Advanced options
Rafter length is the hypotenuse of rise and run; add the eaves overhang. Every roof covering has a manufacturer's minimum pitch, and going below it invites driven rain; always check the specific tile or sheet's datasheet.
What The Roof Pitch Calculator Works Out
- Roof pitch: the angle from horizontal, the arctangent of rise over run, where the run is half the span for a symmetrical roof. A 2m rise over a 4m run is 26.6 degrees. Roofers also quote ratios (rise per 12 of run, here 6:12); degrees are what UK tile datasheets use.
- Rafter length: the hypotenuse of rise and run, 4.47m in the example, before any eaves overhang. Steeper roofs cost more in covering per m2 of plan, exactly the rafter-to-run ratio.
- Suits which coverings: plain tiles need about 35 degrees or more, most natural slates 20 to 25 up depending on size and exposure, interlocking concrete tiles commonly work down to 15 to 17.5, specialist low-pitch systems reach 10 to 12.5, and below 10 degrees is flat-roof territory. Going below a covering's minimum is the classic cause of driven-rain leaks that appear years later, so the manufacturer's datasheet outranks every rule of thumb.
- Known pitch (advanced): if you already know the angle (from drawings, or because an extension must match the existing roof), enter it and the calculator works out the rise from the run instead.
- Eaves overhang (advanced): adds the horizontal overhang to the rafter along the slope and suggests a stock length to buy. 300 to 450mm is typical and keeps rain off the walls.
- Roof area (advanced): both slopes x the length along the ridge, with the plan area alongside, which is the figure for ordering coverings, underlay and insulation.
- Tiles, battens and underlay (advanced): tiles at the chosen covering's rate per m2 (interlocking concrete about 10, large slate 13, standard slate 20, plain tiles 60) plus 5% waste, metres of batten at that covering's gauge, and 1 x 50m underlay rolls at about 45 m2 each after laps. Ridge, hip and verge tiles are extra.
- Loft width with full headroom (advanced): the strip of loft floor with at least your chosen headroom (2m by default) under the rafter line, from the geometry alone. A 45-degree roof over a typical span gives a convertible loft; 22.5 degrees does not. Deduct about half a metre for rafter depth and insulation.
- Pitch as a gradient (advanced): the same slope as a percentage and as 1 in x, the forms used for flat-roof falls and drainage. Permitted development height limits are measured to the ridge, and matching the existing pitch is a standard planning condition on extensions.