Building a 4 × 3 m deck: boards, joists and the span question
One ordinary garden deck, costed properly. The boards are simple. What decides whether the deck feels solid underfoot or bounces is the joist size, and that decision gets made before you buy anything.
The deck
- 4.0 m long × 3.0 m wide, ground level, freestanding
- Boards running across the 3 m dimension
- 144 mm softwood decking boards with a 5 mm drainage gap
- 3.6 m board lengths
- 47 × 150 mm treated joists at 400 mm centres
- Two intermediate bearers, so the joists span 1 m at a time
Step 1 — the boards
Deck area is 12.0 m². Each board occupies its own width plus the gap: 144 + 5 = 149 mm of pitch. Across a 3 m run that is 21 rows.
21 rows at 4.0 m long is 84 linear metres, and with 10% for end cuts, 92.4 m. In 3.6 m lengths that is 26 boards.
Note what the 5 mm gap does. It looks like a detail and it is worth most of a board across this deck — twenty-one gaps at 5 mm is 105 mm, which is close to a whole extra row. Decks get built with no gap by people who find out in August why it matters, when the boards have swollen and there is nowhere for them to go.
Step 2 — the span question, which comes first
The calculator checks the joist against its span and says this:
Span is within limits. Your joists span 1.00 m against an indicative maximum of 2.50 m for 47 × 150 mm at 400 mm centres, with 2 intermediate bearers.
That is a comfortable margin, and it comes from the bearers rather than the timber. Take the two intermediate bearers away and the same joists would have to span the full 3 m — past the 2.5 m limit, and the deck would feel springy in the middle long before anything actually failed.
The three ways out of that are not equal:
- Add bearers. Cheapest by a distance. Two lengths of timber and some more blocks.
- Deeper joists. 47 × 200 mm spans 3.2 m at 400 mm centres and would do it unsupported — but you are buying more timber for every joist, not two extra pieces.
- Closer centres. The weakest lever. Going from 400 to 300 mm centres buys less span than people expect, and costs a quarter more joists.
Bearers first, then depth, then spacing. In that order, almost always.
Step 3 — the frame
At 400 mm centres across 4 m, that is 11 joists at 3.00 m, or 38.7 m of joist timber once 5% waste is allowed. Plus 10 noggins totalling about 4.0 m, staggered between the joists to stop them twisting.
Noggins are the item people leave out. A 150 mm deep joist that is fixed at both ends and nowhere else will rotate under load, and the symptom is a deck that creaks in one particular spot forever.
Step 4 — fixings
Screw count follows crossings, not area. Every board crosses every joist it passes over, and each crossing takes two screws. Across this deck that is 231 crossings, so 462 screws, and with 10% for the ones that snap or strip: 509 screws, or 3 boxes of 200.
Two per crossing is not negotiable. One screw per joist lets the board cup, and a cupped deck board holds water in the middle of its face — which is exactly where you do not want it sitting.
Step 5 — the full order
| Item | Qty | Why this much |
|---|---|---|
| Decking boards, 144 mm × 3.6 m | 26 | 92.4 linear m including 10% end cuts |
| Joists, 47 × 150 mm × 3.0 m | 11 | 400 mm centres across 4 m |
| Bearers, 47 × 150 mm × 4.0 m | 2 | What brings the span down to 1 m |
| Noggin timber | 4.0 m | 10 pieces staggered between joists |
| Decking screws, 65 mm, box of 200 | 3 | 509 needed at two per crossing |
| Structural screws or coach bolts | 1 box | Joist-to-bearer. Deck screws are not a structural fixing. |
| Concrete blocks or deck pads | 12 | Under the bearers, roughly every metre |
| Weed membrane | 15 m² | The whole footprint plus an overlap |
| Sharp sand or gravel | 1 bulk bag | Levelling the pads. Never bed them on soil. |
| End-grain preservative | 1 L | Every cut end. The treatment does not reach the middle of the timber. |
| Joist tape | 1 roll | Over the joist tops. Doubles the frame's life for about £20. |
| Countersink bit and spare drivers | 1 set | 500 screws will eat drivers |
What people forget
- End-grain preservative. Pressure treatment penetrates the outside of the timber, not the core. Every cut you make exposes untreated wood, and cut ends sitting on a bearer are where rot starts. Brush every one.
- Joist tape. A strip of butyl over each joist top stops water sitting in the screw holes. It is the cheapest thing on the list and it is the difference between a frame lasting ten years and twenty.
- The fall. A deck should shed water — roughly 1 in 80 away from the house. Perfectly level sounds right and holds puddles.
- That deck screws are not structural. They fix boards to joists. Joist-to-bearer and anything carrying load needs a proper structural fixing.
- The gap at the edges too. Boards need room to move against a wall or a post, not just against each other.
- Boards crown up. Look down the length of each one before fixing and lay it with the curve rising. A board laid the other way holds water along its whole length.
What this costs in time
- Day 1 — clear, level, membrane, set pads and bearers. All the fiddly work is here.
- Day 2 — joists, noggins, joist tape.
- Day 3 — boards. Fast and satisfying once the frame is right.
The ratio is the point: two days of groundwork for one day of decking. Anyone who tells you a deck is a weekend job is describing day three.
Run your own version
- Decking calculator — change the joist and watch the span check respond
- Waste factor table — where the 10% comes from
- Fixings by substrate — for anything fixed back to a wall
- Concrete calculator — if you are setting posts rather than using pads
Last reviewed 2026-10-17. Span figures are indicative for C16 softwood on a domestic deck and size a frame for planning. They are not a structural design. A raised deck, a deck carrying a hot tub, or anything attached to the house needs proper design and may need building regulations approval.