Wood Truss Calculator: Rise, Rafter Length, Roof Angle and Truss Count
Enter your building’s span and roof pitch to calculate the total rise, rafter (top chord) length, and roof angle using basic right-triangle geometry (run = span/2, rise = run x pitch/12, rafter length = square root of run-squared plus rise-squared), plus how many trusses you need based on building length and on-center spacing.
Quick Answer
Enter your building’s span and roof pitch to calculate the total rise, rafter (top chord) length, and roof angle using basic right-triangle geometry (run = span/2, rise = run x pitch/12, rafter length = square root of run-squared plus rise-squared), plus how many trusses you need based on building length and on-center spacing.
Wood Truss Calculator: Rise, Rafter Length, Roof Angle and Truss Count
Enter your values below for an instant result, then see the formula, worked example, and common mistakes.
Enter your span and pitch, then click calculate.
How to Use This Calculator
This is the total width of the structure, measured outside wall to outside wall — trusses typically bear on the two outer walls.
Pitch is expressed as rise over a 12-inch run (e.g. 6/12 means the roof rises 6 inches for every 12 inches of horizontal run). A 6/12 pitch (about 26.6 degrees) is one of the most common residential pitches.
Trusses are almost always spaced at 16 in or 24 in on-center in residential construction — wider spacing means fewer trusses but requires checking your sheathing span rating.
Combined with spacing, this determines the total number of trusses needed to frame the full roof.
Simple King Post trusses (a single central vertical support) suit shorter spans; wider spans need a Queen Post (two verticals) or a fully engineered web design.
Formula
Run = Span / 2. Rise = Run x (Pitch / 12). Rafter (top chord) length = square root of (Run2 + Rise2), from the Pythagorean theorem, since the rafter is the hypotenuse of the right triangle formed by the run and rise. Roof angle = arctan(Pitch / 12) in degrees.
Reference Table: Common Roof Pitches
| Pitch | Approx. angle | Typical use |
|---|---|---|
| 4/12 | 18.4 deg | Low-slope residential, some ranch styles |
| 5/12 | 22.6 deg | Common residential pitch |
| 6/12 | 26.6 deg | One of the most common residential pitches |
| 8/12 | 33.7 deg | Steeper, common in snow-load regions |
| 10/12 | 39.8 deg | Steep, traditional/craftsman styles |
| 12/12 | 45 deg | Very steep, dramatic roofline |
Common Mistakes to Avoid
- Measuring span as the roof’s total sloped surface rather than the flat horizontal building width — span is always the horizontal distance between the outer wall supports.
- Assuming any simple King Post truss design will work at any span — practical span limits (roughly 26 ft for King Post, up to about 40 ft for Queen Post) exist because tie-beam sag and post buckling become structural problems beyond those ranges without engineering.
- Forgetting truss spacing affects required sheathing thickness — wider on-center spacing (24 in vs 16 in) generally requires thicker roof sheathing to span between trusses without excess deflection.
- Ignoring snow load, wind load, and local building code requirements — this calculator gives basic geometry only, not a structural/engineering truss design, which must account for regional load requirements.
- Rounding the truss count down instead of up — always round up and add an extra truss at gable ends or hip transitions as your specific design requires.
When the Estimate May Be Wrong
This calculator provides basic right-triangle roof geometry (rise, rafter length, angle) and a simple truss-count estimate. It does not perform structural engineering calculations for snow load, wind load, or lumber sizing — pre-engineered wood trusses must be designed and stamped by a truss engineer per local building code, especially for spans beyond simple King Post/Queen Post ranges.
FAQs
What is the difference between a King Post and Queen Post truss?
A King Post truss uses a single central vertical post and works well for spans up to about 26 ft. A Queen Post truss uses two vertical posts standing on the tie beam, allowing clear spans of roughly 26 to 40 ft without the tie beam sagging.
How do you calculate rafter length from span and pitch?
Rafter length equals the square root of (run squared plus rise squared), where run is half the span and rise is run multiplied by the pitch (rise per 12 in of run) divided by 12.
What is the most common roof pitch for residential construction?
A 6/12 pitch (rising 6 inches for every 12 inches of horizontal run, about 26.6 degrees) is one of the most common pitches used in North American residential roofing.
How far apart should roof trusses be spaced?
Trusses are most commonly spaced at either 16 inches or 24 inches on-center; wider spacing reduces the number of trusses needed but requires thicker or higher-rated roof sheathing.
Sources and Methodology
Roof pitch/angle conversions, King Post span limit (~26 ft / 8 m), and Queen Post span range (~26-40 ft / 8-12 m) sourced from Structural Wood Corp’s King Post Truss guide, Firgelli Auto’s Queen Post Roof Truss explainer, and standard roof pitch/rafter geometry (Omnicalculator Roof Truss Calculator, rafter length via the Pythagorean theorem).
