Roof Truss Calculator Online Free

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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