Deck Board Spacing Requirements
Gap specifications for pressure-treated wood, cedar, and every major composite brand. Why the IRC does not publish a gap number but still enforces one.
Quick Answer: Most decks use a 1/8-inch gap between wood boards and 3/16-inch side / 1/4-inch endfor composite. Wet pressure-treated lumber is installed tight and shrinks to the correct gap. The IRC does not specify a number, but IRC R507.1 requires you to follow the manufacturer's install instructions, which do.
Why Deck Boards Need a Gap
Three problems a proper gap solves:
1. Drainage
Rain, snow-melt, and hose water need somewhere to go. A gapped deck sheds water between the boards onto the ground below. A tight deck holds water on the surface, which rots the boards and the joists underneath.
2. Wood movement
Every material expands and contracts — wood with moisture, composite with heat. Tight boards push against each other and lift, cup, or push fasteners out. A gap gives that movement room to happen without damage.
3. Debris clearance
Leaves, pine needles, and dirt need to fall through rather than pack against the boards and trap moisture. A clogged deck rots at the joint line, which is the hardest spot to inspect or repair.
Gap Requirements by Material
Side gap = between boards running the same direction. End gap = butt joint where two boards meet end-to-end.
| Material | Side Gap | End Gap | Source |
|---|---|---|---|
| Wet (green) PT lumber | Tight (butted) | 1/16" to 1/8" | Boards shrink to ~1/8" as they dry. Industry standard. |
| Kiln-dried PT (KDAT) | 1/8" | 1/8" to 1/4" | Already dry, install at target gap. Industry standard. |
| Cedar, redwood, ipe | 1/8" to 3/16" | 1/4" | Kiln-dried installation. Allows minor seasonal movement. |
| Trex (Enhance, Transcend, Select) | 3/16" | 1/4" | Trex Installation Guide, >40°F install temp |
| TimberTech AZEK (PVC) | 3/16" | 1/4" | TimberTech Installation Instructions |
| TimberTech PRO/EDGE (composite) | 3/16" | 1/4" | TimberTech Installation Instructions |
| Fiberon (Concordia, Sanctuary) | 3/16" | 1/4" | Fiberon Installation Guide |
| Hidden clips (CONCEALoc, Phantom) | Clip-set (3/16") | 1/4" (manual) | Clip manufacturer spec, not adjustable |
| Shaded/always-wet decks | 1/4" | 1/4" | Larger gap improves drying; reduces rot risk. |
Manufacturer specs apply at typical install temperatures (40 to 90 degrees F). Composite boards installed in cold weather need larger gaps because the material will expand when it warms up. Always check the install guide for your specific product and temperature.
The "1/8-Inch Convention" Explained
Most builders use a 1/8-inch gap for wood decking as a default. Where does this number come from?
Not from the IRC
The International Residential Code does not publish a specific gap dimension for deck boards. R507.7 covers decking installation but defers to manufacturer instructions.
From wood-movement math
A 5 1/2-inch-wide deck board (a 2x6 in nominal terms) will move roughly 1/16 to 1/8 inch in width as it cycles between its driest and wettest state. A 1/8-inch gap gives that movement room to happen without binding.
From the industry rule-of-thumb
Carpenters pre-1990s used a 16d nail (about 1/8 inch in diameter) as a spacing tool between boards. The shank of a nail as a spacer is still the fastest way to install a 1/8-inch gap.
From composite specs rounding
Composite manufacturers landed on 3/16 inch side and 1/4 inch end. Builders working in mixed materials round everything to 1/8 or 1/4 for simplicity.
Wet vs Dry Pressure-Treated Lumber
This is the single most common source of deck spacing confusion. The rule depends on moisture at install.
Wet (green) PT lumber
What you get when you buy standard PT at a big-box store. The boards are still saturated with treatment solution. They will shrink as they dry over 3 to 6 months.
Install: tight (butted) or 1/16 inch.
After drying, the boards will pull apart to roughly 1/8 inch naturally. That is your target gap.
