Reading a pitch in X:12
Manufacturers' application instructions give the pitch in inches per 12. A 4:12 pitch rises 4 inches for every 12 inches of horizontal run. A 12:12 pitch rises as much as it runs, which is 45° or 100%.
The percentage is the rise divided by the run. Degrees measure the angle from horizontal. A 100% pitch is not a wall. It is a 45° angle.
The thresholds that matter
- Below 2:12, or 16.7%. The City of Montreal treats the roof as flat or low-slope, and your borough's white roof rules apply.
- Also below 2:12. IKO's application instructions ban asphalt shingles. That roof is covered with a membrane.
- From 2:12 to 4:12. This is the low slope range in the application instructions. Shingles are allowed with a double-coverage underlayment, or an ice and water membrane over the whole deck.
- From 2:12 to 3:12. The warranty on IKO's Nordic and Marathon shingles is limited to 12 years.
- 21:12, or 60°, and up. IKO's instructions call for 6 nails per shingle instead of 4.
Thresholds vary from one product to another. The application instructions for the proposed shingle are what count, and our quote names them.
Finding your roof pitch without climbing
Do not climb on the roof to measure the pitch. Look at the gable from the ground or the street. A 12:12 pitch makes a 45° angle. The house plans sometimes show the pitch, and our quote confirms the measured pitch.
Once you know the pitch, it also gives the factor that converts the footprint into roof area.
Frequently asked questions
How do you convert roof pitch to degrees?
Divide the rise by 12, then take the arctangent of the result. A 4:12 pitch gives 0.333, or 18.4°. The converter above does the math.
How many degrees is a 4:12 pitch?
A 4:12 pitch is 18.4°, or 33.3%.
What is the minimum roof pitch for asphalt shingles?
The minimum pitch is 2:12 in IKO's application instructions. From 2:12 to 4:12, installation calls for a special low-slope underlayment.
At what pitch is a roof considered flat in Montreal?
For the City of Montreal, a roof with a pitch under 2:12, or 16.7%, is a flat or low-slope roof.
Official sources, consulted October 16, 2026: