# Roof Truss Calculator

URL: https://speedycalculator.com/calculators/construction/roof-truss/
Last updated: 2026-10-25

> The roof truss calculator counts the trusses for a building from its length and truss spacing, splits them into common and gable-end trusses, and estimates the cost.

## Inputs

- Building length (ft, m, in, yd, cm)
- Truss spacing
- Gable-end trusses
- Span (ft, m, in, yd, cm)
- Price per common truss (/piece)
- Price per gable truss (/piece)

## Results

- Total trusses
- Common trusses
- Gable-end trusses
- Roof footprint
- Truss cost

## Formula

```
Trusses = ceil(building length ÷ spacing) + 1
Common trusses = trusses − gable ends
```

## Explanation

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

FieldWhat to enterBuilding lengthUnits: ft, m, in, yd, cmTruss spacingChoose one: 24 in, 16 in, 19.2 in, 600 mm …Gable-end trussesChoose one: 2 (gable roof), 0 (hip roof), 1SpanUnits: ft, m, in, yd, cmPrice per common trussUnits: /piecePrice per gable trussUnits: /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 trusses201130164021502660318041

## Worked 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.

## FAQ

**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.


## Sources

- [Structural Building Components Association – BCSI](https://www.sbcindustry.com/)