The short answer
Cupa reports that BS 5534 puts the lowest pitch for natural slate at 20 degrees, and its own advice rules out hooks on slopes under 25 degrees. Those are guidance figures, not a guarantee for every slate. The data sheet for the exact slate, with the headlap planned, has the last word.
Twenty degrees and where it comes from
A roof's pitch is its slope angle from horizontal, in degrees. Steeper slopes drain faster, giving water less time to creep under a slate or be blown there. Shallow slopes drain slowly, so every lap works harder.
The slating code is BS 5534:2014+A2:2018, which is current. Cupa Pizarras, a slate producer, reads it as setting 20 degrees as the lowest pitch for natural slate, so that is the working figure for ordinary slating.
It is a floor, not a target. A roof just above 20 degrees is at slate's limit, and conditions matter: a sheltered site is more forgiving than an exposed one, and a longer rafter run sends more water to the eaves. Near the minimum, roofers add margin through headlap and fixings.
Marley notes that working to BS 5534 is not itself required by law, though a careful roofer follows it. A covering laid below its published minimum is a poor way to keep rain out, which is what the Building Regulations expect of a roof. Our answer on slate headlap explains the overlap.
Hooks and pitches below 25 degrees
The fixing method depends on pitch. Cupa advises against hooks on slopes under 25 degrees. A hook is a stainless clip that catches the batten and carries the slate's lower edge, leaving no nail hole through the slate. It looks tidy, but on a shallow roof it holds the slate down less firmly against uplift and movement.
On a pitch from 20 degrees up to 25 degrees, slates are nailed. Each is fixed through holes with copper, aluminium alloy or another non-rusting nail, and a replacement slate in a repair is held with a tingle. Our answer on the slate tingle explains that fixing.
Above 25 degrees, hooks become an option, though many roofers still nail. The choice depends on the slate, the exposure and the look. A fixing method has to be consistent across a slope, and a repair has to use a method that suits the pitch of the roof it joins.
This is why measuring comes first. Our survey takes the pitch in degrees with a digital level on the rafters, and records it with a photograph, since an estimate from the ground can be several degrees out.
If the roof is only just above the minimum, you will hear what that means for the fixing method.
Comparison with tiles
Slate has a higher minimum pitch than some tiles, and a lower one than others. Marley publishes minimum pitches for its tiles, and they vary widely. Its Canterbury clay tile is 40 degrees, Ashdowne 35, and its plain concrete tile 35. Its Double Roman concrete is 22.5 degrees in the smooth version, its Mendip is 15 degrees in the smooth version, and its Lincoln pantile is 15.
The comparison explains why a change of covering needs a pitch check. A roof laid with a low-pitch concrete tile may be below the minimum for slate, so swapping to slate is not simply a matter of taste. A roof built for slate may suit many other coverings, but a roof built for a shallow tile may not.
Weight is the other check. A new covering that is 15 per cent or more heavier needs Building Regulations approval and a structural check, according to the Planning Portal. Our answer on slate or tile compares the two.
Always check the product data sheet. Marley's guidance says so for its tiles, and it applies equally to slate from any maker, because a particular slate, size or exposure may call for more than the generic minimum.
Data sheets, headlap and low-pitch roofs
Headlap is the overlap of each course over the one two below it, and it is the main safeguard on a lower pitch. The lower the pitch, the greater the headlap the slate needs, so a slate that works at 30 degrees with a certain lap may need a longer one at 22 degrees. The data sheet for the slate and size gives the required lap for each pitch and exposure.
That sheet is the authority, and a roofer who works from it, not from habit, is the right one for a low-pitch slate job. Where a roof sits at or near the minimum, the lap, the fixing and the underlay are all chosen with care, and a membrane beneath gives a second line of defence.
Below the minimum, slate is not the right covering. A roof with a low pitch may suit a different material, such as a profiled tile made for shallow pitches, a metal covering or a flat roof system, depending on the angle and the design. The right fix is whichever suits the roof, not whichever suits a preference.
We measure, check the sheet and tell you what the roof will take. A single written price follows, with the slate, size, headlap and fixing named in the scope. See slate roofing for the rest of the service.
