The gap between deck boards lets water drain, lets air dry the boards and the joists under them, and leaves room for the boards to swell. The right gap depends on the board: wet pressure-treated lumber shrinks after it is laid, kiln-dried wood moves a little with the seasons, and composite boards expand with heat. This guide works out how much wood boards actually move, from USDA Wood Handbook shrinkage data and the outdoor moisture levels the Handbook gives for US cities, and turns that into a gap you can set with a spacer. For board counts, use the deck board calculator.
Quick answer
- Wet pressure-treated boards (most treated lumber from the yard): lay them tight or with a gap of no more than about 1/16 in. They will shrink and open a gap of roughly 3/16 to 1/4 in as they dry.
- Kiln-dried wood (cedar, redwood, KDAT treated, hardwoods such as ipe): about 1/8 in, the thickness of a 16d nail shank or a standard deck spacer.
- Composite and PVC: use the side and end gaps in the maker’s installation instructions. They depend on temperature at installation and board length, and getting them wrong can void the warranty.
- Against the house: keep decking and framing off the siding and follow the ledger flashing details in your code and the ledger fastener maker’s instructions.
How much a 5-1/2 in deck board shrinks
| Wood | Tangential shrinkage, green to ovendry | Wet (30%) to 15% | Kiln-dried (19%) to 15% | Season to season (15% to 12%) |
|---|---|---|---|---|
| Southern pine (pressure-treated) | 7.4% | 0.204 in (7/32) | 0.054 in (1/16) | 0.041 in |
| Douglas-fir | 7.6% | 0.209 in (7/32) | 0.056 in (1/16) | 0.042 in |
| Western redcedar | 5.0% | 0.138 in (1/8) | 0.037 in (1/32) | 0.028 in |
| Redwood (young-growth) | 4.9% | 0.135 in (1/8) | 0.036 in (1/32) | 0.027 in |
| Ipe | 8.0% | 0.220 in (7/32) | 0.059 in (1/16) | 0.044 in |
The table explains the usual advice. A treated southern pine board that is still saturated when it is installed loses about 0.23 in of width by the time it reaches a typical outdoor moisture content of 13%. Laid tight, that opens a gap of about 7/32 in, a little over the 1/8 in most people aim for. Laid with a 1/8 in gap, the same boards end up with gaps of about 11/32 in, wide enough to catch heels and drop small items. Kiln-dried boards start close to their service moisture, so they move only a few hundredths of an inch and need the gap from the start.
Seasonal movement where you live
| City | Average outdoor EMC | Monthly range | Seasonal width change, southern pine |
|---|---|---|---|
| Phoenix, AZ | 7.2% | 4.6 to 9.5% | 0.066 in |
| Denver, CO | 10.1% | 9.4 to 11.0% | 0.022 in |
| Boston, MA | 12.2% | 11.6 to 13.1% | 0.020 in |
| Chicago, IL | 13.3% | 12.2 to 14.9% | 0.037 in |
| Atlanta, GA | 13.0% | 11.8 to 14.2% | 0.033 in |
| New Orleans, LA | 14.5% | 14.0 to 15.3% | 0.018 in |
| Los Angeles, CA | 13.7% | 12.1 to 15.1% | 0.041 in |
Outdoors, boards cycle by only a few points of moisture content through the year, so seasonal movement is small, typically a few hundredths of an inch per board. Arid climates are the exception: in Phoenix the average outdoor EMC is around 7%, so boards from a humid mill keep shrinking for months after installation. Rain and sun cause bigger short-term swings at the surface than these averages suggest, which is one reason the top face of a board cups and checks.
Gaps for railings and stairs
- Guard balusters. On decks more than 30 in above the ground below, the IRC requires guards (R312.1.1), and openings in them must not let a 4 in sphere pass (R312.1.3). With 1-1/2 in square balusters, that means a clear gap of under 4 in, so balusters at about 5-1/4 in on centre or closer.
- Open stair risers. Where stairs have open risers, the opening must not let a 4 in sphere through (R311.7.5.1), which usually means adding a riser board or a rail.
- Stair treads. Use the same gap as the deck boards; treads made of two boards drain better with the gap at the back edge kept clear.
Joist spacing for deck boards
The board also sets how far apart the joists can be. The IRC deck provisions (R507.7) limit nominal 5/4 (1 in actual) wood decking to joists 16 in on centre when boards run square to the joists and 12 in when they run diagonally; 2x wood decking can span 24 in square to the joists and 16 in diagonally. Composite and plastic boards use the spacing on their label and evaluation report, often 16 in, or 12 in for diagonal patterns and stair treads. For joist sizes and spans, see the deck joist span calculator.
Practical tips
- Use a spacer, not your eye: a 16d common nail is 0.162 in (just over 5/32 in), an 8d is 0.131 in (about 1/8 in). See our nail size chart.
- Check treated lumber: if it feels cool and heavy and water beads out under a screw, it is wet. Lay it tight, or stack it with spacers for a few weeks and then lay it with a small gap.
- Install boards bark side up or down as the maker recommends; the Handbook shows that flatsawn boards cup away from the heart as they dry, so the choice changes which way water sheds.
- Use stainless or hot-dipped galvanized fasteners and hardware rated for the treatment chemical; see the corrosion notes on our nail size chart.
Frequently asked questions
What is the standard gap between deck boards?
About 1/8 in for kiln-dried wood. Wet pressure-treated boards are usually laid tight because they shrink about 3/16 to 1/4 in as they dry. Composite boards use the maker’s gap.
Should pressure-treated deck boards be butted tight?
If they are wet when installed, yes, or close to it. Our Wood Handbook calculation gives about 0.23 in of shrinkage for a 5-1/2 in southern pine board drying from saturated to 13% moisture content.
Is there a code for deck board spacing?
The IRC does not set a gap between deck boards. It does limit joist spacing for each decking type (R507.7) and the 4 in sphere rule for guard and open-riser openings (R312.1.3, R311.7.5.1).
What gap for composite decking?
Follow the installation guide for your product: side gaps, end gaps and the allowance for installation temperature vary by maker and board length.
Sources
- USDA Forest Products Laboratory, Wood Handbook FPL-GTR-190 (2010): Eq. 4-7 (shrinkage below fibre saturation), Tables 4-3 and 4-4 (tangential shrinkage), Table 13-1 (outdoor equilibrium moisture content of US cities).
- International Code Council, 2021 International Residential Code (IRC). Requirements are paraphrased with their section numbers; local codes and amendments may differ, so confirm with your building department. Sections R507.7 (decking), R312.1.1 and R312.1.3 (guards), R311.7.5.1 (open risers).
- Our nail size chart for spacer diameters (Wood Handbook Table 8-1).
Last reviewed: 24 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.