Roof pitch describes the steepness of a roof slope and is relevant to planning works, choosing appropriate roof covering materials, calculating run-off, and designing new roof structures. There are several ways to express and calculate pitch, and the method used depends on what information you have available.

How Roof Pitch Is Expressed

Roof pitch is most commonly expressed in one of two ways. The ratio method expresses pitch as a ratio of vertical rise to horizontal run, written as X:12 in the traditional imperial system used in construction, meaning for every 12 inches of horizontal run the roof rises X inches vertically. A 4:12 pitch means the roof rises 4 inches for every 12 inches of horizontal distance. In metric contexts, pitch is often expressed as a ratio of rise to span, such as 1:3, meaning the rise is one third of the total span.

The angular method expresses pitch in degrees, with zero degrees being a flat roof and 90 degrees being a vertical wall. A typical domestic roof in the UK has a pitch of between 35 and 50 degrees, with 40 degrees being common for traditional clay tile constructions.

Measuring Pitch from the Roof

The most practical way to measure an existing roof's pitch is with a spirit level and a tape measure or, more accurately, with a digital angle finder or inclinometer. Hold the spirit level horizontally against the roof slope and measure the vertical distance from the end of the spirit level up to the underside of the level. For a standard 600mm spirit level, divide the vertical measurement in millimetres by 600 to get the tangent of the angle, then use the inverse tangent function to calculate the angle in degrees. A digital angle finder placed directly on the roof slope reads the angle directly without calculation.

Calculating Pitch from Known Dimensions

If you know the rise, which is the vertical height from the top of the wall to the ridge, and the run, which is half the total span measured horizontally, you can calculate the pitch angle. The pitch angle in degrees is the inverse tangent of the rise divided by the run. For example, a roof with a rise of 1.2 metres and a run of 3 metres has a pitch angle of the inverse tangent of 1.2 divided by 3, which is the inverse tangent of 0.4, approximately 21.8 degrees.

Minimum Pitch Requirements for Roof Coverings

Different roofing materials have specific minimum pitch requirements to ensure adequate drainage and weatherproofing. Plain clay and concrete tiles generally require a minimum pitch of 35 degrees. Interlocking concrete tiles can be used from around 17.5 degrees with appropriate underlays. Natural Welsh slate is typically laid at a minimum of 25 to 30 degrees depending on the slate size. Flat roofing materials such as EPDM rubber, GRP fibreglass, and felt systems apply from zero degrees with appropriate falls designed in. Always check the manufacturer's minimum pitch recommendation for the specific product being used.

Summary

Roof pitch can be measured directly using a spirit level and tape measure or a digital angle finder, or calculated from the rise and run dimensions using inverse tangent. Pitch is expressed as an angle in degrees or as a rise-to-run ratio. Understanding the pitch of a roof is necessary when planning works, selecting appropriate materials, and ensuring minimum pitch requirements are met for the chosen roof covering.