What roof pitch means and why it matters

Measured against the horizontal, a slope's angle is its pitch, so a flat roof sits near zero and a steep gable might reach 50 degrees or more. In the UK it is normally written in degrees. Older drawings give it as a ratio of rise to run, such as 1 in 2, and people sometimes say "quarter pitch" or "half pitch", which are loose descriptions rather than exact angles.

You can find your own pitch without climbing up. Hold a spirit level or a phone clinometer app against a rafter in the loft, or against a gable end you can see from the ground. Take two or three readings, because old roofs sag and one reading can mislead. Photograph each so you can send it to your roofer.

Pitch is not the same as the overall form. A hipped roof and a gable roof can share one pitch, and our answer on the main roof shapes on UK homes deals with form separately.

Pitch and rafter length together set how much covering you need, but pitch alone decides what can safely go on. That is why we measure the angle first, before anything is priced.

Tiles and slates shed water by overlapping. On a steep slope gravity does the work and rain runs straight down. On a shallower slope water runs slowly, a gust can drive it sideways and up under the laps, and capillary action can pull it through the gaps. Below a certain angle the laps stop working. The manufacturer's minimum pitch is the angle at which its product, laid to instructions, is expected to cope.

The minimum differs from product to product. Marley's published figures include 15 degrees for its Lincoln clay pantile, 35 degrees for concrete plain tiles, 17.5 degrees for its Edgemere concrete tile, and between 30 and 40 degrees for clay plain tiles, depending on the range. Sandtoft lists its Old English clay pantile at 30 degrees.

Natural slate depends on slate size and overlap. Cupa Pizarras cites BS 5534:2014+A2:2018 as indicating a minimum of 20 degrees in moderate exposure for a slate 500 mm long with a 115 mm overlap. Its table gives a 500 by 300 mm slate 115 mm of headlap at 20 degrees, falling to 95 mm at 25 degrees, and advises against hook fixings below 25 degrees. Read the product's own data sheet before you buy.

The trouble usually appears in two settings. One is a replacement where a different covering goes onto existing rafters without anyone checking the angle. A tile with a 35 degree minimum laid on a 25 degree roof looks fine on the day and leaks in the first hard storm. The other is an extension or dormer cheek built shallower than the main roof, with the original covering simply carried across.

If a slope is too low for a tile or slate, there are sensible answers. A covering with a lower minimum, such as a pantile rated at 15 degrees, may suit. Lead, zinc or a single-ply membrane works on a very shallow slope. Or the structure is altered to raise the pitch, though that has planning and structural consequences.

Weight matters as well. Where a new covering is 15 per cent or more heavier than the old one, the Planning Portal says Building Regulations approval is needed, and it advises taking the view of a structural engineer or surveyor. Whether a re-roof needs approval for other reasons depends on the share of roof and building envelope affected, so ask your building control body.

Flat roofs have their own rule. BS 6229 recommends a design fall of 1:40 to achieve a finished fall of 1:80, so that water drains rather than ponds.

Stone slate roofs in the Cotswolds are steeper than most clay or concrete tile roofs. Cotswold District Council's 2000 guidance calls the simplest example a straight pitched roof, usually at 47 to 55 degrees. That reflects how such roofs were traditionally built. It describes traditional practice rather than setting a minimum for stone slate, so the pitch of your own roof is checked against the slate supplier's data sheet before any quote is written.

The reasons are practical. Stone slates are thick and irregular, and each hangs from a peg through a single hole. They overlap less neatly than a machine-made tile, and the stone soaks up some water. A steep slope sheds rain before it can creep through the laps. Weight plays a part too: the guidance gives about 1,000 kg for every 10 m² of stone roof, and a steep slope turns more of that load straight down onto the walls.

Stone slates are also graded by size up the roof. The longest, normally about 600 mm, sit at the eaves, and the tiny ones near the top, called short cocks, are only 150 mm. A steep slope suits that arrangement best.

If you are renewing a stone roof, keep its angle. Changing the pitch changes the building, and on a listed home or in a conservation area it needs the council's agreement first.

On a survey we take the pitch of each slope with a clinometer and note what covering is on the roof now. We compare the reading with the manufacturer's minimum for the covering you want. If they do not fit, the quote says so and offers options, so the problem is found before work starts.

Pitch also affects ageing. Low slopes hold moss and debris, while steep ones shed it but cost more to reach. Typical lifespans by material are set out in how long different roofs last, and those ranges assume the pitch suits the covering.

When a roofer quotes, expect them to ask your pitch and to name the minimum for the product. Our guide to how to check a roofer lists questions worth asking. VAT is shown on the written quote, which covers the whole job.

Send a photograph of the roof and a loft reading through the form, or ring 01453 933613. We clear up afterwards and the finished work carries our written 10-year workmanship guarantee. You can return to our home page at any point.