Slates are not glued or sealed to one another. They shed water because each one is overlapped by the slate two courses above it, and the amount of overlap is set by the roof's angle and by how much rain the wind drives at it. Three numbers therefore decide whether a slate roof is watertight: the size of the slate, the headlap (the overlap down the slope) and the pitch (the angle of the roof). Change one and the others have to follow.
This guide explains how the three relate and where the published figures come from. They are drawn from a Cupa Pizarras guide that reports BS 5534 and NHBC requirements. If you only want the single lowest angle, the answer on the minimum pitch for a slate roof gives it. For the wider service, see slate roofing in Stroud.
Length, width and the three overlaps
A slate is described by its length first and its width second, in millimetres. Two sizes appear in Cupa Pizarras' published headlap tables: 500 x 300 mm and 600 x 300 mm. The length is the dimension that runs up the slope, and it matters most because it governs how far one slate reaches down over those below.
The terms in use
The headlap is the vertical overlap: the distance by which a slate in one course covers the slate two courses beneath it. The sidelap is the overlap across the width, where each slate is positioned over the joint between two slates in the course below, which is why slates are laid in a staggered bond. The gauge is the distance between the tops of successive battens, which are the cross-timbers the slates hang from. The gauge is what the roofer actually sets out with a tape; the headlap is what the gauge produces.
Why length is not the same as cover
A 500 mm slate does not cover 500 mm of roof. Part of its length is hidden under the courses above, and part sits over the headlap beneath. The visible face, called the exposure, is only a fraction of the whole slate, and that fraction shrinks as the headlap grows.
How pitch decides the headlap
On a steep roof, rain runs off quickly and gravity helps. On a shallow roof, water moves slowly and wind can push it up the slope under the slate. The answer is more overlap. As the pitch falls, the required headlap rises.
Cupa Pizarras' summary of BS 5534 shows this clearly. For a 500 x 300 mm slate in moderate exposure, the figures are a 115 mm headlap at 20 degrees, 105 mm at 22.5 degrees and 95 mm at 25 degrees. The table for the longer 600 x 300 mm slate begins at 25 degrees, where the headlap is 95 mm, and falls to 85 mm at 27.5 degrees, 80 mm at 30, 70 mm at 35, 60 mm at 40, and 55 mm from 45 up to 75 degrees. At 85 degrees it is 50 mm.
| Pitch | 500 x 300 mm | 600 x 300 mm |
|---|---|---|
| 20 degrees | 115 mm | No value given |
| 22.5 degrees | 105 mm | No value given |
| 25 degrees | 95 mm | 95 mm |
| 30 degrees | Not listed here | 80 mm |
| 45 to 75 degrees | Not listed here | 55 mm |
The blank cells matter: they mean the tables we hold give no value, not that the combination is allowed. The product data sheet for the slate you are buying is the final authority.
From headlap to batten gauge: the working sum
Once the headlap is chosen, the batten gauge is simple arithmetic for a slate that is nailed near its head. Take the slate's length, subtract the headlap, and divide by two. The division by two is there because the slate is lapped by the course above and sits over the course two below.
Worked examples
For a 500 mm slate at 20 degrees, with the 115 mm headlap from the table, 500 minus 115 leaves 385, and half of that gives a gauge of 192.5 mm. For a 600 mm slate at 30 degrees, with 80 mm of headlap, 600 minus 80 leaves 520, and the gauge is 260 mm. The longer slate at the steeper angle gives a much wider gauge, so fewer courses and fewer battens cover the same slope.
That is also why a roofer cannot switch to a slightly shorter slate and keep the old batten spacing. A shorter slate on the same gauge leaves too little overlap, and the shortfall appears as leaks at the head of the course.
Setting out the slope
The gauge is worked out for the whole slope before the first batten is fixed, so the courses finish evenly at the ridge, with the top course the full width of its gauge and the eaves slate overhanging the gutter by the right amount. Where a slope does not divide into whole courses, the gauge is eased slightly across the slope, never squeezed at one end.
