**Our free roof truss calculator is an invaluable tool helps determine requirements for roof trusses, by considering span, pitch, load, and materials to provide accurate calculations for truss design.**

## Roof Truss Calculator

Span (ft) | Pitch | Truss Spacing (in) | Number of Trusses | Rafter Length (ft) | Truss Angle (degrees) |
---|---|---|---|---|---|

24 | 4/12 | 24 | 13 | 12.65 | 18.43 |

30 | 6/12 | 16 | 23 | 16.97 | 26.57 |

40 | 8/12 | 24 | 21 | 24.33 | 33.69 |

50 | 5/12 | 20 | 31 | 27.95 | 22.62 |

20 | 3/12 | 24 | 10 | 9.54 | 14.74 |

28 | 7/12 | 16 | 17 | 15.63 | 30.26 |

36 | 9/12 | 24 | 19 | 22.64 | 36.87 |

45 | 10/12 | 16 | 29 | 28.73 | 39.81 |

60 | 5/12 | 30 | 25 | 32.83 | 22.62 |

70 | 6/12 | 24 | 36 | 37.92 | 26.57 |

80 | 8/12 | 16 | 45 | 43.01 | 33.69 |

90 | 4/12 | 20 | 55 | 48.10 | 18.43 |

## Roof Truss Calculation Formula

**Truss Load = (Roof Area × Dead Load) + (Roof Area × Live Load)**

Where:

**Roof Area**is the surface area of the roof**Dead Load**includes the weight of the truss, roofing materials, and any permanent fixtures**Live Load**accounts for temporary weights like snow, wind, and maintenance personnel

For a **1,000 sq ft roof** with a dead load of **15 psf** and a live load of **20 psf**:

**Truss Load = (1,000 × 15) + (1,000 × 20) = 35,000 lbs**

## How Many Roof Trusses Do I Need?

The number of roof trusses required depends on factors such as the **building’s width**, **truss spacing**, and **local building codes**. A common rule of thumb is to space trusses **24 inches on center**, but this can vary based on specific requirements.

**To calculate the number of trusses:**

- Measure the length of the building
- Divide the length by the truss spacing
- Add 1 to account for the end truss

For example, consider a **40-foot long building** with trusses spaced **24 inches apart**:

**Number of Trusses = (40 feet × 12 inches/foot) ÷ 24 inches + 1
= 480 ÷ 24 + 1
= 20 + 1 = 21 trusses**

## Rafter Length on a 4/12 Pitch Roof

A **4/12 pitch roof** means the roof rises **4 inches for every 12 inches of horizontal run**. To calculate the rafter length, we use the **Pythagorean theorem**.

For a **24-foot wide building** with a 4/12 pitch:

Half the building width is **12 feet (144 inches)**

The rise is 4 inches for every 12 inches, so total rise = (144 ÷ 12) × 4 = **48 inches**

Now we can calculate:

**Rafter length² = Run² + Rise²
= 144² + 48²
= 20,736 + 2,304
= 23,040
Rafter length = √23,040 ≈ 151.79 inches or 12.65 feet**

## How to calculate roof truss angle?

For a **4/12 pitch roof**:

**Angle = tan⁻¹(Rise ÷ Run)
= tan⁻¹(4 ÷ 12)
= tan⁻¹(0.3333)
≈ 18.43 degrees**

The roof truss angle, also known as the **pitch angle**, is crucial for proper truss design. It’s typically expressed in degrees and can be calculated using basic trigonometry.

## References

- American Wood Council. (2023). “Span Tables for Joists and Rafters.” https://awc.org/codes-standards/span-tables/
- Structural Engineering Institute. (2022). “Minimum Design Loads and Associated Criteria for Buildings and Other Structures.” https://www.asce.org/publications-and-news/asce-7

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