Convert between degrees, ratio, and percentage for any roof pitch
Roof pitch is one of the most fundamental measurements in roofing and determines everything from the type of tiles you can use to the structural loading on your rafters. Whether you are planning a new build, replacing a roof covering, or designing an extension, knowing the pitch in degrees, as a ratio, and as a percentage is essential for ordering materials, complying with manufacturer specifications, and meeting Building Regulations. In the UK, roof pitches typically range from 15 degrees for modern interlocking tiles to 45 degrees or more for traditional plain tile and slate roofs. The pitch affects not only aesthetics but also weatherproofing -- steeper roofs shed rain and snow more effectively, while lower pitches require more careful detailing to prevent water ingress. Many UK planning authorities also have guidance on acceptable roof pitches for extensions and new builds to maintain the character of the local area.
To calculate or convert your roof pitch: 1. Select your input method. If you have physical measurements, choose "Rise & Run" and enter the vertical rise and horizontal run in millimetres. If you already know the angle, choose "Degrees" and enter it directly. If you have a ratio from a plan drawing, choose "Ratio" and enter it as rise:run (e.g. 4:12). 2. For the rise and run method, measure the vertical height (rise) and the horizontal distance (run) of a section of your roof. You can do this from inside the loft by holding a spirit level horizontally against a rafter, measuring a known horizontal distance, then measuring straight down to the rafter. A typical method uses a 300mm level with a vertical measurement at the far end. 3. The calculator instantly converts between all three representations: degrees, ratio (expressed as X:12 for standardisation), and percentage gradient. It also shows the slope factor, which you multiply by plan area to get true slope area, and advises on which tile types are suitable for that pitch angle based on UK manufacturer guidelines and BS 5534 (Slating and Tiling). 4. Use the tile suitability information to verify that your chosen roofing material is appropriate. Using tiles below their minimum pitch without additional waterproofing measures (such as a fully supported underlay) voids the tile manufacturer's warranty and may not comply with Building Regulations Approved Document C.
The roof pitch conversion uses basic trigonometry: From rise and run: pitch (degrees) = arctan(rise / run) x (180 / pi). For example, a rise of 400mm over a run of 1000mm gives arctan(0.4) = 21.8 degrees. From degrees to ratio: the rise per 12 units of run = tan(degrees) x 12. So 45 degrees gives tan(45) x 12 = 12, resulting in a 12:12 ratio (which represents a slope of 1 in 1). Pitch percentage = (rise / run) x 100 = tan(degrees) x 100. A 45-degree pitch is 100%, while a 21.8-degree pitch is approximately 40%. The slope factor (also called the rafter factor) = 1 / cos(pitch). This converts horizontal plan area to actual slope area. For a 35-degree pitch, the factor is 1 / cos(35) = 1.22, meaning a roof that covers 50 m2 on plan actually has a surface area of approximately 61 m2. This is crucial for ordering the correct quantity of tiles, felt, and battens. Tile suitability ranges follow UK industry standards: interlocking concrete tiles are generally rated down to 15-17.5 degrees (depending on manufacturer and exposure zone), most clay and concrete tiles work above 22.5 degrees, and natural slate and plain tiles are ideal from 25-35 degrees upwards. BS 5534 provides the definitive guidance on minimum pitches by tile type and site exposure.
When measuring roof pitch for an existing building, always take multiple measurements at different points along the roof, as settlement and construction tolerances can mean the pitch varies slightly. For planning applications and building control submissions, pitch is normally expressed in degrees. Manufacturers typically specify minimum pitch in degrees, while civil engineering and drainage applications often use percentage or ratio notation. If your roof pitch is close to the minimum for your chosen tile, consider upgrading to a fully supported breathable membrane underlay for additional protection against wind-driven rain.