Offcut WorksDecking board gap calculator
Find the gap between boards that lets whole boards fill the width of your deck, so you avoid a thin ripped board at the edge.
Why this matters
Rows of boards rarely finish exactly at the edge of the deck. If you keep one fixed gap, the last row usually ends up as a ripped strip, and a thin strip is hard to fix and looks poor. Small changes to the gap can make whole boards fit exactly. A change of a millimetre or so between boards is hard to see, but it adds up across 20 or more boards.
How to use it
- Measure the width the boards have to cover, at both ends and in the middle. Use the smallest measurement.
- Enter the board width you are buying, and any gap you want to leave at the edges, such as against a wall.
- Enter the smallest and largest gaps your board maker allows. Check the installation guide, because the allowed range depends on the product and on the temperature when you fit.
- Read the best option, then check the alternatives. If none fits, change the edge gaps or plan to rip the last board evenly.
Hidden clips fix the gap
If you are using hidden clips, the clip decides the gap, so you cannot vary it freely. One retailer describes a T-clip that gives a 6 mm gap and an S-type clip that gives about 3 mm. In that case use this page to choose between clips, to adjust the edge gaps, or to plan how wide the ripped board will be.
Gaps depend on temperature
Boards expand and contract. The guides we checked give different advice: one gives 5 to 7 mm depending on temperature, another 4 to 5 mm at 16 °C and 6 to 8 mm in cold weather. Fit in cold weather and you need a slightly larger gap, because the boards will expand when it warms up. Check your guide for the figure that applies to your boards.
If you have to rip a board
A common approach is to rip the first and last boards to the same width so both edges look deliberate. Avoid very narrow strips. The calculator warns you if the ripped board would be under 40 mm, which is our own rule of thumb rather than a standard.
Questions
What gap should I leave between decking boards?
Follow the guide for your boards. The guides we checked ranged from about 3 mm to 8 mm, depending on the clip, the board and the temperature when fitting.
What if no option fits within my gap range?
Change the edge gaps, choose a different board width, or plan to rip the last board. You can also widen the allowed range if your board maker permits it.
Does this work for diagonal decking?
The idea is the same, but rows are measured across the diagonal. For boards laid at 45 degrees, use the diagonal decking calculator for quantities.
Do I need to leave a gap against walls?
Usually yes, to allow for expansion. One guide we checked recommends 5 mm against a solid structure. Enter the total of both sides in the edge gaps box.
Where the defaults come from
- HD Deck Pro fitting guide (PDF). Joists no more than 300 mm apart, a clip and screw on every joist, and a 5 to 7 mm gap that depends on temperature.
- SAiGE composite decking installation guide. Maximum 400 mm between joist centres, and a 4 to 5 mm gap at 16 °C (6 to 8 mm in cold weather).
- Deckorum, composite decking clips. T-clips giving a 6 mm gap, 3 mm S-type clips, and a table of clips per square metre.
- eDecks composite decking installation guide (PDF). Joists at 300 mm centres, hidden clips, and a 5 mm gap against solid structures.
- The 3 mm and 8 mm default limits come from the range across the guides we checked. They are not a standard, so enter the limits your board maker allows.
- The 40 mm warning for a thin ripped board is our own rule of thumb.