanically fixed as well as bedded. Ideally, all the ridge tiles could be mechanically fixed.

I am confused about which tile batten size I should be using.

The British Standard Code of Practice for slating and tiling, BS 5534: 2003 gives the following recommendations:

  • 38 x 25 mm tile battens for rafter centres up to 450 mm for single lapped tiles
  • 50 x 25 mm tile battens for rafter centres up to 600 mm for single lapped tiles
  • 50 x 25 mm slate battens for all rafter centres up to 600 mm for natural and artificial double lapped slates
  • 38 x 25 mm slate battens for all rafter centres up to 600 mm for concrete and clay double lapped plain tiles

When and why do I need dentil slips?

Where the profile depth of a tile is 25 mm or more, tile slips are inserted into the mortar to help prevent the mortar slumping and cracking. Very often the dentil slips in a bedded ridge are left partially exposed as a traditional aesthetic feature. It is less common to expose the dentil slips in the mortar bed at hips because of the irregular nature of the cut tiles.

How do I decide how often I should nail my tiles?

The level of fixings required for roof tiles depends upon several factors, including the weight and size of the tile, the pitch of the roof and the exposure of the particular site. There are also several factors that determine the exposure of the site, such as the location of the site within the UK, the proximity of buildings and other obstructions, height above sea level, distance away from the sea. The British Standard Code of Practice for slating and tiling, BS 5534: 2003 sets out how to calculate the amount of fixings required, taking all these and other factors into account. In general terms, there are two sets of calculations. The first set is to establish the 'uplift resistance' of the tile and its fixings.
The second set of calculations is to predict the highest wind speed that the particular roof is likely to experience only once in fifty years. By comparing the resulting calculations, a decision on whether to nail and/or clip the tiles can be reached. This is a complex subject requiring specialist knowledge to carry out the calculations involved.
‘Zonal’ fixings tables for all Sandtoft tiles can be downloaded from the Sandtoft website at www.sandtoft.com. Alternatively, Sandtoft Technical Support can provide detailed fixing recommendations upon request, based on information supplied by the client.
Note: BS 5534 recommends that all tiles are nailed when fixed on roof pitches of 45 degrees and above. On roof pitches of 55 degrees and above all tiles should be nailed and clipped.

When and why do I need eaves fillers?

Where there is a gap beneath the laid tiles greater than 16 mm at the eaves then some form of eaves closure is required to prevent birds and rodents entering the batten cavity. Purpose-made, plastic eaves filler units are available for some tiles. Alternatvely, universal bird comb filler can be used. This shapes itself to the contours of a particular tile. Eaves fillers should not be confused with roof space ventilators. Remember that roof space ventilators allow air into the roof space beneath the underlay, whereas eaves fillers are there to prevent access into the batten space, ie between the underlay and the tiles/slates.

How do I decide whether I should clip my tiles? I have been told that I must clip my tiles at low roof pitches to prevent them coming off in high winds. Is this correct?

Sometimes clips are specified by manufacturers for tiles laid at low pitches because the tiles do not have nail holes which would otherwise make the tiles more vulnerable to water leakage at low roof pitches.

With regard to the use of tile clips generally their use depends upon a number of factors such as the location and exposure of the site, as well as the weight of the particular roof tile. ‘Zonal’ fixing tables for all Sandtoft tiles can be downloaded from the Sandtoft website at www.sandtoft.com. Alternatively, Sandtoft Technical Support can provide detailed fixing recommendations upon request, based on information supplied by the client. BS 5534 recommends that all single lapped tiles are both nailed and clipped when fixed on roof pitches of 55 degrees and above.

I always 'kick up' the first course of tiles at the eaves to slow down the water run-off into the gutter. Is this correct?

Although it is quite common to see this detail BS 5534 does not recommended it for use with single lapped tiles. The headlap between tiles is designed to prevent the ingress of water when laid in the correct way, but when the bottom row of tiles are tilted the headlap may then not perform correctly, allowing wind-driven rain enter through the tiling. Where there must be a change of roof pitch then a lead flashing detail can be used to weather the junction. Contact Sandtoft Technical Support for drawings and further information. Such 'sprocketed' or 'bellcast' eaves are better suited to double lapped plain tiles or slates.

