Decking Materials List UK: Boards, Joists and Screws
A 4 × 3 m deck in 144 mm boards with 5 mm gaps needs 31 boards of 3.0 m (10% waste), 11 joists at 400 mm centres, and about 462 screws. The chart below covers other sizes.
The materials list
| What to buy | How much | How we worked it out |
|---|---|---|
| Decking boards, 144 mm wide | 31 boards of 3.0 m | Decking calculator: 21 rows of 4 m with 10% waste. 3.0 m boards mean a butt joint in every row, which must fall over a joist |
| Joists | 11 joists of 3 m | One every 400 mm along the 4 m length, plus one at the end. Check the span tables for your timber: a 3 m span usually needs either deep joists or a support line in the middle |
| Decking screws | About 462 | Two per board at each joist. Use stainless or coated decking screws, 60 mm long for 28 mm boards |
| Posts or bases | Depends on the design | A low deck often sits on pads or concrete blocks; a higher one needs posts set in concrete. Both depend on the ground and the height |
| Weed-control membrane and gravel | 13 m² | Under the deck, so weeds do not grow through, and the ground drains |
| Decking oil or stain | 1 tin | Treat the boards after fitting, then once a year, following the maker's instructions |
Materials for other deck sizes
144 mm boards with 5 mm gaps, joists at 400 mm centres, 10% waste, buying the shortest board that is long enough:
| Deck size | Area | Boards | Joists | Screws |
|---|---|---|---|---|
| 2.4 × 2.4 m | 5.8 m² | 19 × 2.4 m | 7 | 238 |
| 3 × 2.4 m | 7.2 m² | 19 × 3.0 m | 9 | 306 |
| 3 × 3 m | 9.0 m² | 24 × 3.0 m | 9 | 378 |
| 3.6 × 3 m | 10.8 m² | 24 × 3.6 m | 10 | 420 |
| 4 × 3 m | 12.0 m² | 24 × 4.8 m | 11 | 462 |
| 4.8 × 3.6 m | 17.3 m² | 28 × 4.8 m | 13 | 650 |
Boards at least as long as the deck mean no butt joints, but long boards cost more per metre and leave a short offcut. The main example uses shorter 3.0 m boards with a joint in every row, which takes 31 boards for the 4 × 3 m deck, against 24 boards of 4.8 m.
Before you build
- Check the rules. Decking more than 300 mm above the ground generally needs planning permission, and so do decks in conservation areas or on listed buildings.
- Use the right timber. Use UC4 pressure-treated timber for joists and posts that touch the ground, and treat all cut ends.
- Leave gaps. A 5 mm gap between boards, and about 10 mm at the house and at end joints, lets water drain and the boards move.
- Ventilate underneath. A closed-in deck stays damp and rots faster.
- Check for buried services before you dig. The free LSBUD (LinesearchbeforeUdig) service shows the gas, water, electricity and telecoms assets near your address.
Mistakes that cost people money
- Undersized joists for the span. The deck will bounce, and may sag.
- Butt joints that do not fall over a joist, which leaves the board ends unsupported.
- No fall or drainage, so that water pools on the boards.
- Using ordinary screws, which rust and stain the boards.
Frequently asked questions
How many decking boards do I need for a 4m x 3m deck?
31 boards of 3.0 m, using 144 mm boards with 5 mm gaps and 10% waste. That is 21 rows across the 3 m width, each 4 m long.
How far apart should decking joists be?
400 mm centre to centre is standard for most decking, and it gives 11 joists along a 4 m deck. Some thicker boards allow wider spacing, and diagonal boards need closer joists. Check the maker's instructions.
Do I need planning permission for decking?
Usually not, if the deck is no more than 300 mm above the ground. A higher deck, one in a conservation area, or one that takes the outbuildings and extensions past the limit on garden coverage may need it. Check with your council.
How many screws do I need for decking?
Two for each board at every joist it crosses. A 4 × 3 m deck uses about 462, so buy a couple of boxes.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.