How often do you come across the phrase “it’s all in the preparation”, whether it be decorating, baking a cake, or building a shed for example? Roof tiling is no different. Roof tile manufacturers are often called to sites to investigate complaints that their roof tiles are ‘faulty’, when in fact it turns out that the roof substructure is at fault.
One of the most common problems is where a truss or rafter is slightly out of line. This may not be too obvious until the roof tiles are installed and the roofer then finds that the tiles ‘kick up’ instead of lying flat as intended. This can be a particular problem in new-build work where no one expects the trusses to be out of line.
Tiles that are installed ‘straight bonded’ are more forgiving of undulations in the roof slope as they will tend to follow any slight curve. Tiles that lay cross bonded, such as large format interlocking flat tiles, tend to kick up in a ‘zig-zag’ pattern if the roof structure is uneven and it is this phenomenon that often triggers complaints about the tiles. In the 1960s and 70s flat interlocking tiles that are larger than the current models were popular and these were particularly prone to exaggerated ‘kicking’ if there were even the slightest undulations in the substructure. Manufacturers phased out these larger flat tiles in favour of the more popular ten to the square metre roof tiles, though it is still important to have as flat a plane as possible on which to lay the tiles.
When constructing a building it is important to ensure that the gable walls finish at the correct height in relation to the rafters to support the tiling battens in the correct plane, taking into consideration any counterbattens, rigid sarking or insulation over rafters etc. It is equally important to finish the top of the gable wall neatly so that the roof tile cloaks or dry verge components properly lap over the gable wall without any gaps between the masonry and tiling.
Similarly, care must be taken to finish party walls at the correct height, taking into account the fire-break materials, so that the tiling passes over the party wall without any undulations.
Constant weight
When retiling old buildings it is common to find that the rafters are out of line – often because the timbers have sagged under the constant weight of the tiles over the years. Very often, timber roof structures will settle differentially in relation to the masonry gable and party walls, therefore roofers will need to put packers or spacers under the battens to make sure the tiling surface is as flat as possible.
Another common problem occurs where the tiling has to pass over a flashing; for example, when tiling over a roof window, valley or side abutment flashing. Upstands and water channels on the flashings can be set higher than the tile battens, therefore extra care is required to keep the tiles as level as possible. Sometimes this will mean that part of the tile – usually a nib – may be trimmed off, taking care not to compromise weathertightness or security, to ensure the tile sits level or as near level as possible.
Installing the tiles in line and level, as intended, is not only important for aesthetic reasons, but also for the weathertightness of the roof, particularly at lower roof pitches.
Bullcast eaves
There is a commonly held belief that the eaves course tiles should be set at a lower pitch than the tiles above; ie a detail commonly referred to as a ‘bellcast’ eaves. In fact, the correct detail, particularly for single lapped tiles, is to set the eaves course tiles at the same pitch as the tiles above. Therefore, it is important to set the fascia or tilt fillet at the appropriate height to support the eaves course tiles at the correct pitch.
Small double lapped plain tiles are much more forgiving of undulations in the roof plane and, as we all know, are ideal for features where a curve or change in direction is intentional; for example on curved or conical roofs, eyebrow features, curved valleys etc. However, it is still important, for aesthetic reasons to ensure that any unintentional undulations in the roof plane are dealt with prior to tiling.
In summary:
– Make sure gable and party walls finish at the correct height to keep the roof plane level.
– Special consideration should be given to tiling passing over window and abutment flashings – judiciously trim nibs if necessary to allow tiles to site flat but take care not to compromise the tile’s weathertightness.
–Sometimes when retiling old buildings it will be necessary to pack under the tile battens to keep the tiling level.
– Set single lap eaves course tiles at the same angle as the tiles above.
