What is my roof pitch?

Hold a level against the roof or a rafter, mark one metre along it from the point of contact, then measure straight down from that mark to the roof surface. That drop is your rise: a 500 mm drop over a 1,000 mm run is a 26.6° roof. Measuring off a rafter in the loft works just as well and keeps you off the roof.

A 30° pitch is a rise of about 577 mm per metre of run, which is a 57.7% slope — in the American convention, roughly 6.9 in 12. The angle is the arctangent of rise ÷ run, so all three are the same triangle written differently: 22.5° is about 41% and 5 in 12, and 45° is exactly 100% and 12 in 12.

The pitch factor converts the footprint your roof covers into the actual surface area you have to cover. It is the hypotenuse over the run: √(rise² + run²) ÷ run. A 26.6° roof has a factor of 1.118, so it has about 12% more surface than its plan area; a 45° roof has a factor of 1.414, about 41% more. Order tiles off the footprint and you are short on every steep roof.

Multiply the horizontal run by the pitch factor. At 26.6° the factor is 1.118, so a rafter spanning 4 m of run measures 4 × 1.118 = 4.47 m along the top edge. That is the theoretical line length — you still deduct half the ridge thickness, add the tail for the overhang, and cut the birdsmouth.

It depends entirely on the covering. Manufacturers publish a minimum pitch for each product and it varies widely — some interlocking concrete tiles go down to around 15°, while plain tiles and natural slate usually want considerably more. Below the product minimum you move to a different system rather than simply a heavier underlay. Take the figure from the manufacturer for the specific covering, and check what applies locally, before you quote.