What is Roof Slope?
Roof slope is the steepness of a roof expressed as the tangent of the roof angle. It describes how much the roof rises vertically for each unit of horizontal distance. Accurate slope measurement is essential for selecting roofing materials, ensuring proper water drainage, and meeting local building code requirements.
How to Calculate Roof Slope
Here is the formula:
[\text{Slope} = \tan(\theta)]
Where:
- Slope is the roof slope expressed as a Y:1 ratio.
- θ is the roof angle in degrees relative to the horizontal.
If you know the rise and run instead of the angle, calculate the slope directly:
[\text{Slope} = \frac{\text{Rise}}{\text{Run}}]
To find the angle from rise and run:
[\theta = \arctan\left(\frac{\text{Rise}}{\text{Run}}\right)]
Calculation Example
A roof has an angle of 30 degrees.
[\text{Slope} = \tan(30^\circ) \approx 0.5774]
The roof slope is approximately 0.5774 (Y:1), meaning the roof rises about 0.58 units for every 1 unit of horizontal run.
Alternative: Rise and Run
A roof rises 6 feet over a horizontal run of 12 feet.
[\text{Slope} = \frac{6}{12} = 0.5]
[\theta = \arctan(0.5) \approx 26.57^\circ]
The slope is 0.5 (Y:1) and the angle is approximately 26.57 degrees.