What is Roof Pitch?
Roof pitch is the slope of a roof, expressed as vertical rise over horizontal run. The US convention is rise per 12 inches of run: a 6/12 roof rises 6 inches for every 12 inches of horizontal distance. Pitch affects material options, water shedding, snow load, and curb appeal.
Converting Pitch to Angle and Percent
Angle=arctan(RiseRun)
Percent=RiseRun×100
Common Conversions
| Pitch | Angle | Percent | Slope Factor |
|---|---|---|---|
| 2/12 | 9.5° | 16.7% | 1.014 |
| 4/12 | 18.4° | 33.3% | 1.054 |
| 6/12 | 26.6° | 50% | 1.118 |
| 8/12 | 33.7° | 66.7% | 1.202 |
| 10/12 | 39.8° | 83.3% | 1.302 |
| 12/12 | 45° | 100% | 1.414 |
Minimum Pitch by Material
- Below 2/12 - membrane only (EPDM, TPO, PVC, modified bitumen)
- 2/12 to 4/12 - asphalt shingles with double underlayment, metal
- 4/12 and up - standard asphalt shingles, wood shakes, tile
- 6/12 and up - slate, synthetic slate, premium tile
Slope Factor for Ordering
Always multiply footprint area by the slope factor to get actual roof surface area before ordering material. A 1,500 sq ft footprint on a 6/12 roof has 1,677 sq ft of real roof surface.Why Pitch Matters for Design
Roof pitch drives more than aesthetics. It affects:
- Snow retention - pitches under 6/12 retain more snow; steeper roofs shed snow faster
- Wind uplift - flatter roofs experience higher uplift; steep roofs experience drag
- Attic space - doubling the pitch roughly triples usable attic volume
- Water drainage - steeper roofs self-clean leaves and debris
- Material cost - steeper pitches need more material and labor per footprint sq ft
How to Measure Existing Pitch
Three practical methods, no calculator needed:
- Level method - place a 12 inch level horizontally on the rafter, measure the vertical distance from the level's end to the rafter below. That number is the rise per 12 run.
- Speed square - hold a speed square on the rafter. The pivot angle indicates the pitch directly (most speed squares have common rafter pitch marks).
- Phone app - apps like RoofSnap or the built-in angle measure in most smartphones work if you hold the phone flat against the roof surface.
Pitch in Different Regions
Regional climate and tradition influence typical pitch ranges. Desert southwest uses 3/12 to 5/12 for flat Spanish-style looks. Mid-Atlantic prefers 6/12 to 8/12 for colonial aesthetics. Northern snow country often uses 10/12 to 12/12 for faster snow shedding. Always check with local architects for regional expectations.Low-Slope Code Requirements
Roofs 2/12 to 4/12 require a second layer of underlayment and often ice and water shield across the entire surface (not just the eaves). Below 2/12, most asphalt shingle warranties are void - use membrane roofing or metal instead.