Pitch describes how steep a roof is. It changes the roof area, which coverings you can use, how fast water runs off, how safe the roof is to work on, and how much the job costs. It's also one of the most confusing numbers on site, because it's written in three different ways depending on who you ask.
Three ways of saying the same thing
- Rise in 12 (x/12): how many inches the roof rises for every 12 inches of horizontal run. It's the standard in Canada and the US, and on most North American product data.
- Degrees: the angle from horizontal. It's the standard in the UK, in Europe and in tile makers' data everywhere.
- Percent: rise ÷ run × 100. It's used for low-slope roofs, gutters and drainage.
There is also the older idea of a "1/3 pitch" or "half pitch", based on rise over span rather than run. It still turns up in old documents but causes confusion, so convert it before using it.
Conversion table
| x/12 | Degrees | Percent | Area factor |
|---|---|---|---|
| 2/12 | 9.5° | 16.7% | 1.014 |
| 3/12 | 14.0° | 25% | 1.031 |
| 4/12 | 18.4° | 33.3% | 1.054 |
| 5/12 | 22.6° | 41.7% | 1.083 |
| 6/12 | 26.6° | 50% | 1.118 |
| 7/12 | 30.3° | 58.3% | 1.158 |
| 8/12 | 33.7° | 66.7% | 1.202 |
| 9/12 | 36.9° | 75% | 1.250 |
| 10/12 | 39.8° | 83.3% | 1.302 |
| 12/12 | 45° | 100% | 1.414 |
The formulas
- Degrees = atan(rise ÷ run)
- x/12 = tan(degrees) × 12
- Percent = rise ÷ run × 100
- Area factor = √(1 + (rise ÷ run)²)
The area factor turns a plan area into true roof area. It's the single most useful number in roof estimating.
How to measure an existing roof
- Hold a level horizontally with one end touching the roof surface.
- Measure 12 inches along the level from that end.
- Measure straight down from that mark to the roof. That's the rise per 12 inches.
From inside the attic, a digital angle finder on the underside of a rafter gives degrees directly. From the ground, phone apps can estimate pitch from a photo, but treat that as a rough check. On gable ends you can also measure the height from wall plate to ridge and divide by half the span.
Why pitch matters
Materials
Every roof covering has a minimum pitch. Asphalt shingles need at least 2/12, and below 4/12 most makers require extra underlayment. Below 2/12 you need membrane or standing-seam metal.
Cost and safety
Steeper roofs mean more area, more material and slower work, often with extra edge protection or scaffolding. Many roofers add a steep-roof charge above about 8/12. Low roofs are quicker to walk but less forgiving of poor detailing, because water moves slowly.
Attic space and appearance
Pitch decides headroom in the roof space, which matters for attic conversions and storage, and it shapes how the house looks. Matching the existing pitch on an extension is usually a planning or design requirement.
Using pitch in other calculations
Pitch feeds rafter length (run ÷ cos(pitch)), hip and valley lengths, gable areas, solar panel angles and gutter sizing. The roof pitch calculator converts any format to all the others, and the rafter calculator turns span and pitch into cut lengths and angles.
Common questions
Is a 4/12 roof "low"?
For shingles, yes: between 2/12 and 4/12 most makers ask for extra underlayment. For metal and membrane roofs, 4/12 is comfortable.
What pitch can I walk on safely?
Most roofers walk roofs up to about 6/12 or 7/12 in dry conditions with good footwear. Steeper than that needs roof brackets, harnesses or scaffolding. Wet, mossy or frosty roofs are dangerous at any pitch.
Do solar panels need a particular pitch?
Panels are usually fitted flush to the existing roof. Pitch changes output only modestly, and the roof's direction usually matters more.
Pitch on drawings
Canadian drawings usually show pitch as a small triangle with 12 on the horizontal side and the rise on the vertical side, for example 6 over 12. Some engineers' drawings give degrees instead. When an extension has to match an existing roof, measure the existing pitch on site rather than trusting old drawings. Small differences show clearly where the two roofs meet.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.