How to Maintain Proper Deck Board Spacing
The gaps between your deck boards aren't just aesthetic—they're functional. Proper spacing allows water to drain, debris to fall through, air to circulate, and boards to expand without buckling. Here's how to get it right.
Quick Summary
1/8" - 3/16" gap
Install tight (will shrink)
3/16" - 1/4" gap
Why Spacing Matters
Deck board gaps serve several critical functions that affect both the appearance and longevity of your deck:
- Water drainage – Gaps allow rain and splashes to drain through instead of pooling on the surface
- Air circulation – Airflow between and under boards helps them dry quickly, reducing rot risk
- Debris clearance – Leaves, dirt, and debris fall through rather than accumulating
- Expansion room – Wood absorbs moisture and expands; gaps prevent buckling and cupping
- Visual appeal – Consistent gaps look professional; random spacing looks sloppy
Correct Spacing by Material
Kiln-Dried Wood (Cedar, Redwood, Dry PT)
Recommended gap
1/8" - 3/16"
Spacer option
16d nail, 1/8" spacer
Kiln-dried lumber has already released most of its moisture. It will expand and contract slightly with seasonal humidity changes, but the movement is minimal. A 1/8" to 3/16" gap provides adequate drainage and expansion room.
Wet Pressure-Treated Lumber
Recommended gap
0" (touching)
After drying
Gap appears naturally
Freshly treated lumber from the lumberyard is often saturated with water from the treatment process. This lumber will shrink significantly—sometimes 1/8" to 1/4" across a 6" board. Install these boards touching (or nearly touching) and let shrinkage create the gap.
How to tell if it's wet: Fresh PT lumber feels heavy, has visible moisture on the surface, and may have wet ends. Use a moisture meter—above 19% means install tight. Below 15% means it's dry enough for normal spacing.
Composite Decking
Board-to-board gap
3/16" - 1/4"
End gap (at walls)
1/8" - 1/4"
Composite decking doesn't absorb moisture like wood, but it expands and contracts significantly with temperature changes. Hot sun can make boards grow noticeably longer and wider. Larger gaps accommodate this thermal movement.
Important: Always follow your specific manufacturer's spacing requirements. Different composite formulations behave differently. Failure to follow specs may void your warranty.
Tropical Hardwoods (Ipe, Cumaru, Tigerwood)
Recommended gap
1/8"
Movement
Minimal (dense wood)
Dense tropical hardwoods are typically kiln-dried and dimensionally stable. They move less than other woods, so standard 1/8" spacing works well. However, these boards are prone to cupping if installed without proper fastening—use enough fasteners to keep them flat.
Spacing Tools and Methods
Commercial Deck Spacers
Purpose-built spacers are the most reliable option. They're designed to stay in place while you fasten, then pop out easily.
- +Consistent thickness
- +Easy to use and remove
- +Available in multiple sizes
- -Small additional cost ($10-20)
16d Nails
The classic DIY spacer. Standard 16-penny nails are approximately 1/8" in diameter—perfect for wood decking.
- +You probably already have them
- +Consistent 1/8" spacing
- -Can fall through gaps
- -Harder to remove after fastening
Carpenter Pencils
Flat carpenter pencils are approximately 1/4" thick—good for composite decking that needs larger gaps.
- +1/4" spacing ideal for composite
- +Won't fall through gaps
- -May vary slightly in thickness
Hidden Fastener Systems
Many hidden fastener clips have built-in spacing tabs that automatically create the correct gap.
- +Automatic consistent spacing
- +No separate spacers needed
- +Spacing is exactly what clip is designed for
- -Higher material cost
Step-by-Step: Maintaining Consistent Spacing
Step 1: Choose Your Spacer
Based on your decking material, select the appropriate spacing tool. Have enough spacers to place one at every joist—don't skip joists or you'll get wavy gaps.