KDAT (kiln-dried after treatment)
Specialty PT decking sold by brands like YellaWood Extreme, Thompsonized, and some pro lumber yards. Already dry, stable, and will not shrink significantly.
Install: 1/8 inch from day one.
Use a 16d nail or a 1/8 inch spacer tool. This is the day-one gap you want; it will not change.
How to tell wet from dry PT at the store
Pick up a board. Wet PT is noticeably heavier (water weight) and often feels damp on the surface. The end-grain will be dark from treatment solution. KDAT is significantly lighter, the end-grain is paler, and there will be a stamp or tag reading "KDAT" or "kiln dried after treatment." If you cannot tell, ask the yard. When in doubt, assume wet.
Composite Spacing Rules in Detail
Every major composite manufacturer has its own published install guide. The specs below are current for 2026 product lines; always check the guide shipped with your specific product.
Trex (Enhance, Transcend, Select)
Side gap 3/16 inch, end gap 1/4 inch above 40 degrees F. Below 40 F, increase side gap to 1/4 inch and end gap to 3/8 inch because the board will expand when it warms up. Butt joints over a double joist.
TimberTech AZEK (PVC)
Side gap 3/16 inch, end gap 1/4 inch. AZEK PVC expands and contracts more with temperature than composite, so fastener placement and double-joist butt joints are mandatory. Side gap may need to open to 1/4 inch in cold install conditions.
TimberTech PRO and EDGE (composite)
Side gap 3/16 inch, end gap 1/4 inch. Use CONCEALoc hidden clips with grooved boards for automatic 3/16 inch side spacing. Manual end gaps still required.
Fiberon (Concordia, Sanctuary, Good Life)
Side gap 3/16 inch for most lines, end gap 1/4 inch. Sanctuary line allows 1/8 inch side gap at warmer install temperatures. Read the guide for your specific series.
Deckorators (Voyage, Vault)
Mineral-based composite with less expansion than wood-based composite. Side gap 3/16 inch, end gap 3/16 inch. Smaller end gap than most brands because of the stable mineral core.
The 4-Inch Sphere Rule (and Why It Does Not Apply Here)
Do not confuse guard rules with deck surface rules
IRC R312.1.3 requires that a 4-inch sphere cannot pass through the openings in a guard (the vertical balusters on your railing). This is to prevent a toddler from falling through. The rule applies to the guard system ONLY. It does NOT apply to the gap between the deck boards on the walking surface.
Separately, IRC R311.7.5.1 covers the stair rail triangle (formed by tread, riser, and bottom rail) which uses a 6-inch sphere, and open risers on stairs use a 4-inch sphere. None of these are deck-board-gap rules. You will sometimes see inspectors misapply the 4-inch rule to deck gaps — politely cite R312.1.3 and R507.7 to clarify.
Special Case: Shaded and Always-Wet Decks
If your deck is in deep shade, under heavy tree cover, or in a consistently damp climate (PNW, Florida, Gulf Coast), standard gaps may not allow enough drying. Consider going up.
Standard-conditions deck
- • Full sun or light shade
- • Average regional humidity
- • Boards dry within a day after rain
- • Gap: 1/8 inch wood, 3/16 inch composite
Shaded or damp deck
- • Heavy tree canopy or permanent shade
- • Boards take 2+ days to dry
- • Moss or algae forms between boards
- • Gap: 1/4 inch to improve drying
Code Basis
The IRC does not publish a specific deck board gap dimension, but it does make manufacturer install instructions enforceable. Two sections matter:
IRC R507.1 (General)
Wood and wood-plastic composite decking must be installed in accordance with the manufacturer's installation instructions. This clause makes the Trex, TimberTech, and Fiberon install guides code-enforceable. Fail to follow them and you fail inspection.
IRC R507.7 (Deck boards)
Specifies deck board thickness, joist spacing, and fastening. Does not give a gap dimension but cross-references R507.1 for manufacturer instructions. Also restricts deck board species to those approved for exterior use.
Frequently Asked Questions
Why do deck boards need a gap between them?
