Roof Truss Calculator
Estimate common-truss geometry, layout count, chord length, preliminary lumber, and cost.
Truss geometry and materials
Enter the clear span, pitch, building length, spacing, overhang, and a planning lumber rate.
About roof truss calculations
A roof truss calculator provides preliminary geometry and material quantities for a symmetrical common truss. The span is the horizontal distance between the outside supports. Half of that span is the run from one bearing point to the centerline. Roof pitch states how many units the roof rises for every twelve units of horizontal run, so multiplying the half-span by pitch divided by twelve gives the peak height above the bearing line.
The top chord follows the sloped edge from the peak toward an eave. Its length comes from right-triangle geometry, with the entered overhang extending the horizontal run. Two top chords plus one bottom chord form the simplified lumber length shown for each truss. Multiplying this value by the estimated truss count gives a useful comparison quantity, but it is not a fabrication cut list. Actual trusses include webs, connector plates, splices, heel details, bracing, and product-specific allowances that this conceptual estimate does not model.
Truss count depends on building length and on-center spacing. The calculator divides length by spacing, rounds upward, and includes an end truss. This method ensures that the final bay does not exceed the selected spacing. Openings, gable framing practices, girder trusses, setbacks, and local detailing can change the actual count. The optional cost multiplies the simplified chord footage by a user-entered rate. It does not represent a supplier quote and excludes web members, plates, treatment, delivery, cranes, labor, taxes, engineering, and waste.
Roof trusses are engineered structural assemblies. Snow, wind, seismic forces, dead loads, ceiling loads, bearing capacity, lumber grade, moisture exposure, uplift connections, and building-code rules determine whether a design is safe. Never size or build a structural truss solely from this calculator. Use the result to explore proportions, estimate layout quantities, or prepare questions for a truss manufacturer. Final drawings and member specifications should come from a qualified engineer or an approved truss designer and must match the project jurisdiction, loading, supports, and installation bracing requirements.
Roof truss examples
| Inputs | Preliminary result | Planning note |
|---|---|---|
| 30 ft span, 6-in-12 pitch, 40 ft building, 24 in spacing | 21 trusses; 7.5 ft peak height | Includes one truss at each end. |
| 24 ft span, 4-in-12 pitch, 30 ft building, 24 in spacing | 16 trusses; 4 ft peak height | A lower pitch shortens each top chord. |
| 36 ft span, 8-in-12 pitch, 60 ft building, 16 in spacing | 46 trusses; 12 ft peak height | Closer spacing substantially increases the count. |
How to estimate roof trusses
- Measure the supported building span and overall length in feet.
- Enter the specified roof rise per twelve units of run and the desired eave overhang.
- Enter the engineered on-center spacing and an optional planning cost per foot.
- Review the geometry and quantity, then have a qualified designer verify the structural system.
Frequently asked questions
Does this calculator design a structural truss?
No. It estimates simple geometry and quantities but does not analyze member forces, connections, loading, or code compliance.
Why is one truss added to the spacing calculation?
Spacing describes bays between trusses rather than the number of trusses themselves. A run with twenty bays therefore needs twenty-one framing lines.
What does the top chord length include?
It includes the sloped half-span plus the entered horizontal eave overhang. It does not include special heel cuts, peak joints, splices, or fabrication allowances.
Can trusses be spaced more than 24 inches apart?
Some engineered systems use different spacing, but loading and sheathing limitations must be checked. Follow sealed truss drawings and the applicable building code.
Why is estimated lumber not a purchase list?
The estimate counts only simplified top and bottom chords. Webs, bracing, blocking, connector material, and waste must be specified separately.