I want to replace the tiles on my roof with something different.

1. Can the replacement tiles be heavier than the original roof covering?
2. Can the replacement tiles be lighter than the original roof covering?
Building Regulation Approved Document A notes that when stripping and re-tiling an existing roof the replacement tiles or slates may impose a substantially heavier load than the original roof covering. Also, there may be occasions when the new material imposes a substantially lighter load than the original roof covering. If the replacement roof covering was significantly heavier then there is the risk that the roof structure may collapse if it is not designed to carry the extra weight. If a heavier roof covering is to be used, then a qualified structural engineer must carry out a survey and advise on any roof structure strengthening which may be required. If the replacement roof covering was significantly lighter then there is the risk that the roof structure may be lifted or otherwise damaged in high winds. Many older roofs are not physically fixed to the building and instead rely on the weight of the tiles or slates to prevent uplift. If a lighter roof covering is to be used, then a qualified structural engineer must carry out a survey and advise on any roof structure strapping and bracing which may be required.

How do I seal the joint between my new roof and the neighbours existing tiles?

It is quite common when renewing the roofs of old buildings to strip the roof covering and replace it with something different - for example, many terraced houses, which originally had slates on the roof, are re-roofed in concrete single- lapped tiles. Very often each roof in a row of terraced or semi-detached houses is replaced individually, therefore the joint between the old and new tiling or slating must be weathered. Sometimes this is done by simply mortaring a row of tiles or ridge tiles over the joint. Unfortunately, this can often lead to leaks and is not to be recommended. A far better solution is to incorporate a 'bonding' or 'jointing' gutter. This can be formed from lead, or alternatively, there are a number of proprietary systems available. A bonding gutter not only provides an effective means to weather the junction, but it also gives a neat appearance, with old and new materials finishing closely together and the gutter being almost completely hidden.

I am using your tiles on a 'warm roof' ie rigid insulation is laid over the rafters and Cypress VPM is laid over the insulation. Can I rely on the natural gaps between the tiles to provide adequate ventilation into the batten cavity or is supplementary ventilation required?

All Sandtoft tiles and slates are sufficiently ‘air open’ to allow the transmission of water vapour from the batten cavity through the tiling/slating array without the need for extra ventilation. In a ‘warm roof’ design; ie where the insulation is placed on top and/or between rafters, then counterbattens should be used, either above or below the Cypress VPM. If the counterbattens are placed under the membrane then the membrane should be laid to drape slightly between the counterbattens. In warm roof designs where roof space ventilation is not required, a Sandtoft dry ridge system will provide additional ventilation into the batten cavity in any case.

Concrete Tiles

A white 'bloom' has appeared on my concrete roofing tiles. What is it, and will it harm the tiles?

Efflorescence is a naturally occurring phenomenon caused by water in the form of rain, condensation or dew penetrating into the pores of the concrete and dissolving lime. The solution diffuses to the surface of the product, the water then evaporates and leaves behind a white film of lime. The lime naturally occurs in the cement which is used in the manufacture of all concrete products. Since the lime content of the concrete can vary and the weather conditions obviously differ, the level of efflorescence can also fluctuate considerably. The same chemical process which brings the lime to the surface of a tile carries on, enabling it to be degraded and washed away by the rain, so that eventually the efflorescence disappears by itself - usually in a matter of months. Once the lime has disappeared from the surface of the tiles it rarely re-occurs. Unlike some manufacturers, Sandtoft treat the surface of all their concrete tiles with acrylic polymer coatings to not only minimise the formation of efflorescence, but to give stronger and longer lasting colours If efflorescence does appear, it has no detrimental effect on the long-term performance of the tile.

My roofer has laid (Calderdale) Concrete Slates on my roof 'straight bonded'. As all the photos and other roofs I have seen have the tiles laid 'broken-bonded', will I have problems with my roof?

