Roofing battens or laths are strips of timber which are nailed into lines along the roof trusses of a house, from which the roof tiles are hung. They come in different sizes and are usually treated with wood preservative. Their spacing is determined by the size of the tiles and the height and pitch of the roof to be covered. Getting the right layout is essential to prevent leaks.
Read the manufacturer's instructions for your tile. This should specify the maximum tile gauge. The gauge is the exposed section of tile when it sits on the roof, ignoring the overlap with adjoining tiles. The gauge will be the spacing between your battens.
Take two sections of roof batten. Place these on the roof, one above the other. Now take two or three roof tiles. Place these loosely onto the battens. Do not fix into place; you are using them purely for measuring purposes.
Calculate the gutter overhang by resting a tile on the roof so that its bottom edge overhangs the guttering by the required distance. Fix my Roof recommends an overhang of around 50 millimetres, but this will vary according to the depth of the guttering of your house. As a rule of thumb, stop the tile just short of the halfway point. Slide one of your battens under the tile and fix it in place.
Arrange the second batten above the fascia batten to the tile gauge recommended by the tile manufacturer. Check whether this is the appropriate gauge for the pitch of your roof. According to Fix my Roof, if the roof is shallow pitched, or faces into the wind, the tile lap should be increased 25 millimetres, which will give a smaller tile gauge. Fix the second batten in place.
Measure the tile gauge with a tape measure.
Measure the height of the roof from the top of your second batten to about 30 millimetres short of the top of the roof. This is where you will place your final batten, to allow the tile lugs to fit without pinching or breakage.
Divide your height measurement by your tile gauge. The result is the number of additional battens you require. Round any part numbers up, as obviously you cannot install part of a batten. For example, if your roof measures 450cms and your tile gauge is 33cms, you would need an additional 13.63 battens. Round this up to 14.
Recalculate your tile gauge based on the number of battens specified in step 7. So, using the previous example, divide your roof height of 450cms by the batten number of 14. This gives 32.14cms, which is the tile gauge you should use for the remaining battens to ensure correct spacing between them.