Deck joist calculator
The joist spacing your boards allow, then the joists, bearers, posts and footings.
Your deck and the result
Your deck
Framing for a 4.80 m × 3.60 m deck
- Joist centres
- 396mm 90 × 45 MGP10 H4 (Treated pine MGP10 (H3 / H4))
- Longest joist span
- 1,170mm
- Bearer lines
- 4 2 / 190 × 45 MGP10 H4 (double)
- Posts
- 12 90 × 90 H4 post
- Footing pads
- 12 300–400 mm square
- Deck area
- 17.3m²
How the boards are laid: every board runs the full length in one piece.
Concept plan, not for construction
The full list
Framing, footings and the rest
| Item | Specification | Qty | Unit |
|---|---|---|---|
| Joists — 90 × 45 MGP10 H446.8 lm. 13 joists in total — see the cut list; H4 treated. | 3.6 m lengths | 13 | pcs |
| Rim joists — 90 × 45 MGP10 H49.6 lm. 2 rims / ledgers in total — see the cut list; H4 treated. | 4.8 m lengths | 2 | pcs |
| Bearers — 190 × 45 MGP10 H438.4 lm. 4 bearers in total — see the cut list; each bearer is 2 sticks nailed or screwed together; H4 treated; H4 isn't confirmed for this section in our supplier data, so check with your merchant. | 4.8 m lengths (2 per bearer) | 8 | pcs |
| Posts — 90 × 90 H4 post2.4 lm. 12 posts in total — see the cut list; cut posts to height on site; H4 treated. | 2.4 m lengths | 1 | pcs |
| Concrete mix12 pad footings: 10 × 300 × 300 mm, 450 mm deep; 2 × 400 × 400 mm, 600 mm deep. 0.60 m³ in total. At this volume, ready-mixed concrete delivered to site may be easier than bags. | 20 kg bags | 63 | bags |
| Post stirrups / post anchorsOne per post, set in the footing concrete so the post stands clear of the ground. | Galvanised, to suit 90 × 90 posts | 12 | pcs |
| Fascia board3 exposed edges: 3600 + 4800 + 3600 mm. Buy in lengths to suit. | Matching composite fascia — Charcoal, ≈ 113 mm deep | 12 | lm |
| Weed mat / geotextileLow decks are hard to weed under once built. | Laid under the deck, includes 10% for overlaps | 20 | m² |
The boards on top
| Item | Figure |
|---|---|
| Board rows | 26 |
| Boards on the materials list | 26 pieces |
How this is worked out
Each row of Composite 137 × 23 (any brand) covers its width plus the gap beside it: 137 + 6 = 143 mm. The engine lays rows across the deck. Composite boards come in one length, so a deck longer than one board gets breaker boards (boards laid across the others) and the rest run in one piece. It cuts each row from the stock lengths that leave the least offcut, and counts a clip (or a pair of screws) at every place a board crosses a joist.
The board allows joists at up to 400 mm centres when laid straight; the engine spaces them evenly at or under that (396 mm here), then sizes the bearers, posts and footings from its span tables for a deck 450 mm above the ground.
The figures come from the same engine as the free deck designer, run on the server for a rectangular deck against the house. The span tables behind its framing have been checked by a structural engineer, but every framed figure is still a concept figure: concept plan, not for construction. Have a builder, building surveyor or certifier check any design before you build it.
Cutting and waste
Cut plan
| Material | Stock bought (m) | Cut (m) | Waste |
|---|---|---|---|
| Composite 137 × 23 — Charcoal | 140.4 | 124.8 | 11% |
| 90 × 45 MGP10 H4 — joists | 46.8 | 45.6 | 3% |
| 90 × 45 MGP10 H4 — rim joists | 9.6 | 9.6 | 0% |
| 2 / 190 × 45 MGP10 H4 (double) — bearers | 19.2 | 19.2 | 0% |
| 90 × 90 H4 post — posts | 2.4 | 1.8 | 27% |
The engine’s checks (3)
Checks
| Check | Detail |
|---|---|
| Extra rot protection near the ground | 19 frame pieces sit 400 mm or less above the ground. Wood this close to the ground gets extra rot protection (H4 treatment) instead of H3. Posts are always H4. Nothing for you to do. |
| Last row of boards ripped to 25 mm | The last row of boards has to be ripped (cut down lengthwise) to 25 mm wide — a full board is 137 mm. To finish on a full board, make the deck depth (out from the house) 31 mm smaller (3569 mm) or 112 mm larger (3712 mm). Strips narrower than about 69 mm are hard to fix down and can split or cup. |
| Council approval — VIC | In Victoria every deck attached to the house needs a building permit from a building surveyor, whatever its size. A planning permit from the council is usually not needed when the floor is 800 mm or less above the ground, unless a heritage or other overlay applies. Your deck is 17.3 m² and 450 mm high — confirm the requirements with your council before you build. |