What is headlap on a slate roof?

Slating goes up the slope in horizontal rows, which roofers call courses. Each course sits on a batten, a thin timber strip fixed across the rafters. Within a course the slates lie edge to edge with a small gap between them, and the slate in the next course up bridges that gap so water cannot run through. That is the cover from above, and it deals with the sideways joints.

Headlap is the third piece, and it deals with the end joints. Picture a slate in the third row up. Its bottom edge reaches down past the top of the slate in the first row, covering it by a set distance. Without that overlap there would be a straight path from the sky to the underlay. The distance is measured along the slope, not across it, and it is the first number a roofer works out, before a single batten goes on. Get it wrong and every course above inherits the mistake, which is why it is checked at the start.

The spacing between battens, called the gauge, is not chosen freely. It follows from two numbers, the length of the slate and the overlap it needs: subtract the overlap from the length, then halve the result. Using the figures from the manufacturer's table above, a 500 mm slate with a 115 mm headlap leaves 385 mm, and half of that gives a gauge of 192.5 mm.

Everything on the slope hangs from that gauge, so an error at setting out repeats on every course. It is also why headlap cannot be added to a finished roof by shifting a slate or two. A different overlap means a different batten spacing, and that means stripping the slope back to the rafters. Our guide to re-slating a roof shows where the setting out fits into a strip and re-lay, and the main slate roofing page covers the whole job.

On a steep roof, water runs off fast and gravity does most of the work. On a shallow one it moves slowly, and wind can push it uphill under the slate above. The remedy is more overlap. The same manufacturer gives 115 mm for the 500 by 300 mm slate at 20 degrees, and adds that under BS 5534 the lowest pitch it indicates for natural slate is 20 degrees for that slate in moderate exposure.

Below that limit, extra overlap does not make the roof a good idea. Our answer on the minimum pitch for a slate roof explains what to do when an extension or dormer is flatter than the limit, which usually points to a different covering altogether. The pitch and the overlap are two halves of the same decision, and we settle both at the survey with a tape and a pitch gauge.

Headlap tables are split by how exposed a roof is. Slate makers point out that the right fixing and overlap turn on the site: its position, its height above sea level and the wind that actually reaches that one building. A house standing in a gap between two taller ones has a different exposure from its neighbour across the road, even in the same street.

For that reason we do not read an overlap off a town name. We look at the roof in its setting, then check the figure against the slate maker's published table for the exact product and pitch. The data sheet beats a rule of thumb, and it beats a measurement taken from the old roof, which may have been laid to an older standard or crept over the years. Where a roof is open to the sea weather, that goes into the judgement too, but we do not put a number on it that nobody has measured.

Too little headlap shows up as staining. Look at the ceilings and at the timbers under the slope for dark tide marks that follow the rows of slates, particularly after a spell of driving rain. The damp can appear a long way from the slate that let it in, because water tracks along the underlay before it drops. That is why a stain tells you little about where the fault is, and why we look at the whole slope.

Slipped slates are a separate fault, usually down to the fixings, and our answers on slipped slates and the slate tingle cover them. If the leak does come from the overlap, a patch will not cure it. You get the verdict with photographs, and the cost is then set down in a single written figure.