Rafter Angle Calculator – Calculate Rafter Length and Angle Easily

Building a roof requires precise calculations, especially when determining the rafter length and rafter angle. This Rafter Angle Calculator helps you quickly find both values using only two simple measurements: the roof span and rise.

Whether you’re a DIY enthusiast or a construction professional, accurate rafter measurements ensure your roof has the correct slope and structural balance.

1. Rafter Length Formula

To find the rafter length, we use the Pythagorean theorem because the rafter, rise, and half of the roof span form a right-angled triangle.

$\text{Rafter Length}=\sqrt{(\text{Rise}{)}^2+{\left(\frac{\text{Span}}{2}\right)}^2}$

2. Rafter Angle Formula

The rafter angle (or roof pitch angle) is calculated using the tangent function:

$\text{Rafter Angle}={tan}^{-1}\left(\frac{\text{Rise}}{\frac{\text{Span}}{2}}\right)$

Example Calculation

Given:

• Roof Span = 20 ft
• Rise = 5 ft

Step 1: Find Rafter Length

$\text{Rafter Length}=\sqrt{(5{)}^2+{\left(\frac{20}{2}\right)}^2}=11.18\text{ ft}$

Step 2: Find Rafter Angle

$\text{Rafter Angle}={tan}^{-1}\left(\frac{5}{\frac{20}{2}}\right)={26.57}^{\circ }$

Final Results:

• Rafter Length: 11.18 ft
• Rafter Angle: 26.57°

Frequently Asked Questions

1. What is a rafter angle in roof framing?

Rafter angle is the incline or pitch angle formed between the rafter and the horizontal plane, determining the roof's steepness. It's crucial for proper load distribution, water runoff, and material cuts, typically expressed in degrees or as a rise-over-run ratio like 4:12. Accurate angles ensure structural stability and aesthetic appeal in construction. 

2. How do you calculate the rafter angle from roof span and rise?

Use the formula: angle = arctan(rise / (span / 2)) in degrees. For example, a 20 ft span with 8 ft rise yields about 25 degrees. Convert units consistently, then apply Pythagoras for rafter length: √(rise² + (span/2)²). This helps precise cuts for common rafters.

3. Why is the correct rafter angle important for roofing?

Proper rafter angles ensure efficient drainage, prevent leaks, and support snow/wind loads without sagging. Incorrect angles lead to weak framing, higher material waste, or safety risks during installation. They also influence shingle overlap and ventilation, impacting roof longevity and energy efficiency.

4. What tools are used to measure or mark rafter angles?

Rafter angle squares or framing squares mark precise cuts based on pitch. Digital angle finders provide exact degrees, while construction calculators compute from rise and run. For on-site verification, use a spirit level on a vertical line along the rafter for accuracy.