Prefabricated trusses are usually set 24 in on centre for roofs (sometimes 16 in for heavy tile or snow loads). The two end trusses are often special gable-end trusses with vertical studs for sheathing. The truss maker designs each truss for your span, pitch and loads.
How to use the Roof Truss Calculator
- Building length
- Truss spacing
- Gable-end trusses
- Span
- Price per common truss
- Price per gable truss
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the building length, truss spacing, the number of gable ends, the span and prices for common and gable trusses.
Tips for an accurate estimate
- Measure the footprint from the ground and apply the pitch multiplier rather than climbing on the roof.
- Add 10% waste for simple gable roofs and 15–20% for hips, valleys and dormers.
- Confirm design snow and wind loads with your local building department before changing framing or roofing.
- Always work with fall protection; roofing is one of the most dangerous trades.
Common mistakes to avoid
- Using the footprint area instead of the sloped roof area when ordering materials.
- Forgetting starter strips, ridge caps, drip edge and underlayment.
- Mixing up pitch (rise per 12 in of run) with the roof angle in degrees.
Inputs explained
| Field | What to enter |
|---|---|
| Building length | Units: ft, m, in, yd, cm |
| Truss spacing | Choose one: 24 in, 16 in, 19.2 in, 600 mm … |
| Gable-end trusses | Choose one: 2 (gable roof), 0 (hip roof), 1 |
| Span | Units: ft, m, in, yd, cm |
| Price per common truss | Units: /piece |
| Price per gable truss | Units: /piece |
What each result means
- Total trusses = ceil([Building length]÷[Truss spacing] – 0.000001) + 1
- Common trusses = [Total trusses] – [Gable-end trusses]
- Roof footprint = [Building length]×[Span]
- Truss cost = [Common trusses]×[Price per common truss] + [Gable-end trusses]×[Price per gable truss]
Example values
How the main result changes when Building length changes and everything else stays at the example values above:
| Building length (ft) | Total trusses |
|---|---|
| 20 | 11 |
| 30 | 16 |
| 40 | 21 |
| 50 | 26 |
| 60 | 31 |
| 80 | 41 |
Formula
Trusses = ceil(building length ÷ spacing) + 1Common trusses = trusses − gable endsWorked example
Inputs — Building length: 40 ft; Truss spacing: 24 in; Gable-end trusses: 2 (gable roof); Span: 24 ft; Price per common truss: $150 /piece; Price per gable truss: $200 /piece.
Results — Total trusses = 21; Common trusses = 19; Gable-end trusses = 2; Roof footprint = 960 ft²; Truss cost = $3,250.00.
Frequently asked questions
How far apart are roof trusses?
Usually 24 in on centre; 16 in for heavy roofing or high snow loads.
How many trusses for a 40 ft building?
At 24 in spacing, 21 trusses (20 spaces + 1).
Can I cut or modify a truss?
No – trusses are engineered; never cut chords or webs without the designer's approval.
What formula does the Roof Truss Calculator use?
Trusses = ceil(building length ÷ spacing) + 1.