Free Roof Pitch calculator with clear step-by-step results.
Expresses the same pitch in the three conventions in use - degrees, rise in twelve, and percentage - and derives the slope factor, which is the multiplier that converts plan area into the real surface area you have to cover with material.
Pitch
Degrees = arctan(rise / run); rise in 12 = rise / run x 12
Slope factor
Factor = square root of (1 + (rise / run)^2)
Because the roof is the hypotenuse. A 30 degree roof has about 15.5% more surface than its footprint, which is exactly how material orders come up short.
It depends on the product - plain tiles typically need 35 degrees or more, interlocking concrete tiles about 17.5, and profiled metal sheets can go to 5 or lower with sealed laps.