In the latest instalment of his regular column, Technical Roofing Consultant John Mercer addresses some of the common pitched roofing mistakes with regards to design and poor installation, and explains how these can be avoided…
During my career inspecting roofs and problem solving, I have come across many recurring faults and mistakes. Some are design-related and others are simply due to poor installation. However, they all have the potential to cause a roof failure, whether that be rainwater ingress, wind damage or simply an unacceptable appearance.
A good roof must start with good design. Ideally, a roof should be designed with the roof tiles in mind, considering factors such as roof pitch, detailing and rafter length.
Avoid designs where water is discharged from a higher roof slope onto small lower roof areas, or into another junction such as a side abutment. If practical, design the building to break up the roof into smaller sections. For example, allow the upper roof to shed directly into a gutter, rather than onto a lower roof. When building a lean-to extension, do not be tempted to drain the gutter of the main roof onto the extension roof through a down pipe. Extension roofs, by their nature, may have quite a low roof pitch, therefore the tiling can be overwhelmed by water being discharged from the roof above in a point load.
Overly long rafter lengths can be problem, particularly when using single lap flat tiles at low pitches. Tile manufacturers give maximum recommended rafter lengths for their products, with guidance on increasing the roof pitch for long rafter lengths. Roof tiles are subjected to rigorous wind-driven rain testing in wind tunnels and on roof rigs that usually simulate a 6 or 7 metre rafter length. Therefore, if a rafter length is longer than this, there is a risk of water ingress through the tiling, particularly in the tile courses near the eaves.
It is often assumed that rainwater should never penetrate the roof tiles. This is generally true for perhaps 99% of the time, but there is always the risk, particularly during a heavy storm, that rain may be driven through the tiles. Condensation may also develop occasionally in the batten cavity (the space between the tiles and underlay). Therefore, underlay must act as a second line of defence, as well as making the building weathertight before the tiles are installed. A common mistake is to lay the underlay too tight, ie without a drape. Any water in the batten cavity then becomes trapped behind the battens and cannot flow toward the gutters.
Where this happens, there is a risk that water will penetrate through the batten nail holes. Where the underlay is laid unsupported over the rafters, it must have enough drape to allow water to drain under the battens, up to a maximum drape of 15mm.
Alternatively, consider the use of nail tape and counterbattens. Nail tape seals between the underlay and battens and counterbattens encourage air flow within the batten cavity and provide a gap for any water to flow under the tile battens.
The underlay must also be installed to be weathertight at junctions such as side abutments, valleys and around roof windows and chimneys, by turning the underlay up against abutments and lapping over valley sides.
Another detail prone to incorrect installation is the eaves. Firstly, it is common to see the first tiling course set at the wrong angle; if using double lapped plain tiles, then it is permissible to have a ‘sprocketed’ or ‘bellcast’ eaves provided that the tiles within the sprocketed section are laid at or above their minimum recommended roof pitch. For single lap tiles, BS 5534 recommends that the tiles are not sprocketed at eaves and should be set at the same angle as the general tiling above.
Single lap roof tiles rely on their headlaps to be set correctly, therefore a sprocket will run the risk of leakage through the headlaps.
It is common to find that the underlay is not supported behind the fascia or tilt fillet, creating a ‘trough’ in the underlay in which water can collect. If water pools behind the fascia, it will find its way through any cuts or holes and eventually rot the underlay, causing leaks. Therefore, the underlay must be properly supported at the eaves, with adequate fall, behind the fascia or tilt fillet using proprietary underlay support trays.
Valleys are one of the most likely areas of a roof to fail because water is directed into a valley from the adjacent roof planes. Unfortunately, they are also one of the most likely details to be constructed incorrectly. There is a common misconception that the mortar in a valley is there to seal it. But think of plain tile or slate valleys that are installed without mortar. When constructing a lead valley, it is important to install tile battens to create upstands in the lead each side of the valley. The lead valley lining can then be dressed over the upstands, with a clear gap between the mortar and upstand and its outer edges finished in a welt, thus providing two defences against water ingress.
The valley tiles must not be mortar-bedded directly onto the lead; lead expands and contracts with changes in temperature, therefore mortar must be laid onto an undercloak of slate or fibre cement board to allow differential movement between the lead and undercloak.
Side abutments must be constructed to suit the tile type they are being used with. For example, individually soakers should be used with double lapped plain tiles and cover flashings are fine for profiled single lapped tiles. But a common mistake is to use a cover flashing solely with single lap flat tiles. Without a tile profile, or some form of upstand in the tile shape, water can penetrate between a flat tile and the cover flashing. Therefore, it is important to install a lead-lined secret gutter or proprietary soakers. Ideally, if using a secret gutter or continuous GRP soaker, a step and cover flashing should also be installed to prevent debris getting into the water channel and clogging it over time.
Lastly, but just as important, is the tile fixing specification. There are still roofs being installed without a wind load calculation having been carried out. Tile manufacturers can supply written fixing specifications for a project. This service is free and generally available as an instant online service.
• Make sure the roof design is correct and appropriate for the chosen roof tile.
• Install the underlay to provide a continuous waterproof layer.
• Ensure the eaves course tiles are set at or above the minimum recommended roof pitch and do not use a sprocket detail with single lap tiles.
• Take care with lead valleys to install upstands and welts to protect against water ingress.
• Use the side abutment detail appropriate for the roof tiles.
• Make sure you obtain a written manufacturer’s fixing specification for each project.