Roof Pitch Calculator
Pitch is how steep a roof is, written as the number of inches it rises for every 12 inches it runs horizontally. It decides which materials are allowed on the roof, how much surface there is to cover, and how much of the labour bill is really about getting to the work. This converts between every form it gets quoted in.
Pitch, angle and slope
Convert between rise over run, degrees and slope percentage, and see what the pitch means for materials and access.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Roof pitch The rise expressed over a run of 12, which is how pitch is quoted.
- Angle from horizontal
- Slope
- Pitch multiplier Multiply a footprint area by this to get the sloped area.
Below 2 in 12 this counts as a low-slope roof. Asphalt shingles are not rated for it and manufacturers will not warranty them there, because water moves slowly enough to work back under the courses. This pitch needs a membrane system: TPO, EPDM, modified bitumen or standing-seam metal.
From 2 in 12 up to 4 in 12, shingles are allowed but only over a doubled underlayment, and most manufacturers state that condition in the warranty. If a contractor quotes a single layer on a pitch this shallow, ask them to point at the installation instructions.
A conventional pitch. Shingles go on in the standard way, water sheds quickly, and most roofers can walk it without staging. This is the range roofing products are designed around.
Steep. Roof jacks or staging are needed to work on it safely, everything takes longer, and labour is priced above a walkable roof. Budget for that when you compare estimates, and never go up there yourself.
Very steep, the range of Victorian roofs, turrets and mansards. Scaffolding and harness work are the norm, the labour surcharge is significant, and some materials call for extra fasteners at this angle. This is specialist work.
Pitch is a ratio, so the units cancel: 6 over 12 and 150 over 300 are the same roof. The safest way to measure it is from inside the attic against a rafter, with a level and a tape, rather than on the roof itself.
How to measure it, safely
From inside the attic, against a rafter, with a spirit level and a tape. Hold the level horizontally with one end touching the underside of the rafter, mark 12 inches along it, and measure straight up from that mark to the rafter. That measurement is the rise, and you now have the pitch.
Do not climb onto the roof to measure it. Nothing on this page is worth a fall, and the attic gives you the same answer standing on a joist.
Why the units do not matter
Pitch is a ratio, so any unit works as long as the rise and the run are measured in the same one. Six over twelve and 150 over 300 describe the same roof. The calculator normalises whatever you enter to a run of 12, because that is how the trade quotes it, which is also why this is the one calculator here with no feet-and-metres switch.
What each range means
- Under 2 in 12. Low slope. Asphalt shingles are not rated for it and manufacturers will not warranty them there, because water moves slowly enough to work back under the courses. This needs a membrane system: TPO, EPDM, modified bitumen or standing-seam metal.
- 2 to 4 in 12. Shingles are permitted, but only over a doubled underlayment, and most manufacturers write that condition into the warranty. If a contractor quotes a single layer on a pitch this shallow, ask them to show you the installation instructions.
- 4 to 8 in 12. Conventional. Roofing products are designed around this range, water sheds quickly, and most roofers can walk it without staging.
- 8 to 12 in 12. Steep. Roof jacks or staging are needed to work safely, everything takes longer, and labour is priced above a walkable roof.
- Over 12 in 12. Very steep. The range of Victorian roofs, turrets and mansards. Scaffolding and harness work are normal, the labour surcharge is significant, and some materials call for extra fasteners at this angle.
Pitch and the size of the job
The pitch multiplier the calculator returns is the number that turns a building footprint into a roof area. At 4 in 12 it is 1.054, so the roof is about five per cent larger than the ground it covers. At 12 in 12 it is 1.414, or forty-one per cent larger. Two houses with identical floor plans can therefore need materially different quantities of material, and that is before the steeper one attracts an access charge.
Degrees, percentages and x in 12
Roofers quote x in 12. Architects and engineers often work in degrees. Drainage and groundwork use slope as a percentage. They describe the same thing: 6 in 12 is 26.57 degrees is a 50 per cent slope. If a drawing gives you one and a contractor wants another, this converts between them without the trigonometry.
One roof, several pitches
Plenty of houses have more than one. A main roof at 8 in 12 with a porch at 3 in 12 is common, and the porch may need a different material entirely for the reason set out above. Measure each plane separately, and treat the shallow one on its own terms rather than assuming what covers the main roof will cover it.