Open methodology
Every number here can be checked by hand.
Nothing is estimated behind the scenes. The 3D model and the material list read the same geometry, and the geometry follows the building practice of the country you pick. This page lists the rules, the sources and the formulas so a builder, a customer or an inspector can verify them.
Rules used per country
| Country | Joist centres | Max joist span | Post spacing | Guard from / height | Max step rise |
|---|---|---|---|---|---|
| United States | 16.0 in | 13.1 ft | 8.0 ft | 30.0 in / 36.0 in | 7.8 in |
| Canada | 406 mm | 3.90 m | 2.44 m | 599 mm / 899 mm | 200 mm |
| United Kingdom | 400 mm | 3.00 m | 1.80 m | 600 mm / 900 mm | 190 mm |
| Ireland | 400 mm | 3.00 m | 1.80 m | 600 mm / 900 mm | 190 mm |
Where each figure comes from
United States
IRC / DCA 6: joists at 16 in on centre, stair rise up to 7 ¾ in
- IRC 2021 R507.6 — joist spans
- IRC 2021 R311.7.5 — rise and going
- IRC 2021 R312.1 — guards above 30 in
- IRC 2021 R507.5 — beam spans
Canada
NBC Part 9: joists at 16 in on centre, stair rise up to 200 mm
- NBC 9.23.4 — joist spans
- NBC 9.8.4 — stair dimensions
- NBC 9.8.8 — guards above 600 mm
- NBC 9.23.4 — beam support
United Kingdom
BS 8579 / Approved Document K: joists at 400 mm centres, rise up to 190 mm
- BS 8579 / TRADA span tables — 47×150 at 400 mm centres
- Approved Document K — rise up to 190 mm
- Approved Document K — guarding
- BS 8579 — support spacing
- Planning Portal — decking as permitted development (300 mm)
Ireland
TGD Part K: joists at 400 mm centres, rise up to 190 mm
- TGD Part A / TRADA span tables — 400 mm centres
- TGD Part K — rise up to 190 mm
- TGD Part K — guarding
- TGD Part A — support spacing
- Planning and Development Regulations — decking over 300 mm
The formulas
- Deck area
- width × length, minus any corner cut, plus the lower platform
- Decking rows
- depth ÷ (board width + gap), rounded up — each row is counted at its real length
- Decking to buy
- total row length × (1 + waste ÷ 100) ÷ stock length, rounded up. Waste defaults to 10%
- Joists
- width ÷ joist centres, rounded up, plus one
- Beams
- one at each end plus an extra whenever the joist span passes the limit
- Posts and footings
- beam length ÷ maximum post spacing, rounded up, plus one, per beam
- Steps
- deck height ÷ target rise, rounded so every step is equal
- Stringers
- three on any flight, at the standard going for the country
- Railing
- open perimeter only — the house edge is never counted
- Fixings
- board-to-joist crossings × 1 hidden clip, or × 2 face screws
- Joist hangers
- two per joist carried by a beam
- Concrete
- three bags of post-set per footing — 20 kg in the UK and Ireland, 80 lb in the US and Canada
- Cut list
- pieces packed longest first into the lengths yards actually sell
Waste. The allowance on decking follows the board layout rather than one flat figure: straight 5%, picture frame 8%, diagonal (45°) 15% and herringbone 20%. An L-shape adds 2 points, a U-shape 3, and a second level 1, because the cut rows leave more offcut. It is shown on the line, and you can type your own instead. Framing is not inflated: it is cut from real stock lengths and the offcut is listed.
Prices. There are none in the software. No national averages, no supplier feed, no affiliate links — we do not sell timber and take no cut. Money appears only from rates you type in yourself.
Limits. This is a take-off and a sanity check, not a structural design or an approval. Level ground is assumed. Ledger fixing, ground conditions, drainage and local consent still need a competent check.