What Is a Truss Calculator?
A truss calculator works out the key dimensions of a roof truss — the rise, rafter (top chord) length, and roof angle — from the building span and the roof pitch. Trusses form the triangular framework that supports a roof, and getting the geometry right is essential for ordering materials and framing accurately. Enter your span and pitch, and the calculator returns the rise, rafter length, and angle.
How to Use the Calculator
- Enter the span — the full width the truss spans.
- Enter the roof pitch — as a ratio (such as 6:12) or angle.
- Calculate — see rise, rafter length, and roof angle.
Truss Geometry Basics
A common truss is a triangle: the span is the base, the run is half the span, the rise is the vertical height at the peak, and the rafter (top chord) is the sloped side. Pitch expresses how many inches of rise per 12 inches of run.
Rise = Run × (Pitch ÷ 12) and Rafter Length = √(Run² + Rise²) (Pythagorean theorem).
Common Roof Pitches
| Pitch (rise:run) | Approx Angle | Description |
|---|---|---|
| 4:12 | 18.4° | Low slope |
| 6:12 | 26.6° | Conventional |
| 8:12 | 33.7° | Steeper |
| 12:12 | 45° | Very steep |
Why Truss Dimensions Matter
- Material ordering: rafter length determines lumber needed.
- Roof clearance: rise affects attic space and headroom.
- Drainage and snow load: pitch influences performance.
- Code compliance: structural trusses must meet engineering specs.
Note: For load-bearing structural design, always follow engineered drawings and local building codes.
Frequently Asked Questions
How do you calculate roof truss dimensions?
From the span and pitch: the run is half the span, the rise is run × (pitch ÷ 12), and the rafter length is the square root of run² plus rise². The calculator computes all three.
What is roof pitch?
Pitch is the steepness of a roof, expressed as inches of vertical rise per 12 inches of horizontal run — for example, 6:12 means 6 inches of rise per foot.
How do I find rafter length?
Use the Pythagorean theorem: rafter length = √(run² + rise²), where the run is half the span and the rise is the peak height.
What is the run of a truss?
The run is the horizontal distance from the outside wall to the center peak — half of the full span for a symmetrical truss.
Is this truss calculator free?
Yes — it is completely free, requires no signup, and gives rise, rafter length, and angle.