The overlap you cannot see from the ground
A slate roof is built upward in horizontal rows named courses, starting at the eaves (the lowest edge). Each row covers the joints of the row below it, in the way bricks are staggered. The exposed part of a slate is what shows. Everything tucked under the slates above is the headlap.
More exactly, headlap is the amount by which the tail of a slate in one course overlaps the head of the slate two courses below it. Two courses are involved because water that gets past the first joint should meet a second slate before it reaches the batten, the thin timber strip the slates are fixed to.
Water gets under slates by three routes: it runs down the face and into side joints, it is blown up the slope by wind, and it creeps up a narrow gap by capillary action, the same pull that draws water up a tissue. Headlap is the main defence against the second and third.
You will never see a slate's headlap once the roof is laid, which is why it is something a surveyor has to check from the loft or by lifting a slate. A neat looking roof can still have too little lap behind its tidy rows.
Cupa's worked example, in numbers
Cupa, a slate producer, publishes guidance on minimum headlap and fixing. Its example is set out here, with the layout it implies.
| Item | Figure |
|---|---|
| Roof pitch | 20 degrees |
| Slate length | 500 mm |
| Headlap | 115 mm |
| Exposure | Moderate |
Cupa gives 20 degrees as the lowest pitch BS 5534 indicates for natural slate, so the example sits right at that limit.
Those figures also show how a roof is set out. Take the 115 mm lap from the 500 mm slate and 385 mm remains. That length has to cover two courses, so about 192 mm of each slate is on show. The roofer marks the battens at that spacing before the first slate goes up, and every row after it follows the marks.
The figures describe one case. A different slate length, pitch or site changes the answer, so they should not be copied onto another roof. A shorter slate with the same lap would show less of itself, and the battens would sit closer together as a result.
Why a flatter roof needs more lap
A steep slope drains quickly, and gravity does most of the work. As a slope flattens, water moves more slowly and sits on the slate surface longer, and wind has a better chance of pushing it up the joint. More overlap puts more slate in the way.
Exposure counts too. Cupa grades sites on slopes, hills and the coast as more exposed, and the more exposed a site, the more lap or the steeper a pitch the slates need. The Met Office's Manston climate station gives a mean wind speed at 10 metres of 9.86 knots, with January at 11.21 knots, and about 613 mm of rain over a year, all averages for 1991 to 2020. They describe one station, so a roof's own exposure is still assessed at the house.
The roofer's checklist for headlap, whenever a slate roof is laid or relaid:
- the slate length chosen for the pitch;
- the exposure grade of the site;
- the lap that results, then the batten spacing from it;
- the fixings, calculated under BS 5534 for the site's exposure.
Where a pitch is under 20 degrees, it is below the figure Cupa gives from BS 5534 for natural slate, so unless the slate's data sheet allows that pitch, the covering itself is reconsidered.
Lap and exposure on an existing roof
On a roof that has stood for decades, headlap is not usually something you set. It was set on the day the slates went up, and your task is to check it has not been lost. Slates that slip downwards cut their own lap, because the tail now sits lower than the row intended.
Signs the lap has shrunk or was always short:
- Damp patches in the loft below a slope after driving rain.
- Daylight showing through the roof when the loft is dark.
- Slipped slates, often with a gap above the bottom edge.
- Staining on the sarking, battens or rafters under one slope only.
Relaying slates later, whether to reuse old ones or fit new, is the moment to reset the lap properly. A roofer will state the length of slate, the lap and the gauge, which is the batten spacing, in writing. Weighing a repair against starting afresh? Our page on re-slating or repairing sets out the choice, and the slate roofing page covers the rest.
The angle side of the question has its own page, on the minimum pitch for slate.
