A Roof Truss Calculator helps you estimate the number of trusses needed for a roof, the total truss expense, and the length of a rafter. Use the Truss Count option to calculate truss quantity and cost, or use the Rafter Length option to calculate a rafter from its rise and run.
These calculations are useful for early roofing material planning. Confirm the final truss design, sizing, loads, connections, and installation requirements with a qualified professional.
What Is a Roof Truss?
A roof truss is a structural framework that supports the roof and transfers its load to the building walls. Trusses are commonly installed at regular on-center spacing along the length of a building.
The required number of trusses depends on the roof length and the selected spacing between trusses.
How to Use the Roof Truss Calculator
Select the calculation you need:
- Truss Count: Calculate the number of trusses and total truss expense.
- Rafter Length: Calculate the length of one rafter using the rise and run.
How to Use the Truss Count Calculator
Enter the following values:
- Roof length
- On-center spacing
- Single truss cost
The calculator returns:
- Truss count
- Total expenses
Calculate Truss Count
Divide the roof length by the on-center spacing, round up, and add one truss to account for both ends of the roof.
Truss Count=⌈On-Center SpacingRoof Length⌉+1
For example, if the roof length is 30 feet and trusses are spaced 2 feet on centre:
Truss Count=⌈230⌉+1=16 trusses
The roof requires approximately 16 trusses.
Calculate Total Truss Expenses
Multiply the truss count by the cost of one truss.
Total Expenses=Truss Count×Single Truss Cost
If one truss costs 150:
Total Expenses=16×150=2400
The estimated truss expense is 2,400.
How to Use the Rafter Length Calculator
Enter the following values:
- Rise
- Run
The calculator returns:
- Rafter length
The rise is the vertical distance from the wall plate to the roof peak. The run is the horizontal distance from the wall plate to the roof peak.
Calculate Rafter Length
The rafter, rise, and run form a right triangle. Use the Pythagorean theorem to calculate the rafter length.
Rafter Length=Rise2+Run2
For example, if the rise is 8 feet and the run is 15 feet:
Rafter Length=82+152
Rafter Length=64+225=289=17 ft
The rafter length is 17 feet.
Important Planning Notes
- Use the same unit for roof length and on-center spacing.
- Enter the truss cost as the cost of one truss.
- Use the rise and run of one side of the roof for rafter-length calculations.
- Include any overhang in the run if it is part of the required rafter length.
- Add allowances for cutting, connections, bracing, delivery, and installation.
- Do not use this calculator as a substitute for structural engineering or approved truss drawings.
Frequently Asked Questions
What does on-center spacing mean?
On-center spacing is the distance from the centre of one truss to the centre of the next truss.
Why does the truss count calculation add one truss?
Adding one truss accounts for trusses at both ends of the roof length.
What should I enter as the roof length?
Enter the distance along the building or roof where trusses will be installed. This is not the sloped rafter length.
Does the calculator include the cost of installation?
No. Total expenses estimate the cost of the trusses based on the single truss cost entered. Delivery, labour, crane hire, hardware, bracing, and installation may cost extra.
What is the difference between rise and run?
Rise is the vertical height from the wall plate to the roof peak. Run is the horizontal distance from the wall plate to the roof peak.
Does rafter length include the overhang?
Only if you include the overhang in the run measurement. Add the overhang to the run when you need the full rafter length beyond the wall.
Can I use feet, inches, metres, or centimetres?
Yes. Use any unit, but make sure rise and run use the same unit. Likewise, roof length and on-center spacing must use the same unit.
Is this calculator suitable for final truss design?
No. It provides a planning estimate only. Final truss design must consider roof loads, span, pitch, wind, snow, materials, connections, and local building requirements.
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