Although it is recommended that Calderdale Slates are laid 'broken bond', this is mainly for aesthetic purposes - they are a flat tile emulating the appearance of a double lapped slate. But as Calderdale Slates are a single lapped, interlocking tile there is no technical reason why they cannot be laid 'straight bond' in the same way as other single lapped tiles such as, for example, Double Roman tiles. However, it should be noted that all Sandtoft specifications for Calderdale Slates are based on the tiles being laid broken bond. Also, technical data such as the minimum recommended roof pitch are based on wind-driven rain testing carried out on tiling arrays laid broken bond.

My concrete tiles have a painted finish. Why? & Will this weather off?

All concrete tiles are coloured to give them the appropriate appearance. Unlike clay tiles this colour will eventually fade away over time. However Sandtoft, unlike some other concrete tile manufacturers, colour the tile body, the tile surface and then add an acrylic coating, which gives the following benefits: -

Enhances the intensity of the tile colours - Prolongs the effective life of the cement/pigment rich tile surface - Minimises water loss during the curing stage to allow improved concrete strengths - Protects the surface from rain water erosion and corrosion (acid rain) - Minimises dust and dirt and inhibits lichen/moss contamination - Minimises the risk of surface salts and efflorescence formation during the critical early life of the product - Ensures colour between product batches and main tiles and fittings always match - Reduces the effect of the scuffing which occurs during handling and transportation - Increases rain and snow dissipation rate from the roof - Reduces surface water capillarity, ie improved performance

Clay Tiles

My clay tiles appear to have a sprayed on finish. Why? & Will this weather off?

The beauty of Natural Red clay tiles lies in the way in which they retain the natural colour of the clay material, whilst maturing to a darker, richer shade. Colours such as Flanders, Tuscan and Antique are created by applying an 'Engobe' to the surface of the tile prior to firing in the kiln. This is a ceramic material, as is the tile itself. Therefore when the tile is fired the engobe and the tile 'fuse' together and become one. This means that the surface colours of the tiles are permanent will not fade under any circumstances and the slow change that does take place will be a steady darkening of the red colour. It should be noted that Sandtoft surface-coated clay tiles easily pass the European Frost Resistance test, BS EN 539-2: 1998, with no damage to the tile or its surface.

My clay tiles have small white-centred chips on their surface. What is causing this and will it affect the durability of the tiles.

The small pits that are occasionally visible on the surface of clay products are created when pockets of lime immediately below the surface expand, causing the surface above the lime to be pushed up or 'blown'. This expansion takes place as soon as the tiles leave the kiln as the tiles absorb moisture from the atmosphere. Lime occurs naturally in most clays and can usually be neutralised, ie prevented from expanding, by submerging the tiles in water. Sandtoft make every effort to prevent lime blows, although occasionally the process can still occur before the tiles have been fully soaked. The expansion action of the lime only occurs immediately after the tiles leave the kiln. The process stops once the tiles have absorbed moisture and can not re-occur. Therefore there is no risk of further 'pitting' to the tile surface after the tiles have been laid on the roof. It is a common misconception that clay products can be attacked by frost action due to irregularities within the surface finish. But there is no possibility that the small pits, or 'lime blows', will affect the future durability of the tiles. All Sandtoft clay tiles pass the European Standard test for frost resistance, BS EN 539-2: 1998. Under the terms of the European Standard for clay roofing tiles, BS EN 1304: 1998, small pits or chips 7 mm or less in size in the surface of the tiles are not regarded as faults.

My new clay pantile roof looks wonderful but I am concerned about the gaps between the tiles. Will my roof leak?

Clay pantiles have been around for about 400 years now and have long proved themselves to be an extremely efficient form of roof covering. In early methods of clay production there was very little control during firing over the final shape of a product. This meant that the design of a roof tile was such that it had to be weathertight whatever its final shape - the shape of a traditional pantile is a wonderful testimony to early roof tile designers. Not only do they keep the water out but the gaps around the tiles minimise the suction effect of the wind, helping to prevent wind damage. Because modern clay tiles are pressed, rather than extruded in the way that concrete tiles are, far more weatherproofing features, such as top interlocks and weather barriers, can be designed into the surface of the tiles.

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