Gap between boards
| Decking | Typical gap at installation |
|---|---|
| Kiln-dried wood | About 1/8 to 3/16 in (3 to 5 mm) |
| Wet pressure-treated pine | Tight or nearly tight; gaps open as it dries |
| Hardwoods | Often 3/16 to 1/4 in (5 to 6 mm) |
| Composite and PVC | Follow the manufacturer; often set by hidden clips |
Gaps let rain drain and debris fall through, and give the wood room to swell in wet weather. Without them, water sits on the surface and boards can cup or buckle.
End gaps and butt joints
Where two boards meet end to end, the joint must land on a joist. Many builders add a second joist or blocking at butt joints so each board end has full support. Composite boards expand lengthwise with temperature, so manufacturers specify an end gap that depends on the temperature when you install.
Joist spacing
Joists at 16 in (about 400 to 450 mm) centers are standard for most wood decking laid at right angles to the joists. Some thinner boards and many composites call for 12 in spacing, especially on diagonal layouts, where the board spans a longer distance between joists. Check the decking manufacturer's maximum span before you frame.
Fasteners
The usual pattern is two screws at every joist, set about 3/4 in from each edge of the board. Use screws rated for outdoor use with your decking and for treated lumber, such as coated or stainless steel. Hidden fastener systems clip into grooved board edges and set the gap automatically.
Board count example
A 20 × 14 ft deck with 5.5 in boards and 3/16 in gaps: 168 in ÷ 5.6875 in = 29.5, so 30 rows. With boards running the 20 ft length and 20 ft boards, that is 30 boards, or 33 with a waste allowance. On joists at 16 in centers there are 16 joists, so about 960 screws. Try your own deck in the deck calculator.
Structure is a separate job
Deck boards are the easy part. Joist sizes, beam spans, ledger attachment, footings and railings must meet local building code. Many areas require a permit for decks above a certain height or attached to a house.
Frequently asked questions
How much gap should be between deck boards?
About 1/8 to 3/16 in for dry wood decking. Wet treated pine is often laid tight, and composite boards follow the manufacturer's spacing.
Can deck joists be 24 inches apart?
Some thick decking products allow 24 in spans, but most wood and composite boards are rated for 16 in or less. Check the product rating.
How many screws do I need for a deck?
Two per joist for every board. For a typical 12 × 12 ft deck with joists at 16 in, that is roughly 520 screws.
Do the math for your own project.
Open the deck calculator