Introduction
Every roof starts with two questions: how many trusses do I need, and how long are the members? A roof truss is a prefabricated triangular frame carrying roof load to the exterior walls, and counting them is simple: divide the building length by the on-center spacing, then add one.
The Roof Truss Calculator does that core math: enter the building length and truss spacing (16/24 in or 400/600 mm o.c.) and it returns the truss count, plus rafter length for stick-framed sections and ridge board length. This guide covers everything behind those numbers — truss types, spacing, loads, gable vs. hip layouts, and worked examples in metric and imperial.
What a Roof Truss Is and Why the Count Matters
A roof truss is an engineered, factory-built triangular frame — typically 2×4 or 2×6 lumber joined with steel gusset plates. The triangle converts roof weight into forces that travel to the bearing walls, letting a truss span distances no single board could cross.
Trusses are set at regular intervals — 16 or 24 in o.c. (400/600 mm metric) — matching sheathing sizes so panel edges land on a truss. The count matters because trusses are made to order with 2–4 week lead times. The estimating guide covers the full takeoff this count feeds into.
The Truss Count Formula
Number of trusses = (Building length ÷ On-center spacing) + 1
Work in consistent units — convert building length to inches (feet × 12) or millimeters (meters × 1000) before dividing. The “+1” is the piece beginners miss: the first truss sits at the zero mark and the last closes the far end, so N spaces always need N+1 trusses.
Worked example — imperial
A 30 ft building at 24 in o.c.: 30 × 12 = 360 in; 360 ÷ 24 = 15 spaces; 15 + 1 = 16 trusses. At 16 in o.c.: 360 ÷ 16 = 22.5 + 1 = 23.5 → round up to 24 trusses.
Worked example — metric
A 9.14 m building at 600 mm o.c.: 9140 ÷ 600 = 15.23 → +1 = 16.23 → 16 trusses, with bay spacing adjusted to 9140 ÷ 15 ≈ 609 mm. At 400 mm o.c.: 9140 ÷ 400 = 22.85 + 1 = 23.85 → 24 trusses. Each step is derived in the formula-explained guide.
Common Truss Types
The type you order depends on span, pitch, and whether you want usable attic space:
| Type | Typical span | Best for |
|---|---|---|
| King post | Up to ~16 ft (5 m) | Sheds, porches — one center vertical |
| Queen post | ~16–24 ft (5–7.5 m) | Garages — two verticals, open center |
| Fink (W) | ~20–36 ft (6–11 m) | The standard residential truss |
| Howe | ~25–45 ft (8–14 m) | Flatter pitches, heavier loads |
| Attic | ~20–36 ft (6–11 m) | Livable space under the roof |
| Scissor | ~20–32 ft (6–10 m) | Vaulted ceilings — sloped bottom chord |
| Mono | Up to ~20 ft (6 m) | Single-slope roofs, sheds, additions |
| Gable end | Matches main truss | End walls — sheathed like a wall |
Span ranges are typical, not limits — the truss manufacturer engineers each design for your actual span, pitch, and loads. If you’re comparing stick framing against trusses for a small project, the DIY vs. pro guide lays out the trade-offs.
Spacing: 16 vs. 24 Inches On Center
Spacing is a structural and economic choice:
- 24 in (600 mm) o.c. — the residential default. Fewer trusses, lower cost, and 4×8 ft sheathing breaks cleanly on the layout. Most truss designs are optimized for this spacing.
- 16 in (400 mm) o.c. — for heavier loads (high snow regions), tile or slate roofing, or matching existing 16-in wall framing. Roughly 50% more trusses than 24-in spacing.
Whichever you choose, the truss spacing and the wall-stud spacing don’t have to match — trusses bear on the top plate, which distributes the load. What must match is the design load: a 24-in spacing with a heavier truss design is often cheaper than 16-in spacing with lighter trusses. Your manufacturer runs that comparison when they engineer the job.
Rafter Length for Stick Framing
Not every roof uses trusses — porches, dormers, and small additions are often stick-framed. The calculator also returns rafter length from span and pitch:
Rafter length = √(run² + rise²), where run = half the span and rise = run × (pitch ÷ 12).
