Enter the length of the building, the span between the walls, the roof pitch and the truss spacing. The calculator counts the roof trusses for a gable roof and gives the rise, the top chord length and the roof area to help you plan and price the job.
How to use the roof truss calculator
- Enter the building length along the ridge, from one gable end to the other.
- Enter the span, the distance across the trusses from the outside of one wall to the outside of the other.
- Enter the pitch in inches of rise per 12 in of run, and the eave overhang measured level.
- Choose the spacing shown on your truss design drawings. 24 in on center is common for house roofs.
Roof truss formulas
Trusses = (building length in inches ÷ spacing), rounded up, + 1 Gable end trusses = 2; common trusses = trusses − 2 Run = span ÷ 2; rise = run × pitch ÷ 12 Pitch factor = √(1 + (pitch ÷ 12)²) Top chord (each side) = (run + overhang) × pitch factor Roof area = 2 × top chord × building length
Rounding the spaces up means no gap between trusses is wider than the spacing you chose; the last space is simply a little shorter. The top chord length is a line length along the slope and does not include the heel height or tail cut.
Worked example: a 40 × 28 ft house, 6/12 pitch
- Trusses at 24 in on center: 480 ÷ 24 = 20 spaces, so 21 trusses: 19 common and 2 gable end trusses.
- Run: 28 ÷ 2 = 14 ft. Rise: 14 × 6 ÷ 12 = 7 ft.
- Pitch factor at 6/12: 1.118. Top chord with a 12 in overhang: (14 + 1) × 1.118 = 16.77 ft per side.
- Roof area: 2 × 16.77 × 40 = 1,342 ft², about 13.4 squares before rake overhangs.
How many trusses for common building lengths
| Building length | 24 in on center | 16 in on center |
|---|---|---|
| 20 ft | 11 | 16 |
| 24 ft | 13 | 19 |
| 30 ft | 16 | 24 |
| 40 ft | 21 | 31 |
| 50 ft | 26 | 39 |
| 60 ft | 31 | 46 |
Trusses must be engineered
This calculator is for counting and planning, not for designing a truss. The International Residential Code, section R802.10, requires truss design drawings to be approved by the building official before installation, and metal-plate-connected wood trusses to be designed and made to ANSI/TPI 1, as Brent Maxfield explains in STRUCTURE magazine. Your truss supplier designs each truss for your span, pitch, snow and wind loads, and the drawings set the spacing, bracing and how the trusses attach to the walls.
What you need besides the trusses
- Gable end trusses carry the siding at each end and are usually built differently from the common trusses.
- Bracing is shown on the truss drawings and the BCSI guide your supplier provides; order it with the trusses.
- Sheathing: divide the roof area by 32 ft² for 4 × 8 sheets, or use the plywood calculator.
- Hurricane ties or clips at each truss, as the drawings and your local code require.
Questions
How many trusses do I need for a 40 ft building?
At 24 in on center, 21 trusses: 20 spaces plus one. At 16 in on center, 31 trusses.
How do you calculate the number of roof trusses?
Convert the building length to inches, divide by the spacing, round up and add one. A 30 ft building at 24 in on center is 360 ÷ 24 = 15, plus one, or 16 trusses.
How tall is a roof truss?
The rise is half the span times the pitch ÷ 12. A 28 ft span at 6/12 rises 7 ft at the peak, plus the heel height of the truss.
What is the standard spacing for roof trusses?
24 in on center is the most common in houses, but trusses are also made for 16 in or 19.2 in spacing. Always build to the spacing on your truss drawings.
Sources
- Code Requirements for Residential Roof Trusses, Brent Maxfield, STRUCTURE magazine.
Used for: IRC R802.10 truss design drawings approved before installation; trusses designed and made to ANSI/TPI 1. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact foot and inch definitions.