WORKBENCH GUIDE
Estimating deck boards, gaps and waste
Count rows, boards and joints from the deck size, board width and gap, then add waste. With board weights for common decking species.
Rows and lengths
The number of rows is the deck width plus one gap, divided by the board width plus the gap, rounded up; the last row is usually ripped narrower. Along the length, each row needs as many boards as fit end to end with their end gaps. The deck calculator does this geometry and reports the width of the last row.
When boards are shorter than the deck, rows need joints, and staggering them leaves offcuts. A waste allowance covers those offcuts, defects and mistakes; 10% is the calculator’s default, not a rule. The calculator gives quantities only: it does not size joists, beams or fixings.
Board sizes
In North America, radius-edged decking is most often 25 or 38 mm thick (actual 1 or 1-1/2 in) and 89 to 140 mm wide (actual 3-1/2 to 5-1/2 in), in lengths of 2.4 to 4.9 m (Wood Handbook, chapter 6). Elsewhere sizes depend on the supplier and the species, so enter the actual width and length you can buy.
The example follows the calculator’s default: a 4,000 × 3,000 mm deck with 140 mm boards, 5 mm gaps and 2,400 mm stock. The weight column multiplies the boards to buy by the board volume (2,400 × 140 × 20 mm) and each species’ density at 12% moisture content, which helps with transport and handling, not with structural loads.
Result: 21 rows, 42 board pieces to cut and 47 boards of 2,400 mm to buy with 10% waste.
| Species | Decay resistance of heartwood | Weight of the boards |
|---|---|---|
| Cumaru | Class 1 (EN 350) | 337.9 kg |
| Ipê | Class 1 (EN 350) | 328.5 kg |
| Garapa | Class 3 (EN 350) | 249.5 kg |
| Teak | Class 1–3 (EN 350) | 211.6 kg |
| European larch | Class 3–4 (EN 350) | 189.5 kg |
| Western red cedar | Resistant (Wood Handbook) | 113.2 kg |
Durability
Decking needs durable heartwood or treated wood. The Wood Handbook groups western red cedar heartwood as resistant to decay, suitable above ground; Tropix rates cumaru and ipê class 1 (very durable) for ground or freshwater contact, while larch heartwood, class 3–4, is for outdoor use out of ground contact with the sapwood removed.
Garapa is rated class 3 and its durability is very variable, so ask about the origin of the boards (Tropix).
Finishing a deck
The Wood Handbook notes that decks are usually replaced because of weathering rather than decay, and that film-forming finishes adhere poorly to decks; penetrating stains or water repellents need refreshing every few years but are easy to renew. Use the finish calculator with the deck area.
With 20 mm cumaru boards, the 47 boards of 2,400 × 140 mm weigh about 47 × 0.00672 m³ × 1,070 kg/m³ ≈ 338 kg.
Sources & references
- USDA FPL · Wood Handbook (2021), ch. 6: Commercial lumber, round timbers, and ties
- USDA FPL · Wood Handbook (2021), ch. 14: Biodeterioration of wood
- USDA FPL · Wood Handbook (2021), ch. 16: Finishing of wood
- CIRAD · Tropix: Cumaru
- CIRAD · Tropix: Ipê
- CIRAD · Tropix: Mélèze d'Europe
- CIRAD · Tropix: Garapa