Worked example — imperial
A 24 ft span at 6/12 pitch: run = 24 ÷ 2 = 12 ft; rise = 12 × (6 ÷ 12) = 6 ft; rafter = √(12² + 6²) = √180 = 13.42 ft.
Add the eave overhang (the “tail”) and half the ridge board thickness for the cut length. A 2 ft overhang at 6/12: tail run 2 ft, tail rise 1 ft, tail = √(4+1) = 2.24 ft — so the rafter cuts at about 13.42 + 2.24 = 15.66 ft before the ridge deduction.
Metric check: 7.32 m span at 6/12 — run 3.66 m, rise 1.83 m, rafter = √(3.66² + 1.83²) = 4.09 m, the same geometry.
Gable vs. Hip Roofs
The count formula covers the main run, but roof shape changes what you order:
- Gable roof — triangle at each end. Order the formula’s full trusses plus two gable-end trusses (confirm whether your supplier counts them in the total). Gable ends are sheathed like walls.
- Hip roof — slopes on all four sides, using a girder truss plus jack trusses toward the corners. Plan-specific — have the manufacturer design it from your drawings.
Loads the Truss Must Carry
Trusses aren’t sized by span alone — they’re engineered for four load types. You don’t calculate these yourself (the manufacturer’s software does), but the terms help you read a quote:
- Dead load — permanent weight: roofing, sheathing, truss, ceiling drywall. Shingles run ~2–3 psf (10–15 kg/m²); tile/slate can exceed 10 psf (50 kg/m²), changing the design.
- Live load — temporary: workers and equipment during construction/maintenance. Residential designs commonly use 20 psf (~1.0 kN/m²).
- Snow load — the big regional variable: near zero in warm climates to 70+ psf (3.4+ kN/m²) in mountain regions, from local code maps.
- Wind load — pressure plus uplift, which tries to peel the roof off. High-wind regions need hurricane ties at every truss-to-wall connection.
Give the manufacturer your site address — they pull the code-prescribed snow and wind values for the location. See the code compliance guide for what a design assumes.
Mistakes That Distort Results
The arithmetic is simple, which makes input errors easy to overlook: forgetting the +1, mixing feet and inches without converting, measuring building length without the gable overhangs you intend to frame, and assuming 24-in spacing suits a tile roof in a snow zone. Each is dissected with fixes in the common mistakes guide.
Related Guides
- Formula Explained — every equation the calculator runs, derived step by step.
- Material Guide — lumber grades, gusset plates, sheathing, connectors.
- Cost Guide — per-truss pricing, delivery, crane, and labor.
- Estimating Guide — full quantity takeoff with waste allowances.
- Ordering & Install Checklist — what to verify before the truck arrives.
Frequently Asked Questions
What does “16 on center” mean?
Trusses are spaced so the centers of adjacent trusses sit 16 inches (406 mm) apart. “On center” keeps layout consistent regardless of member thickness.
Can I use the calculator for a hip roof?
For the main rectangular run, yes — but hip ends need girder and jack trusses the simple count doesn’t cover. Have the manufacturer design the hip layout from your plan.
How long does it take to get trusses made?
Typically 2–4 weeks from approved drawings, longer in peak season. Order as soon as the design is finalized — walls can’t wait for trusses.
Do trusses need an engineer?
The manufacturer supplies sealed design drawings for your span, pitch, and local snow/wind loads. You don’t engineer them yourself, but you need those drawings for the permit.
What’s the difference between a truss and a rafter?
A truss is a prefabricated triangular frame spanning wall to wall; rafters are individual boards (stick framing) meeting at a ridge board, often needing ceiling joists or a structural ridge. Trusses go up faster; rafters leave the attic open.
Get Your Number
Roof truss estimating is simple arithmetic on careful inputs: measure the full building length, pick the spacing your design calls for, divide, and add one. The Roof Truss Calculator runs the count plus rafter and ridge math in metric and imperial — then send span, pitch, and load details to a manufacturer for engineered drawings and a quote.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Roof trusses are engineered structural products — always order from a manufacturer that supplies sealed engineering drawings, verify spans and loads with a licensed engineer, and check your local building code before construction.
