Roof Truss Calculator
Truss count and lumber for a roof — an estimate, not an engineering design.
Estimate only — this is not an engineering sign-off
This tool applies a published code equation or standard beam formula to the numbers you enter. It does not know your building, your local code amendments, your soil, your connections, or your load path. The result is a planning estimate for sizing conversations and material take-offs only.
- Every design value (loads, allowable stresses, factors) is your input — a wrong value in gives a wrong answer out.
- Quantity and rough lumber estimate only — this performs NO truss engineering.
- Member sizes, web layout and bracing come from the truss fabricator's sealed shop drawings per ANSI/TPI 1.
- No load path, connection, lateral, uplift, bearing, notch, hole, fire, or serviceability check beyond the one stated formula.
A licensed engineer or your local building official must review and stamp the final design before you build or order structural members.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Common trusses = ⌊ building length ÷ spacing ⌋ + 1; total = common + gable-end trusses
Slope factor = √(pitch² + 12²) ÷ 12
Top chord (both sides) = 2 × (span ÷ 2 + overhang) × slope factor
Chord lumber ≈ (top chords + bottom chord) × truss count
Run the calculation to see each step with your own numbers.
40 ft long building, 28 ft span, 6-in-12, trusses 24 in o.c., 2 gable ends, 12-in overhang.
Common = ⌊40 ft ÷ 24 in⌋ + 1 = 21; total = 21 + 2 = 23 trusses
Slope factor = √180 ÷ 12 = 1.118; top chord = 2 × (14 + 1) × 1.118 = 33.5 ft
Chord lumber ≈ (33.5 + 28) × 23 ≈ 1,415 lineal ft (chords only)
- Quantity and layout estimate only. Truss design — member sizes, web layout, plate sizing, camber, bearing and bracing — is done by the truss fabricator's engineer per ANSI/TPI 1 and sealed shop drawings.
- Simple gable roof, single span, trusses perpendicular to the ridge at uniform spacing. Hip sets, girder trusses, valley sets and step-downs need a truss designer's layout.
- The chord-lumber figure is a rough order-of-magnitude number for comparison only — it omits webs, wedges, plates, permanent bracing and any load-driven member upsizing.
- The supporting walls, beams and hold-downs for the truss reactions (including uplift) are a separate design.
- Can this tell me what size lumber my trusses need?
- No — and no honest calculator can. Truss member sizes and the web pattern come from the fabricator's engineering software using your loads, span, spacing and profile, and are issued as sealed shop drawings. This tool estimates how many trusses and roughly how much chord lumber for budgeting.
- How far apart should roof trusses be?
- 24 in on centre is the common default for a residential gable roof; 16 in is used for heavier loads or thinner sheathing. The fabricator's engineer confirms the spacing against your loads and profile.
- Do I count the gable-end trusses separately?
- Yes. Gable-end trusses are heavier — usually built with vertical studs to carry the end wall — and are ordered as a distinct item. There are normally two, one at each end.
Layout
Standard truss layout (members on-centre + one). Top-chord length is roof-slope geometry. Unit conversions per NIST SP 811.
Truss design authority
ANSI/TPI 1 'National Design Standard for Metal Plate Connected Wood Truss Construction' — truss engineering is performed by the fabricator, not by this estimate.
About the Roof Truss Calculator
The Roof Truss Calculator is a quantity and layout estimate — how many common and gable-end trusses a gable roof needs at your spacing, plus a rough chord-lumber figure for budgeting. It does no truss engineering: member sizes, the web pattern, plate sizing and bracing come from the fabricator's sealed shop drawings under ANSI/TPI 1.
What you enter
- Building length (ridge direction)
- Truss span (out to out of walls)
- Roof pitch (rise-in-12) — For the top-chord length estimate.
- Truss spacing
- Gable-end trusses — Usually 2 — heavier, with vertical studs.
- Eave overhang
- Waste allowance — Default 0%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Trusses (total) — Common trusses + gable-end trusses.
- Common trusses — the exact result of the calculation.
- Top-chord length (per truss, both sides) — the exact result of the calculation.
- Chord lumber estimate (total lineal) — Top chords + bottom chord only. Webs, plates and bracing are extra and depend on the truss design.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Snow Load Calculator — Estimated roof snow load for planning — an estimate, not an engineering determination.
- Roof Pitch Calculator — Roof angle, slope factor and rise-in-12 pitch from a rise and run.
- Rafter Length Calculator — Common rafter length from the building span and roof pitch.
- Framing Calculator — Stud count and lineal feet for a wall at a given stud spacing.
Part of the Roofing calculators.