Wood Roof Truss Calculator — m, lumber
How to use this calculator
Pick a preset or enter span, pitch (degrees), spacing, and building length. The output is your timber yard order — truss count for the crane day, volume of C24 chord and web stock for the PO, and rafter length so you can size top chords to standard CLS lengths (2.4/3.0/3.6/4.2/4.8/5.4/6.0 m) without a wasteful splice.
Truss types — King Post (simple, short spans), Queen Post (medium spans), Fink/W (most common residential), Howe (heavier loads).
Lumber — volume = width(m) × depth(m) × length(m).
Saved Calculations
| Time | Type | Span | Pitch | Trusses | Total m³ | Cost |
|---|
How to size a wood roof truss (metric)
Pick a truss type (King Post for short spans, Queen Post for medium spans up to ~9 m, Fink/W for typical 7–11 m residential roofs, Howe for higher snow loads), enter the clear span between wall plates, the roof pitch in degrees, truss spacing on-centre in mm, and the building length along the ridge. Chord and web sizes use standard CLS / C24 structural softwood sections (38 × 89 mm, 38 × 140 mm, 38 × 184 mm, 38 × 235 mm, 38 × 286 mm) per EN 1995-1-1.
Add 10–15 % to the calculated volume for cuts, defects, and temporary bracing. The result is a structural takeoff — sheathing, fascia, and battens are separate. For roof sheathing on top, use the plywood / OSB sheets (metric). To price the framing lumber on its own, see the lumber volume calculator (m³).
Formulas
Rise = (span / 2) × tan(pitch). Rafter length Lr = (span/2) / cos(pitch) + overhang / cos(pitch). Number of trusses N = ceil(building length / spacing) + 1. Volume per piece V = width(m) × depth(m) × length(m). Total volume = 2 Lr Vchord + span Vchord + Σ web Vweb, multiplied by N.
Pitch & spacing in practice
15° is a low-slope cold roof; 25° is a typical mainland European tile pitch; 35°–45° suits heavy snow or slate. 600 mm o.c. is standard with C24 chords and 18 mm sheathing; drop to 400 mm o.c. for higher loads or longer spans. For floor framing under the roof, see the joist span calculator (m). For decks and outdoor framing use the deck calculator (m).
FAQ
Why does my volume look high? The top chord is counted twice (one per side of the ridge). Heel and seat cuts are not subtracted — the small overrun covers waste.
Are metalwork and bracing included? No. The total is structural sawn timber only. Add metal plate connectors (factory MPC trusses) or plywood gussets plus nails on top of this estimate.
Which currency selector should I use? EUR is the default; GBP, PLN, CZK and SEK are available for site-specific quotes — the symbol updates in the cost row and the result block.
On install day
The number this calc drives is your timber-yard purchase order: volume of C24 chord and web stock plus the truss count for crane scheduling. Three things estimators miss. Stock length — CLS sections come in 2.4/3.0/3.6/4.2/4.8/5.4/6.0 m; an 8.5 m span needs 5.4 m top chords (with a scarf joint) or 6 m stock cleanly; round up at the merchant. Permanent bracing — lateral bracing along the bottom chord and webs per EN 1995 and TRA-style guidance is not in the takeoff; add ~5% of total volume. Pre-fab vs. site-built — for spans over ~7 m, factory MPC (gang-nailed) trusses with an engineer's certificate (typically €90–250 per truss in 2025 EU) almost always beat site-built once you cost the plates, labour, and the structural sign-off. Cross-check with the lumber volume calculator before placing the order.
