Roof Truss Calculator
How to Use This Roof Truss Calculator
Using this roof truss calculator is simple:
- Enter your building span and roof pitch
- Enter your roof length and truss spacing
- Review your truss height, rafter length, and truss count
The calculator will automatically size your trusses and estimate how many you’ll need for the full roof length.
For best results, make sure your measurements match how the structure will actually be framed.
You Will Need:
- Building span (ft)
- Roof length (ft)
- Roof pitch (X:12)
- Truss spacing (on-center, in inches)
Enter Your Roof Details
Roof Truss Results
Get More Accurate Measurements & Estimates with RoofSnap.
How this Roof Truss Calculator Works
This roof truss calculator turns your span, pitch, roof length, and spacing into truss dimensions and a truss count.
- Truss height is calculated using half the span and your roof pitch
- Rafter (rake) length is calculated using the Pythagorean theorem on the half-span and truss height
- Truss count is calculated by dividing roof length by truss spacing, rounding up, and adding one for the end truss
Common Roof Truss Types
This calculator estimates dimensions for a standard gable truss. Other common truss types include:
King Post Truss
- Simplest, most affordable design
- Best for shorter spans
Fink (W) Truss
- Most common residential truss
- Efficient across mid-range spans
Attic Truss
- Creates usable attic space
- Best for homes wanting bonus storage or living area
When to Use a Roof Truss Calculator
Use this truss calculator when:
- Planning a new roof, addition, or outbuilding
- Estimating truss count and material needs before ordering
- Comparing truss spacing options for your project
This tool provides a planning estimate, not engineered drawings. Final truss design should be stamped by a licensed truss manufacturer or structural engineer per local code.
Roof Truss Measurements Glossary
Span
The distance between the outer walls that a truss must cover
Truss Height
The vertical distance from the bottom chord to the peak of the truss
On-Center Spacing
The distance between the centers of adjacent trusses, typically 16″, 19.2″, or 24″
Truss Count
The total number of trusses needed to cover the roof length
Rafter (Rake) Length
The sloped length from the peak to the outer edge of a truss
Rafter Length
The distance along the sloped roof surface from eave to ridge, calculated from rise and run
Roof Truss Calculator FAQs
Divide your roof length by your truss spacing, then add one for the end truss. This roof truss calculator does the math for you automatically based on the spacing you enter into the calculator.
Trusses are typically spaced 16, 19.2, or 24 inches on-center. Tighter spacing (16″) adds strength and cost; wider spacing (24″) uses fewer trusses but may require thicker sheathing.
Multiply half of your building span by your roof pitch (rise per 12 inches of run). The result is the vertical height from the top plate to the peak.
A truss is a pre-engineered, factory-built triangular frame that includes both the top chords and internal webbing. A rafter is a single structural beam, typically cut and assembled on-site as part of stick framing.
Yes. A steeper pitch increases truss height for the same span, which also increases the rafter length and the amount of lumber needed per truss.
Roof truss costs vary by span, pitch, and truss type, but typically range from $75–$400 per truss installed. Use our roof replacement cost calculatorfor a broader project cost estimate.
No. This calculator provides planning estimates only. Actual truss design, member sizing, and connector plates must be specified by a licensed truss designer or structural engineer per local building code.
Yes. Contractors can use this roof truss calculator for quick planning estimates and material takeoffs. For stamped engineering drawings, work with a certified truss manufacturer.