Question answered
What is the minimum pitch for a slate roof?
The short answer
The minimum pitch for a natural slate roof is 20 degrees under BS 5534, as Cupa Pizarras reads the standard. That figure assumes moderate exposure, a 500 mm slate and a 115 mm headlap. Flatter slopes need a different covering. Steeper slopes allow a smaller headlap. Cupa's tables give no value below 25 degrees for some smaller slate sizes.
Slate keeps water out by overlap, so the slope it sits on matters as much as the slate. The minimum pitch depends on slate size, headlap and exposure, and the manufacturer's data sheet has the last word.
What pitch means and why slate needs a minimum
Pitch is how steeply a roof rises, given as degrees above level ground. A shallow roof sheds water slowly. A steep one sheds it quickly and lets the wind do less.
Slate is a small-unit covering. Rain does not pass through it; it travels over it, with every course lapping the one beneath. If the slope is too flat, water lingers on the surface, and wind can push it sideways and upward into the gap between courses. The underlay then does the work the slates should do.
That is the reason for a minimum. Below it, the overlap cannot be made large enough to stop water creeping up and in. A lean-to or porch pitched below the limit for slate is a common reason a slate roof leaks only in storms, which our page on slate roof leaks explains in more detail.
The 20 degree figure and the conditions behind it
Cupa Pizarras takes BS 5534:2014+A2:2018 to put natural slate's floor at 20 degrees of slope. That is for moderate exposure, with a 500 mm slate and a 115 mm overlap, the combination at the lowest end of the standard's range.
Every part of that sentence matters. A 20 degree roof built with a smaller slate or a shorter overlap does not meet the figure, and a survey should confirm which case you have. The pitch of 20 degrees is also not a rule that makes every roof safe at that angle; it is the lowest slope at which the stated slate and overlap are expected to work.
The maker's own data sheet settles any doubt. A manufacturer can specify a higher minimum for a particular slate, and the roof should follow whichever figure is stricter. We read the sheet for the slate going on your roof instead of quoting a general number from memory.
Headlap grows as the slope flattens
The headlap is the distance by which a slate overlaps the second course down. As the pitch falls, it has to grow, so that the same rain finds a longer path to the gap.
Cupa's table for sheltered and moderate exposure, for a 500 by 300 mm slate, shows how steeply the figure changes: 115 mm at 20 degrees, 95 mm at 25, 80 mm at 30, 70 mm at 35, 60 mm at 40, and 55 mm from 45 degrees up to 75. For 600 by 300 mm and 500 by 250 mm slates, Cupa lists nothing under 25 degrees.
In practice, a bigger headlap means more slate for each square metre of roof. A low-pitched slate roof is therefore a heavier, dearer roof to lay than a steep one. That is one reason a shallow slope is often better covered another way; see fixing slate on an exposed roof for the wind side of the question.
Exposure and fixing change what is acceptable
The table above applies to sheltered and moderate exposure. On an open site, the standard calls for more caution, and the category for your address depends on the site itself. A slate maker, or a roofer working to BS 5534, can assess that for your house, and we do it as part of the survey.
Fixing changes as the pitch falls. For hook fixings Cupa sets a higher bar of 25 degrees, so a low slope means nailing and possibly extra fixings. The choice between the two is covered under slate hooks or nails.
Where a roof dips below the minimum, a roofer may improve matters with a better underlay, but that is a cure for the symptom, not a licence to lay slate on a flat slope. If the pitch is wrong, the covering usually needs to change.
Measuring your own roof and what to do with the answer
Nobody needs to climb up for this. A phone with a level app, laid along a rafter in the loft, gives a rough reading, though a rafter is not always a perfect guide to the finished slope. Photographs of the gable end, taken square-on, can also be measured. We would rather work it out properly on a survey and show you the result.
If your slope is close to the limit, small things matter: a thin headlap from an earlier repair, a mismatched replacement slate, or a lead flashing that has tilted the courses. Our guide to delaminating slates in the main slate roofing section covers a different reason slates fail on shallow roofs, where water sits and frost works on them.
If your slope falls short of what slate allows, we tell you and set out the options, which might be a different covering or a lead-covered section. The right job is the one that keeps water out, not the one that reuses the old material at any cost.
Slate pitch, two more questions
Can I put slate on a roof below 20 degrees?
Not safely as a plain laid covering. Below the pitch in the standard the headlap cannot be made big enough. A different covering, or a purpose-designed system from a slate manufacturer, is the better route, and its data sheet should be followed.
Does a steeper roof need fewer slates?
It needs less overlap, so a steeper pitch lets the same slate cover more roof per course. Cupa's table shows the headlap for a 500 by 300 mm slate falling from 115 mm at 20 degrees to 55 mm at 45 degrees and above.
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