Common Mistakes Building Decking (UK): Boards, Joists, Gaps
A deck is a frame plus a surface, and most mistakes are in the order. A 4 × 3 m deck with 144 mm boards, 5 mm gaps and 3 m lengths takes 31 boards in 21 rows, with 11 joists at 400 mm and 462 screws. The decking calculator sizes the surface, and the decking materials calculator the whole job.
Mistake 1: forgetting that board width changes the count
Narrower boards mean more rows, more boards and many more screws:
| Board width | Rows | Boards | Screws |
|---|---|---|---|
| 144 mm | 21 | 31 | 462 |
| 120 mm | 24 | 36 | 528 |
| 95 mm | 30 | 44 | 660 |
Leave a gap of about 5 mm between boards, so that water drains and the boards can move.
Mistake 2: joists too far apart
Wide spacing leaves boards bouncy. The example by spacing:
| Joist spacing | Joists |
|---|---|
| 400 mm | 11 |
| 450 mm | 10 |
| 600 mm | 8 |
Mistake 3: choosing the board length last
Short boards mean joints, long ones mean cuts:
| Board length | Boards |
|---|---|
| 2.4 m | 39 |
| 3 m | 31 |
| 3.6 m | 26 |
| 4.8 m | 24 |
Mistake 4: a poor base and no air
Level the ground, lay a membrane and make a firm support. Leave a gap for air to circulate under the deck, and fall the surface gently away from the house. Check the framing with the joist calculator.
Mistake 5: wrong fixings and no care
- Use decking screws, two for each board on each joist, and pre-drill near the ends.
- Seal the cut ends of treated timber with the maker preservative.
- Oil or treat the boards as advised, and brush off leaves to keep the surface from turning slippery.
Estimates only: check the span and the planning rules.
Frequently asked questions
How many deck boards do I need?
A 4 × 3 m deck with 144 mm boards, 5 mm gaps and 3 m stock needs 31 boards in 21 rows, with 10% extra.
How far apart should joists be?
Typically 400 mm, and some boards allow 450 or 600 mm. Check the maker's span: 11 joists at 400 mm for the example.
Do I need a membrane?
A weed membrane under the frame slows weeds. The ground should drain, and the deck should have air under it.
Do I need planning permission?
A low deck often does not, but a raised deck usually does. Check with your local planning authority.
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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.