Slate pitch
What is the minimum roof pitch for slate?
The lowest safe pitch is not one number: it moves with the slate, the overlap and the site. Below are the published figures and what a survey measures.
The short answer
Twenty degrees is the lowest pitch Cupa, a slate maker, quotes from BS 5534 for natural slate, and only for a 500 mm slate with a 115 mm overlap on a site with moderate exposure. A more exposed site needs more help from the design. Hook fixings should not go on any pitch below 25 degrees. Man-made slates follow their own data sheet, so the figure for your roof is worked out from the slate, the overlap and the site.
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Where the 20 degree figure comes from
The number is a published minimum, not a rule of thumb. Cupa reports that BS 5534:2014+A2:2018 indicates a minimum roof pitch of 20 degrees for natural slate on a project with moderate exposure, using a slate 500 mm long and a 115 mm overlap. Every part of that sentence carries weight. Alter the slate length, the lap or the weather the site takes, and the lowest workable pitch moves with it.
BS 5534 is the code of practice that sets out how slating and tiling is done in Britain. Marley, a tile maker, points out that the law does not require a roof to be installed to it, and lists batten selection, methods of fixing, wind uplift formulas and a means of calculating fixing specifications among its contents. It earns its place because it gives a method for working out a specification for each roof, which a rule of thumb cannot do.
Pitch means the angle of the rafters measured from the horizontal. A slope that looks gentle from the pavement can sit well under 20 degrees, so the angle is taken on site with a level or an angle finder and never judged from the ground. Each slope is measured on its own.
What pushes the minimum higher
Cupa names two main factors: exposure to wind and exposure to rain. It says a building is graded with higher exposure if it stands on high or sloping ground, sits near the coast, or is tall. It also advises that small slates suit steep roofs, while on a more exposed site or a lower pitch, wider slates should be used and the lap increased. Its head-lap advice is built on the same inputs: the roof pitch, wind uplift and exposure to driving rain.
- A longer overlap on a shallow slope, so each slate covers more of the one below.
- A wider slate where the site is exposed.
- A steeper pitch where the slate and overlap you want will not work at the angle you have.
- Fixings calculated for the site, not taken from a catalogue.
Notice that a lower pitch and a more exposed site push in the same direction, towards a longer lap and a wider slate. A roof that is both shallow and exposed has the least room for error.
On a coastal roof the fixing specification is calculated for the site under BS 5534, because coastal areas are graded with higher exposure. That is a calculation for your address, not a general rule about the sea.
What we measure before quoting
A survey on a slate roof starts with the angle of each slope, the size of the slates now on it and the overlap between courses. Those three readings are set against the maker's data and the exposure of the site. The table shows the published figures that frame the check.
| Situation | Published figure |
|---|---|
| Natural slate, moderate exposure, 500 mm slate, 115 mm overlap | 20 degrees minimum |
| Any slate held with hook fixings | Not below 25 degrees |
| More exposed site or lower pitch | Wider slate, longer lap |
| Man-made slate | Whatever the maker's own data sheet gives |
The data sheet habit applies to any covering. Marley's own list shows how wide the spread is: the lowest pitch it gives runs from 12.5 degrees for its Mendip 12.5 concrete tile up to 40 degrees for its Canterbury clay tile, which is a Marley product name and nothing to do with the city. A figure that suits one product says nothing about another, so the sheet for the product on your roof, or the one you are considering, is the one that counts.
A low pitch is a reason to check
If the angle comes out under the figure for your slate and overlap, you hear it before any price is written. The options are then a longer lap, a different slate or a covering with a lower minimum, each with its own rules. Changing the pitch itself is a much bigger job and is only worth discussing if the others do not work.
Re-roofing in slate meets the usual tests as well. Under the Planning Portal's guide, re-roofing does not normally need a building regulations application if it affects less than 25 per cent of the building envelope and less than 50 per cent of the roof. Separately, new slates that increase the weight of the roof covering by 15 per cent or more bring in an approval requirement of their own, so the weight of the slate you choose is compared with the covering it replaces.
Our slate roofing page shows how a re-slate runs. Our answers on slate hooks or nails and on natural against man-made slate cover the fixing and product questions that follow from pitch.
More questions
Does a lower pitch always rule slate out?
No. A lower pitch means a longer overlap, a wider slate or a different covering has to be worked out, and the data sheet for the product holds the figures. We measure the roof first and tell you which options are open before any price is written.
Is the minimum pitch the same for hooks and nails?
No. Cupa rules out hook fixings on any pitch below 25 degrees, while its 20 degree figure is for natural slate in the case it describes. A roof between 20 and 25 degrees would therefore be nailed, not hooked, and the specification would still be checked for the exposure of the site.
Tell us what's wrong with the roof.
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