The short answer
Roof pitch is the steepness of a roof slope. In the UK it is usually given in degrees, such as 30 degrees, measured between the sloping surface and a level line. In other places it is given as a ratio of rise to run. You can measure it with a spirit level and a tape from inside the loft, or with a pitch finder or phone app. Pitch matters because each tile and slate has a minimum angle below which it will leak.
Degrees, ratios and percentages
There are three ways to describe pitch. The first is degrees from the horizontal, and it is the standard in the UK. A 30 degree pitch means the roof rises thirty degrees from a level line. The second is a ratio, such as a rise of six units for every twelve units of horizontal distance. The third is a percentage grade, which is more common on flat roofs and drains.
The three can be converted, but you will mostly meet degrees on British roof data sheets, and ratios on imported products. Keep to one system when you compare specifications, so you do not mix them up. A steeper roof has a bigger number in degrees, up to ninety, which would be a vertical wall.
How to measure it
The simplest way is from the loft. Hold a level against the underside of a rafter, horizontal, with one end touching the rafter. Measure the vertical gap from the other end of the level to the rafter, and the length of the level. The ratio of the gap to the length gives the rise over the run, which converts to an angle. A digital angle finder or a phone app placed on the rafter reads the angle directly and is quicker.
From the outside, you can estimate pitch from a photograph taken square on to the gable end, or measure the rise and the half-span of the gable. A roofer will normally measure it with a pitch gauge when surveying. Do not climb the roof yourself. Our answer on walking on roof tiles explains why.
Why pitch matters
Pitch controls how fast water runs off and how far it can be driven under a tile by wind. On a steep roof, water runs off fast and the laps can be shorter. On a shallow roof, water moves slowly, and wind can push it up the slope under the tiles. That is why every covering has a minimum pitch given by the maker.
Typical minimums differ. Concrete interlocking tiles can often be used at fairly low pitches, plain clay tiles need a steeper slope, and slates depend on size and lap. Always use the maker's data for your product. Our guide on the minimum roof pitch for tiles goes through the typical figures and the options below them.
Pitch and the other roof measurements
Pitch is different from the span, which is the width between the walls, and from the rise, the height from the wall plate to the ridge. The three are linked: for a given span, a steeper pitch means a taller rise and a larger roof area. A steeper roof needs more material, more labour and more scaffold, so it costs more.
Rafter length also depends on pitch. For fixing, steep slopes need extra care with fixings, because gravity pulls tiles downwards harder. Our page on whether roof tiles need clips covers that.
Pitch of common roof types in Britain
Pitch tends to follow the age and style of the house. Georgian and Victorian roofs covered in slate are often fairly steep, because slate laps need the slope to shed water. Interwar and post-war houses with plain tiles sit in the middle of the range. Modern estates with concrete interlocking tiles often go lower, because those tiles are tolerant of shallow slopes. Extensions with a lean-to roof may be shallower still, and some are barely above the minimum.
If your extension roof is a lean-to at a low angle, check the covering is suitable for it. A tile used below its minimum pitch can let wind-driven rain in, and the result is a stain on the ceiling that is blamed on a failed flashing.
Rafter pitch and ridge height
Pitch also decides the height of the ridge above the eaves. A steeper pitch means a taller ridge, a larger loft volume and a bigger roof area to cover. Planners often look at the ridge height when assessing an extension or loft conversion, so a change in pitch can alter the planning position.
When the pitch has to change
A change of pitch is a design decision. Lowering the pitch of an existing roof can make the old covering unsuitable, and raising it adds height that may need planning permission. If you are planning a loft conversion or an extension, tell us the pitch early, because it decides the covering and the detailing. We measure on site and photograph the result. More on our tiled-roof work is on the pitched roofing in Basingstoke page.
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