Step 2: Position Board Against Spacers
Place spacers against the installed board at each joist location. Position the new board against all spacers simultaneously. The board should touch every spacer along its length.
Tip: If using nails as spacers, insert them vertically so they're easy to pull out after fastening.
Step 3: Secure the Board
While holding the board firmly against the spacers, drive your first fastener near the center of the board. This anchors it in position. Then work toward the ends, keeping the board pressed against spacers as you go.
Step 4: Remove Spacers
Once the board is fully secured, remove all spacers before installing the next board. Check visually that the gap is consistent along the entire length.
Step 5: Verify Alignment Periodically
Every 5-10 boards, measure from your reference point (usually the first board or house) to check that spacing hasn't drifted. Small errors compound—better to catch them early.
If you find drift, you can make small adjustments over the next few boards rather than one large correction that would be noticeable.
Dealing with Inconsistent Board Widths
Real-world lumber isn't always consistent. Even boards from the same bundle can vary by 1/16" or more in width. Here's how to handle it:
Sort Boards by Width
Before starting, sort your boards into groups: narrow, average, and wide. Install boards of similar width together, making gradual transitions between groups.
Adjust Spacing Slightly
If a board is 1/16" narrower than average, you can add that to the gap (now 3/16" instead of 1/8") without it being noticeable. Spread variations across multiple boards rather than making one big adjustment.
Rip Wide Boards
If a board is significantly wider than others, rip it down to match. This is especially important for the last board at the deck edge, where the full width is visible.
Use the Widest Boards First
Starting with wider boards and working toward narrower ones means you're more likely to end with an acceptable last board. Running out of width at the end is better than running out of space.
Common Spacing Mistakes
Installing wet PT lumber with gaps
Wet boards shrink as they dry. If you add a gap to already-wet boards, you'll end up with excessive gaps—sometimes 1/2" or more.
Skipping spacers at some joists
Using spacers only at every other joist leads to wavy gaps that are obviously inconsistent. Use spacers at every joist.
Not checking alignment
Small spacing variations compound across the deck. A board that's 1/32" tight at each joist is 3/16" off by the time you've installed 6 boards.
Ignoring composite manufacturer specs
Composite materials vary widely. Using wrong spacing can cause buckling in summer heat or void your warranty.
Forgetting end gaps for composite
Composite expands in length too. Leave 1/8" to 1/4" at board ends where they meet walls, posts, or other fixed objects.
Temperature Considerations for Composite
Composite decking is especially sensitive to temperature during installation. The gap you create will change as the temperature changes.
| Installation Temp | Gap Adjustment | Why |
|---|---|---|
| Cold (below 40°F) | Increase gaps slightly | Boards will expand in summer heat |
| Moderate (40-80°F) | Use standard gaps | Middle ground for expansion/contraction |
| Hot (above 80°F) | Decrease gaps slightly | Boards are already expanded |
Pro Tip: If possible, install composite decking at moderate temperatures (50-70°F). This minimizes the extreme variations you'll see between winter and summer.
FAQ
What is the correct gap between deck boards?
For kiln-dried wood, use 1/8" to 3/16". For wet pressure-treated lumber, install boards tight (touching)—they'll shrink to create the gap. For composite, use 3/16" to 1/4" per manufacturer specs.
Should I install wet pressure-treated boards tight together?
Yes. Wet PT lumber (above 19% moisture) shrinks significantly as it dries. Installing tight or nearly tight ensures you end up with appropriate gaps after drying. Installing with gaps when wet results in excessive gaps later.
How do I keep deck board spacing consistent?
Use a consistent spacer at every joist: commercial spacers, 16d nails, or carpenter pencils. Check alignment every 5-10 boards by measuring from a reference point to catch drift early.
Does composite decking need larger gaps than wood?
Yes. Composite expands and contracts with temperature more than wood does with moisture. Most manufacturers specify 3/16" to 1/4" gaps between boards, plus end gaps at fixed objects. Always follow your specific manufacturer's guidelines.