Severe exposure and the 30-mile rule
The headlaps in the table assume moderate exposure. Cupa Pizarras defines that as a driving rain index below 56.5 litres per square metre. Where driven rain is heavier, the roof needs more overlap, and the lap tables rise accordingly.
The same guidance adds a blunt rule: anywhere inside 30 miles of a coast facing west counts as severe exposure. Severe exposure calls for a longer headlap than the standard one, and the exact lap for each slate length comes from the slate supplier's table or the architect's specification for your address, which is why it is confirmed before any slates are ordered. Which category a given address falls into depends on the location, and those figures decide it.
What this means for a quote
If the rule applies to your house, the headlap increases, the gauge narrows, and the same slope takes more battens and more slates. It is therefore not a detail to settle on the day. The effect shows in the cost of materials in the written quote, and we check the exposure category with the slate supplier at the planning stage.
We have not found a sourced statement of which exposure category Stroud falls into, so we do not claim one. If you want to know where your own roof sits, ask for it to be stated in writing alongside the slate size and lap.
Reading the size and lap of an existing roof
You can work backwards from what is already on the house, and this is the first thing we do at a survey. A slate taken from a hidden spot, such as behind a chimney or under the eaves, is measured for length and width. The batten spacing is measured with a tape from the underside in the loft, or from a stripped section.
Finding the headlap
The headlap of an existing roof follows from two measurements. Take the length of the slate, subtract twice the gauge, and the answer is the headlap. A 500 mm slate on a 190 mm gauge, for example, has 500 minus 380, so 120 mm of lap. If the result is far below the table figure for that pitch, the roof is under-lapped, and that is a likely reason for leaks that repairs do not cure.
Why old figures differ
The tables quoted here are modern and tied to a standard. Older roofs were laid by custom and many work perfectly well on laps that would not meet today's tables, helped by their weight of slate and by decades of settling. We do not condemn a roof for that alone. We note it, we photograph your own slopes, and the survey records whether the laps are the cause of any leak or just a difference from current practice.
Measuring at the survey also tells us whether the slates are a standard modern size, which makes matching new ones easy, or an old irregular size, which means sourcing or sorting reclaimed stock before work starts.
Minimum pitch and the fixing that goes with it
Per Cupa Pizarras, BS 5534:2014+A2:2018 indicates 20 degrees as the floor pitch for natural slate, provided exposure is moderate and the slate is 500 mm long with a 115 mm overlap. NHBC's standards, in clause 7.2.19, Table 12, give 20 degrees subject to headlap. So 20 degrees is not a free-standing number; it comes tied to a slate size and a lap, and cutting either one makes it invalid.
Fixing changes with pitch
The same source warns against hook fixings on any pitch below 25 degrees. A slate on a shallow roof is nailed, and may need a second fixing. That is one reason a low-pitch slate roof is more demanding to build and to repair than a steep one.
What about a pitch below the minimum
Below the minimum, slate is not the right covering unless a specialist design applies. A roof with a shallow pitch may suit a different covering, and a roof designed for another material and later re-covered in slate needs the pitch checked first. Do not assume an old roof meets the figure; measure it. A survey records the angle on each slope, since an extension or a dormer roof can sit at a different angle from the main roof.
What this means on a repair or a re-slate
On a repair, size and lap decide whether the new slate fits the old roof. A replacement that is shorter than the original, even by a few millimetres, will not lap properly, and the fault shows in the next storm. Matching slates by size, thickness and colour is therefore part of the repair and not a cosmetic extra; our slate conservation page covers the extra care on protected buildings.
Older roofs
Victorian roofs often carry slates of mixed or unusual sizes, laid to the stock of the day, and the gauge was set by eye. Re-laying them to a modern table can be a poor fit. Our guide to Victorian slate roofs explains the details we look for before stripping.
On a re-slate
Before a roof is stripped for re-laying, the old gauge is recorded, the pitch is measured, and the lap is checked against the published table and the exposure. If the old roof was laid with too little headlap, re-laying it the same way repeats the fault.
Re-roofing that touches over half the roof can bring in Building Regulations, as set out in the planning portal rules. The work carries our 10-year workmanship guarantee, in writing.