Roof pitch calculator

Pitch is what makes water leave the roof. Flatter roofs shed slower, and below a cladding's minimum pitch the water sits long enough to track back under the laps and flashings and find its way inside. That is why every roofing product has a stated minimum, and going under it is not a grey area.

There are three reasons to respect it. Water: a roof under its minimum pitch leaks, sooner or later. Warranty: run the cladding below the manufacturer's minimum and the warranty is void, so a failure is on you. Code: the Building Code requires the roof to keep water out, and pitch is a big part of how it does that. Get the pitch right first, then choose the cladding, not the other way around.

You do not need to climb onto the roof to get a good reading. The safest way is from the ground or the gable end with a spirit level and a tape.

• From the ground: hold a spirit level dead flat, lined up with the roof slope, measure a set run along the level (300mm is easy), then measure straight down from the far end of the level to the roof line. Punch that rise and run into the measurement mode above.
• In the roof space: if you can safely get into the ceiling cavity, do the same against the underside of a rafter. It is often easier and out of the wind.

Either way you are measuring a rise over a run and letting the calculator turn it into degrees. Never lean a ladder against a spouting or step onto a wet roof to save five minutes.

The minimum pitches in the table are the baseline for standard conditions. Where you build changes them. In a high wind zone, or on a long sheet run, water and wind can push moisture back up the roof, so the NZ Metal Roofing Code of Practice bumps the minimum up in those cases. Snow country adds load and slower melt, which is a structural and a drainage question both. If your site is exposed, on a hill, or in an alpine or high-rainfall area, treat the table minimum as the floor and add a degree or two, or ask your roofer what the code requires for your zone.

This is the trap on flat-looking roofs. You measure the pitch, fall in love with corrugated iron, and find the roof is flatter than the 8 degrees corrugate needs. Now you have a choice, and it is not to ignore the number. You either switch to a profile rated for the pitch you have, like a trapezoidal or trough section that goes down to about 3 degrees, or you go to a membrane system rated for near-flat falls of 2 degrees and under. Both are proper answers. Forcing a high-pitch cladding onto a low-pitch roof is how you end up with a leak and a void warranty. Measure first, then buy the cladding that suits the pitch, not the pitch you wish you had.