The short answer
For natural slate, BS 5534 indicates a minimum pitch of 20 degrees, but only in moderate exposure and with a 500 mm slate laid to a 115 mm headlap, as reported by Cupa Pizarras. NHBC gives the same 20 degrees subject to the headlap. A longer slate, a harsher site or a thinner overlap moves the figure up, so 20 degrees is a starting point rather than a licence.
Where the 20 degree figure comes from and what it assumes
Cupa Pizarras, a slate producer, reads BS 5534:2014+A2:2018 as setting 20 degrees as the lowest pitch where natural slate is laid. It attaches conditions: moderate exposure, a slate 500 mm long and a headlap of 115 mm. The NHBC standards, in section 7.2.19, say the minimum pitch for a slate roof is 20 degrees subject to headlap.
Read those together and the figure is less a flat rule than a point on a curve. Change the slate length, the overlap or the exposure and the lowest safe angle changes with it. A roofer quoting "20 degrees" without asking what size of slate is going on, and how much it will lap, is quoting half an answer.
Headlap is the distance by which each slate covers the slate two courses below it. It is what stops wind-driven rain being pushed up the joint between slates. A flatter roof sheds water more slowly and lets it travel further sideways, so it needs more overlap to stay dry.
How the headlap grows as the slope flattens
Cupa's tables for moderate exposure, which it defines as under 56.5 litres per m² of wind-driven rain, show the pattern clearly. These are manufacturer figures, so the data sheet of the product you actually buy has the last word.
| Slate size | Pitch | Headlap |
|---|---|---|
| 500 x 300 mm | 20 degrees | 115 mm |
| 500 x 300 mm | 22.5 degrees | 105 mm |
| 500 x 300 mm | 25 degrees | 95 mm |
| 600 x 300 mm | 25 degrees | 95 mm |
| 600 x 300 mm | 30 degrees | 80 mm |
| 600 x 300 mm | 40 degrees | 60 mm |
Notice that the 600 x 300 mm slate has no value at all below 25 degrees. A bigger slate does not buy a lower pitch; on that table it does the opposite. At the steep end the same slate needs only 55 mm of lap between 45 and 75 degrees.
Why 25 degrees is a second threshold
There is another number to keep in mind. Cupa also advises against hook fixings anywhere under 25 degrees of slope. A hook fixing holds the slate with a hook at its tail instead of a nail through its head. Below that angle a slate laid in the usual way is nailed, and the nails need to be of a type and spacing set out for the job.
So a roof at 20 or 22 degrees has two limits at once: a bigger headlap, and nails rather than hooks. That constraint rules out some of the slating methods covered in our guide to slate sizes, headlaps and pitch, which explains how the three interact; this page only answers the single minimum.
On a low roof we also look hard at the underlay beneath the slates, because at shallow angles more water reaches it. It is the back-up layer when the slates alone do not quite keep the weather out.
Exposure can raise the figure again
Everything in the table above assumes moderate exposure. Cupa treats any ground within 30 miles of a coast that faces west as severely exposed. A severe site needs a bigger headlap for the same slate and pitch, or a steeper slope.
We do not give a category for any particular Stroud street here, because it depends on the exact site and the maps used. What we will do on a survey is ask which category applies, check it against the manufacturer's data, and put the headlap into the written quote. That matters because headlap is not free. More overlap means more slates per m² and more weight, so it feeds straight into the price.
If a roof is already below 20 degrees, such as a dormer cheek, a porch or a lean-to extension, slate may simply be the wrong covering. a metal sheet or single-ply membrane then does the work, and the slates are kept for the steeper slopes beside it.
Stone slate follows different traditions
Do not carry the 20 degree figure over to Cotswold stone slates. The Cotswold District Council technical guidance from 2000 describes a simple stone roof as usually between 47 and 55 degrees, but that is how they were traditionally built, not a minimum. We hold no verified minimum for natural stone slates, so none is given here.
What the figure does tell you is that stone roofs in the area are typically steep. If you are looking at a shallower roof on an extension and wondering about matching it, ask for the product data sheet and check the slate before you commit to a look.
Finding out your own roof's pitch
You can get a reasonable estimate from the loft. Hold a spirit level against the underside of a rafter, measure 1 metre along it horizontally, and measure the drop. A drop of about 36 cm per metre is roughly 20 degrees. For anything near the limit, a proper reading from a surveyor's digital level is better than a tape measure.
When we survey, we photograph the slopes and record the pitch of each, because a hipped roof or a rear extension can sit at its own angle. Your written quote then lists slate size, lap and fixing slope by slope, so you can see why the overlap changes from one part of the roof to another.
For the other practical questions on slate, our answers on moss on slate and slate against tile cost are good next reads, and the full picture is on our slate roofing page. Our work carries a 10-year workmanship guarantee in writing, covering what we lay rather than how the roof was first designed.