What Gap Should Be Between Deck Boards?
Gap by material
| Decking | Side gap | End gap | Note |
|---|---|---|---|
| Kiln-dried or air-dried lumber | 1/4 in | 1/8 in | Stable — the gap you lay is the gap you keep |
| Wet ("green") pressure-treated | tight to 1/8 in | tight | Shrinks ~1/4 in in width as it dries — the gap makes itself |
| Composite / PVC | 3/16–1/4 in | per chart | Moves with temperature, not moisture; end gaps grow with board length |
| Hardwood (ipe etc.) | 3/32–1/4 in | 1/16–1/8 in | Follow the supplier — dense species move differently |
What the gap is for
- Drainage. Water has to leave the surface between the boards. A tight deck holds puddles, and standing water is what rots wood and stains composite.
- Debris. Leaves and grit need somewhere to fall through, or they sit in the joints holding moisture against the board edges — the first place a deck board fails.
- Movement. Wood swells and shrinks with moisture; composite expands and contracts with temperature. The gap is the room that movement needs.
- Airflow. The underside of the boards needs to dry too. Gaps are part of how a deck ventilates itself.
The wet lumber exception, and why people get it backwards
Most pressure-treated decking arrives saturated from the treatment plant — noticeably heavy, cool to the touch. As it dries over its first months it shrinks across the grain by roughly a quarter inch on a 5.5 in board. That has two consequences:
- Lay wet boards nearly tight, and the drying shrinkage opens a normal gap on its own.
- Lay wet boards with a 1/4 in gap, and a season later you have half-inch gaps that swallow chair legs and phone screens.
Boards that have sat in a dry warehouse or your garage for months are already down to a stable moisture content — treat those as dried lumber and gap them at 1/4 in. The test is weight and feel, not the label.
Keeping the gaps consistent
Uneven gaps are what make a deck look amateur, and they compound — being 1/16 in off per row is over an inch by row twenty. Practical fixes:
- Use spacers, not eyeballing: deck spacer tools, a stack of washers, or 16d nails all give a repeatable gap.
- Measure to the far edge every few rows and check the remaining distance divides evenly by board-plus-gap. Adjust by a hair per row long before the last board — a wedge-shaped final board against the house is the visible version of this mistake.
- Plan the last board. The deck calculator's row count tells you whether the final row is a full board or a rip; if it is a sliver, split the difference between the first and last rows.
Butt joints and picture frames
End-to-end joints on long decks need their own gap on composite — manufacturers publish end-gap charts by board length and installation temperature, and ignoring them buckles boards in summer. Every butt joint must land on a joist, ideally doubled so each board end has full bearing. A picture-frame border hides all the end grain and is the tidiest way to finish composite edges.
Enter the deck size and your board width to get the board count, the linear footage to order, the joists underneath and the screws to fix it all down — with the gap between boards included, which is what most quick estimates forget.
Frequently asked questions
What gap do I leave between pressure-treated deck boards?
If the boards are still wet from treatment — heavy and cool to the touch — lay them nearly tight, because they shrink about a quarter inch in width as they dry. If they have dried in storage, gap them at a quarter inch like any dried lumber.
Why are the gaps in my deck too wide now?
The boards were probably gapped a quarter inch while still wet, then shrank as they dried, roughly doubling the gap. The fix at build time is to lay wet boards tight; after the fact, the only real fix is relaying the field.
Do composite deck boards need a gap?
Yes — typically 3/16 to 1/4 inch at the sides and a temperature-dependent gap at the ends, from the manufacturer chart. Composite moves with heat rather than moisture, so end gaps matter more the longer the board and the colder the install day.
How do I keep deck board gaps even?
Use physical spacers rather than judging by eye, and check the remaining distance to the wall every few rows so you can adjust gradually. Plan the final row before you start — a wedge-shaped last board is the classic sign nobody measured ahead.