What is Roof Pitch?
Roof pitch is a numerical measure of the steepness of a roof. In construction and architecture, it is traditionally defined as the vertical "rise" over a specific horizontal "run." In the United States, this is almost always expressed as a ratio over 12 inches. For example, a "6/12 pitch" means that for every 12 inches of horizontal distance (run), the roof rises 6 inches vertically.
Understanding your roof pitch is critical for several reasons, including selecting appropriate roofing materials, calculating the area of the roof for shingle estimation, and ensuring proper drainage.
How to Calculate Roof Pitch
The formula for calculating roof pitch depends on the metric you need (ratio, angle, or percentage). Our calculator uses the following logic:
- Pitch Ratio: (Rise ÷ Run) × 12. This gives you the "X" in "X/12".
- Angle (Degrees): ArcTangent(Rise ÷ Run). This converts the slope into a degree angle.
- Slope Percentage: (Rise ÷ Run) × 100. This is often used in commercial roofing.
To measure this manually on an existing roof, you can use a level and a tape measure. Mark 12 inches on the level, hold it perfectly horizontal against the roof slope, and measure the vertical distance from the 12-inch mark down to the roof surface. That measurement is your rise.
Common Roof Pitches and Materials
The steepness of your roof dictates what materials can be safely installed to prevent leaks. Here is a general guide:
| Pitch Category | Pitch Ratio | Angle (approx) | Suitable Materials |
|---|---|---|---|
| Flat / Low Slope | 0/12 – 2/12 | 0° – 9.5° | Built-up roofing, Torch-down, TPO, EPDM rubber |
| Low Slope | 2/12 – 4/12 | 9.5° – 18.5° | Metal standing seam, rolled roofing, special shingle installs |
| Conventional Slope | 4/12 – 9/12 | 18.5° – 37° | Asphalt shingles, clay tiles, slate, metal |
| Steep Slope | 9/12 and up | 37°+ | Asphalt shingles, wood shakes, slate |
Why the Rafter Length Multiplier Matters
The calculation output includes a "Rafter Length Multiplier." This is an incredibly useful number for framing. If you know the total horizontal run of a building span, you can multiply that run by this multiplier to find the exact length of the rafter needed (excluding overhangs).
For example, if you have a 6/12 pitch (Multiplier ≈ 1.118) and a building run of 10 feet, the rafter length is 10 × 1.118 = 11.18 feet.
Frequently Asked Questions
Is a 4/12 pitch walkable?
Generally, a 4/12 pitch is considered low enough to be walkable for most people with proper footwear and caution. Pitches above 6/12 or 7/12 become significantly more dangerous and usually require safety harnesses and roof jacks.
How do I convert degrees to pitch?
If you know the angle in degrees, you can find the pitch using the tangent function: tan(angle) × 12. For example, for a 30° roof: tan(30) is approx 0.577. 0.577 × 12 = 6.9, so it is essentially a 7/12 pitch.