The short answer
Pitch means how steeply the roof leans. The easiest way to find it is from inside the loft: hold a spirit level horizontal against a rafter, measure the vertical drop from the level to the rafter at a set distance, then work out the angle. A phone inclinometer app held along the rafter does it faster. Stay off the roof itself, because tiles crack and slopes are dangerous.
What pitch means and how it is written
Pitch describes how steep a roof slopes. Builders write it in two ways. One is degrees from horizontal, so a pitch of 30 degrees rises noticeably but is far from steep. The other is a ratio of rise to run, such as 6 in 12, meaning six units up for every twelve across.
The two measure the same slope. A rise of 300 mm over a run of 500 mm gives roughly 31 degrees. A rise of 400 mm over a run of 600 mm gives roughly 34 degrees. Most calculators and phone apps will convert between them, and the sum behind it is the arctangent of rise divided by run.
The pitch matters because the pitch decides what can safely cover the roof. Rain crawls down a low slope, and a gust can push it back up under the laps. A steep slope clears it quickly. Every covering has a pitch below which its maker will not recommend it.
Measuring from inside the loft
This is the safest and usually the most accurate method. You need a spirit level, a tape measure and a way to light the loft.
- Pick a rafter within easy reach. These are the inclined timbers under the slope.
- Hold the spirit level level, with one end touching the underside of the rafter.
- Measure along the level a convenient distance, say 500 mm, from the rafter to the point where you read the gap.
- Measure straight down from the level to the rafter at that point. That is the rise.
- Divide the rise by the run, and look up the angle with a calculator.
Take readings on two or three rafters, since old roofs are not always true. A spread of more than a degree or two between readings suggests the roof has settled, and pitched roof problems lists the likely reasons. Write each reading down with the rafter it came from, so you can compare them later or pass them to a roofer.
Measuring with a phone or from the ground
A phone with an inclinometer or level app can read the slope directly. Lay the phone edge along a rafter in the loft, or along a straight batten pressed against the underside, and read the angle. Keep the phone steady and check it against a known flat surface first.
From the ground, use a photograph. Stand square-on to the gable end of the house, far enough back that the camera is not tilted, and photograph the gable. The angle of the edge of the roof against the horizontal line of the eaves gives the pitch. Free apps and a simple protractor on a printed photo both work. This method is approximate and suits a quick check, not a design decision.
If the roof has a hip, the slopes may not be shown clearly on the end, which is where our page on the difference between hip and gable roofs may help. For mansards, each slope has its own pitch, and the lower is far steeper than the upper.
Why the number matters for tiles and slates
Manufacturers publish minimum pitches, and they differ by covering. The figures are typical, they depend on exposure, and BS 5534 and the maker's data govern each job.
| Covering | Minimum pitch quoted |
|---|---|
| Natural slate (NHBC, moderate exposure) | 20 degrees |
| Marley concrete plain tile | 35 degrees |
| Marley interlocking concrete tiles | 12.5 to 25 degrees, by product |
| Marley clay plain tile, Hawkins | 30 degrees |
| Sandtoft Old English clay pantile | 30 degrees |
| Bradstone reconstituted Cotswold slate | 30 degrees |
Cupa Pizarras, a slate supplier, adds two points for slate. Hook fixings stay out below 25 degrees, and the headlap, meaning the overlap at the head of each slate, has to grow as the slope flattens. For 500 x 250 mm slates in moderate exposure, its table runs from 60 mm at 40 degrees to 95 mm at 25 degrees. A roof close to the minimum leaves little margin, so a low-pitched slope is where leaks from wind-driven rain start. Our page on breathable membranes also shows how pitch affects ventilation.
Pitch, roof windows and what to do next
A few practical cautions apply to all of these methods. Never lean out of a window or step onto the slope to take a reading, because tiles crack underfoot and a wet slope is slippery. Measure each main slope on its own, since an extension or dormer may sit at another angle to the house. On a mansard, write down the steep lower slope and the gentle upper one as two numbers. A reading taken along a hip or valley rafter is not the true pitch, since that timber runs diagonally across the roof, so use a common rafter that runs straight from ridge to eaves.
Pitch also decides which roof windows can be fitted. Each window model is rated for a range of pitches, and the maker will state it. A window used outside its range may leak, so measure before you choose. We do not restate the planning rules for roof windows here, and our skylights pages explain them.
Once you know the pitch, compare it to the covering you have or want. If the figure is at or below the minimum for the product, talk to a roofer before ordering. We note the pitch early in any survey and set it against the maker's data for the covering. A fixed written price follows from there.
If the slope looks uneven or the ridge dips, the pitch may not be the only problem, and a sagging line points to timber trouble. The wider pitched roofing service page sets out what we look at on a pitched roof.
