Deck boards· 3 min read
Deck Board Joints: a 5 × 4 m Deck from 4 m Boards
When the deck is longer than a board: where to put the joints, how to stagger them, how many boards to buy and how to cut them. A worked 5 × 4 m deck example.
The deck is 5 m long, but the yard only has 4 m boards. So every row gets a joint. Where should it go, how do you keep the joints of neighbouring rows from lining up, and how many boards do you buy? Let’s work through an example.
The brief
- Deck length along the boards 5,000 mm, width across them 4,000 mm (about 16 × 13 ft).
- Boards 145 mm wide, 5 mm gap, sold only in 4,000 mm lengths.
- Joists at 400 mm centre to centre, 2 screws where each board crosses a joist.
- 3 mm saw kerf, staggered joints on.
Where the joints go
Joists stand at 0, 400, 800 … 4,800 and 5,000 mm — 14 in total. The last bay is only 200 mm, because 5,000 is not a multiple of 400. A board joint must land on a joist centre line, or the ends would hang in the air.
- Rows 1, 3, 5 … (14 rows): a full 4,000 mm board + a 1,000 mm piece, joint on the joist at 4,000 mm.
- Rows 2, 4, 6 … (13 rows): a 3,600 mm piece + a 1,400 mm piece, joint one joist earlier, at 3,600 mm.
Joints in neighbouring rows are one joist bay apart and never meet. There are 27 rows — the last one has to be ripped to 100 mm (how to avoid that is covered in our article on board width and gap).
Cutting plan
| 4,000 mm boards | Cut into | Offcut from each |
|---|---|---|
| 14 | 4,000 mm (whole) | 0 |
| 13 | 3,600 mm | 397 mm |
| 6 | 1,400 + 1,400 + 1,000 mm | 191 mm |
| 1 | 1,400 + 1,000 + 1,000 mm | 591 mm |
| 2 | 1,000 + 1,000 + 1,000 mm | 991 mm |
36 boards in total, 6.3 % waste. The offcuts already allow for the saw kerf — that is what you will really have left.
Without staggering, and with longer boards
Without staggering you would also buy 36 boards (27 whole and 9 cut into three 1,000 mm pieces), but every joint would sit in one line on the joist at 4,000 mm. Manufacturers recommend staggering joints for looks alone — a long seam across the whole deck catches the eye.
If the yard has 5,000 mm boards, 27 of them do the job with no joints and no waste. You also save screws: 756 instead of 810. On a joint joist both board ends are screwed down, so every joint adds one crossing.
How to support a joint
- Board ends at a joint must rest firmly on a joist. A single joist leaves little room for two ends, so some clip manufacturers require a double joist under every butt joint.
- In this plan the joints fall on just two lines — 3,600 and 4,000 mm — so only those two joists need doubling.
- Take the end gap at a joint from the board manufacturer’s instructions; the material moves with temperature and moisture.
- Square-cut the ends even on new boards — factory ends are often damaged or out of square.
Laying sequence
- 1Set the joists to the plan and add a second joist under the joint lines if the manufacturer requires it.
- 2Sort the boards into stacks by the cutting plan, so you don’t cut a 1,400 mm piece from a board meant for 3,600 mm.
- 3Lay row by row: odd rows start with a full board, even rows with a 3,600 mm piece.
- 4Keep the gap with spacers of the same thickness along the whole row.
The calculator works out materials, not structure. Joist spacing and size, joint support, gaps and fixings must follow the board manufacturer’s installation guide and local building rules.