Three reasons: drainage (water needs somewhere to go so it does not pool on the surface and rot the boards from the top), wood movement (lumber expands and contracts with moisture and temperature, so tight boards will push against each other and cup or buckle), and debris clearance (leaves, dirt, and pine needles need to fall through the gap rather than dam up on the surface and trap moisture against the wood). A deck installed with no gap traps water, rots faster, and often cups within a single season.
Does the IRC 4-inch sphere rule apply to deck board gaps?
No. This is the single biggest misconception about deck spacing. IRC R312.1.3 requires that a 4-inch sphere cannot pass through the openings on a guard (railing) system. That rule applies to the balusters of the guard, not the deck surface. There is no deck-surface gap rule for the 4-inch sphere. Stairs have a separate 4 3/8-inch rule for stair rail openings and a 6-inch rule for triangles formed by tread, riser, and rail. None of these apply to the gaps between your deck boards.
What gap should I use for wet vs dry pressure-treated lumber?
Wet (green) pressure-treated lumber is still saturated with treatment fluid when you buy it and will shrink as it dries. Install these boards tight (almost no gap) and they will open up to about 1/8 inch naturally as they dry over the first few months. Kiln-dried-after-treatment (KDAT) PT lumber is already dry and stable, so install with a 1/8-inch gap from day one. Putting a 1/8-inch gap on green lumber will result in 1/4-inch-plus gaps after it dries, which looks sloppy and lets small debris fall through your feet.
Can I install deck boards with no gap at all?
Not recommended for any material. Even perfectly dry lumber expands slightly with humidity. Composite boards expand significantly with heat. A deck installed tight will buckle, cup, or push the perimeter boards off their fasteners. The only exception is green (wet) PT lumber, which is installed nearly tight because it will shrink and create its own gap. All other materials need at least 1/8 inch.
What gap do hidden-fastener systems create?
Hidden fasteners like Camo, Cortex plugs, or the plastic clips used with grooved composite boards (Trex Universal Fastener, TimberTech CONCEALoc, Fiberon Phantom) create fixed gaps of either 3/16 inch or 1/4 inch depending on the clip. This takes the guesswork out of spacing. If your boards are grooved for hidden clips, the clip dictates the gap and you do not get to choose.
Is the end gap different from the side gap?
Yes. End gaps (butt joints between two boards running end-to-end) typically need more room than side gaps because end-grain absorbs moisture faster and expands more. Trex specifies 1/4 inch end gap vs 3/16 inch side gap. TimberTech specifies similar. For wood, leave at least 1/8 inch on sides and 1/4 inch at butt joints. Also stagger butt joints across the deck so you do not have a row of cracks across a single joist.
Is the stair tread gap different from the deck-surface gap?
The gap on stair treads follows the same rule as the deck surface: about 1/8 inch for wood, 3/16 to 1/4 inch for composite. What stairs DO have that the deck does not is the IRC R311.7.5 rule that open risers cannot pass a 4-inch sphere. That is the riser opening, not the tread gap. Two separate things that commonly get confused at inspection.
What gap should I use for composite decking?
Follow the manufacturer. Trex calls for 1/4-inch end gap and 3/16-inch side gap at installation temperatures above 40 degrees F, with larger gaps in cold weather because composite expands when it warms up. TimberTech specifies similar (1/4 inch end, 3/16 inch side). Fiberon requires 1/8 to 1/4 inch depending on line and temperature. Deviating from the manufacturer spec voids the warranty AND is a code violation under IRC R507.1, which requires decking to be installed per manufacturer instructions.
Joist Span Tables
Joist sizing that supports the deck surface.
Fastener Code
Screws, nails, and hidden-clip requirements.
Railing Height
Where the real 4-inch sphere rule applies.
Deck Electrical
Receptacle, lighting, and GFCI requirements.
References IRC Sections R507.1 and R507.7 (2021 edition), plus published install guides from Trex, TimberTech, Fiberon, and Deckorators current for 2026 product lines. Manufacturer specs change; verify the guide shipped with your boards. Local amendments may apply. Not engineering advice.