1. How do you calculate rafter length?
Calculating the length of a rafter relies on basic geometry—specifically, the Pythagorean theorem (A² + B² = C²). We view half of the truss as a right triangle:
- A (Run): Half of the total span width.
- B (Rise): The vertical height from the center to the peak.
- C (Rafter): The hypotenuse. The formula is:
Rafter = √(Run² + Rise²)
After finding the base rafter length, you must add the length of the Overhang (Eaves). The horizontal overhang distance must be converted to match the angle of the roof to find the exact rafter tail length.
2. Rise vs. Pitch
Roof slope can be expressed in two ways:
- Pitch (Imperial): Expressed as a ratio of X inches of rise for every 12 inches of run (e.g., 6/12 pitch).
- Angle (Metric): Expressed simply in Degrees (e.g., 22.5°).
Our calculator lets you seamlessly toggle between inputting the total vertical Rise, or the specific Pitch/Angle. It automatically solves for the missing variable.
3. Common Truss Types Explained
Different truss designs require different amounts of internal webbing (the zigzag lumber inside the triangle) to distribute weight loads:
- King Post: The simplest truss. Uses a single vertical post in the center. Good for short spans (like sheds).
- Fink (W-Truss): The most common residential truss. Uses a "W" shaped web configuration to transfer loads evenly, excellent for spans up to 40 feet.
- Howe: Similar to a W-Truss but uses vertical webs under compression and diagonal webs under tension. Extremely strong for commercial spans.
This calculator approximates the internal webbing length based on the geometric nodes of these standard shapes to give you a highly accurate lumber estimate.
4. How many trusses do I need?
To find the total number of trusses, take the Total Roof Length and divide it by your Truss Spacing (usually 24 inches on center in the US). Finally, add 1 to account for the starter truss on the gable end.