The short answer
Slate producer Cupa Pizarras, reading BS 5534, puts the floor for natural slate at 20 degrees, in moderate exposure with a 500 mm slate and a 115 mm headlap. Shallower roofs need a different covering. Steeper roofs can use a smaller overlap, and exposed sites need a deeper one.
Pitch, headlap and why they go together
Pitch is the angle of the roof slope from the horizontal, measured in degrees. Headlap is the depth by which each slate is covered by the course two rows above, and that overlap stops rain being blown or drawn up under the slates. On a steep roof, water runs off fast and a small overlap is enough. On a shallow one, water moves slowly and can creep upwards, so the overlap has to be deeper.
That is why no single pitch limit works without the headlap. A roof at 20 degrees needs a long slate and a headlap of 115 mm. The same slate at 35 degrees needs only 70 mm. Sizing, pitch and exposure are decided together at the design stage, and a roofer who quotes you a minimum pitch for slate should be able to tell you the headlap that goes with it.
Our page on slate sizes and headlaps explains the terms in more detail.
The figures from BS 5534 via Cupa
Cupa Pizarras, a slate producer, publishes guidance based on BS 5534:2014+A2:2018. In it, 20 degrees is the lowest pitch for natural slate, assuming moderate exposure, a 500 mm slate and a 115 mm headlap. The table gives its moderate-exposure figures for the 500 x 300 mm size.
| Pitch | Minimum headlap, 500 x 300 mm slate |
|---|---|
| 20 degrees | 115 mm |
| 25 degrees | 95 mm |
| 30 degrees | 80 mm |
| 35 degrees | 70 mm |
| 45 degrees | 55 mm |
For 600 x 300 mm and 500 x 250 mm slates, Cupa gives no value below 25 degrees. Read the data sheet for the product you intend to use.
Exposure changes the answer
The figures above assume moderate exposure. Cupa treats any area within 30 miles of a west-facing coastline as severely exposed. For severe exposure, the headlap gets bigger or the minimum pitch moves up, and your roofer should work it out from the standard rather than guessing.
We do not quote local weather statistics for Swindon or Wiltshire, because there are no figures we can stand behind. The principle holds, though: an exposed hilltop home on the downs needs a more cautious design than a house in a sheltered street, and the roofer has to read the exposure, not assume it.
If you are building an extension or replacing an old roof and you do not know the exposure category, ask the designer or the supplier to confirm which one has been used. Our written quote names the slate size and headlap we intend to use, so you can check them against the data sheet.
Fixings at low pitch
Pitch affects fixings as well as laps. Cupa adds that hook fixings are not for pitches under 25 degrees. At the low end of the range, slates are nailed, and the nails are the main defence against wind lift and slippage. That makes the choice and quality of nail more important at 20 or 22.5 degrees than at 40.
At low pitches, slate is also less forgiving of poor workmanship, because a small gap or a cracked slate lets water travel further. Underlay and battens matter too. A good underlay with correctly lapped joints and flashings gives a second line of defence when a gust drives rain in.
Our slate fixing methods page explains nails, clips and hooks in more detail, and our page on keeping a slate roof waterproof sets out the other layers that protect it.
What to do if the roof is shallower than 20 degrees
If a roof is below 20 degrees, natural slate is not the right choice unless the design is altered. Options include a different covering that is rated for the pitch, such as a profiled metal sheet or single-ply membrane, or raising the pitch of the roof structure, which usually means major work and may need approval. Lead, zinc and some interlocking tiles work at lower pitches. The product data sheet gives each one's limit, for example Marley's Lincoln clay pantile is listed at 15 degrees.
Do not stretch a slate to a pitch it is not rated for. Roofs that fail early do so because water gets under the slates in wind-driven rain, and that is hard to cure without stripping the roof.
If you are replacing a roof that already has slate at a low pitch, a survey will tell you what it has and whether it is working. Some old roofs have survived for generations through good maintenance and deep headlaps, and that is different from a new design.
Checking a roof you already have
You can estimate pitch from a loft or a photograph, but a survey measures it properly. We take the angle, the slate size and the headlap on a sample of courses, and compare them with the standard for the building's exposure. If the numbers are tight, we flag it and explain what that means for re-roofing.
Look at the page about walking on slate before you go on a roof. Shallow slate roofs are slippery and slates crack under weight. Our slate versus tile cost page can help if you are comparing coverings, and the Victorian slate roofs page covers older buildings.
For a survey, ring 01793 626096 or drop us a WhatsApp. You get the quote in writing afterwards, and any work we do is under our ten-year written workmanship guarantee. Browse the wider slate roofing hub for related jobs.