Deck boards· 3 min read
Deck Board Width and Gap: How to Avoid Ripping the Last Board
A 3.5 m wide deck and 120, 140 or 145 mm boards: how to choose the board width and gap so the last row does not have to be ripped into a thin strip.
The last row is where deck planning shows. If it comes out as a thin strip, you have to rip a whole board lengthwise and the edge of the deck ends with an awkward sliver. On a 4 × 3.5 m (about 13 × 11.5 ft) deck we show how to avoid it by choosing the board width, the gap or the deck size.
How it works
N rows of boards have N − 1 gaps. The width covered by full boards is N × board width + (N − 1) × gap. If more than one gap is left after the last full board, the calculator adds a row and rips it to what is left.
Three boards, 5 mm gap
| Board | Rows | Last row | 4 m boards | Screws |
|---|---|---|---|---|
| 120 mm | 28 | full | 28 | 616 |
| 140 mm | 25 | rip to 20 mm | 25 | 550 |
| 145 mm | 24 | rip to 50 mm | 24 | 528 |
The 120 mm board fits exactly: 28 boards and 27 gaps make 3,495 mm, and the remaining 5 mm is just one more gap. With 140 and 145 mm boards you are left with a 20 or 50 mm strip — hard to cut and hard to fix. A narrower board does mean more rows and more screws, though (at 2 screws per crossing and joists at 400 mm).
Fix 1: adjust the gap
| Board / gap | Rows | Last row |
|---|---|---|
| 140 mm / 6 mm | 24 | full |
| 145 mm / 6 mm | 24 | rip to 27 mm |
| 145 mm / 8 mm | 23 | rip to 134 mm |
With a 140 mm board, one extra millimetre of gap gives 24 rows with no ripping. With 145 mm it does not help — at 8 mm you would get a wide 134 mm strip, but a gap that wide may be outside what the manufacturer allows. Only change the gap within their instructions.
Fix 2: adjust the deck width
If the deck is not boxed in between walls, size it to the board. With 145 mm boards and a 5 mm gap, 23 full rows cover 3,445 mm and 24 rows 3,595 mm. A deck 55 mm narrower saves one board and all the ripping.
Fix 3: split the ripping between both edges
If the size cannot change, do not leave a 50 mm strip on one edge. The last full row and the strip add up to 195 mm of board (not counting the gap between them) — split them into two edge rows of 97.5 mm. Both edges look the same and no piece is too narrow.
Step by step
- 1Measure the deck width and the actual board width.
- 2Enter the manufacturer’s gap in the calculator and look at the last row.
- 3If it comes out as a thin strip, try a nearby gap within the allowed range, another board width or another deck size.
- 4If none of that works, split the ripping between the two edge rows.
Before you buy
- Measure the actual width of a few boards — that is what counts, not the catalogue figure.
- Wood changes width with moisture; pressure-treated boards keep shrinking after installation. The gap is never exact to a tenth of a millimetre.
- For a deck against the house wall, leave the clearance at the wall from the board manufacturer’s instructions and include it in the width.
The calculator works out layout and materials. Take the board gap, joist spacing and fixing method from the board or clip manufacturer’s installation guide.