Common Mistakes When Building a Deck (Boards, Joists, Gaps)
A deck is a stack of small decisions that add up. A 16 × 12 ft deck with 5.5 in boards running along the 16 ft side takes 29 boards in 26 rows, and 13 joists at 16 in on center. The deck board calculator sizes the surface, and the deck materials calculator the whole frame.
Part of the project planners: Planning and building a deck and Summer projects for the house and the yard.
Mistake 1: forgetting that board width changes the count
Narrow boards mean more rows, more boards and many more screws. The 16 × 12 ft example by board width:
| Board | Rows | Boards | Screws |
|---|---|---|---|
| 2 × 4 (3.5 in) | 40 | 44 | 1040 |
| 5/4 × 6 or 2 × 6 (5.5 in) | 26 | 29 | 676 |
| 2 × 8 (7.25 in) | 20 | 22 | 520 |
Leave a gap of about 1/8 in between wood boards, or what the maker says for composites, so that water drains and the boards can move.
Mistake 2: choosing the joist spacing last
Closer joists stiffen the deck and are needed for diagonal boards, but they add joists and screws. The example by spacing:
| Joist spacing | Joists | Screws |
|---|---|---|
| 12 in | 17 | 884 |
| 16 in | 13 | 676 |
| 24 in | 9 | 468 |
Mistake 3: buying the wrong stock length
Boards that are too short leave butt joints, and ones that are too long are wasted in cuts. The same deck by board length:
| Stock length | Boards |
|---|---|
| 8 ft | 58 |
| 12 ft | 39 |
| 16 ft | 29 |
| 20 ft | 23 |
Mistake 4: weak footings, ledger and flashing
Most deck failures are at the ledger or the footings, not the boards. Fasten a ledger to the house with the right bolts, flash it properly, and set posts on footings that suit the frost depth. Check the local rules, and see the joist calculator for the framing.
Mistake 5: skipping the finish
- Seal or stain wood boards soon after the deck dries, then keep it up: the deck stain calculator gives the gallons.
- Use deck screws or fasteners rated for treated wood.
- Keep boards off the ground so air can dry them, and slope the surface slightly away from the house.
Estimates only: check spans, fasteners and the permit with the board maker and local authority.
Counting materials: 12x12 deck material list.
Frequently asked questions
How many deck boards do I need?
A 16 × 12 ft deck with 5.5 in boards, a 1/8 in gap, 16 ft stock and 10% waste needs 29 boards in 26 rows.
How far apart should deck joists be?
Often 16 in on center, and 12 in for diagonal boards or some composites. Check the board maker's span rating. At 16 in the example has 13 joists.
What gap do I leave between boards?
About 1/8 in for dry wood that will shrink, and more for wet wood or composites as the maker says. Gaps let water and debris out.
Do I need a permit for a deck?
Often yes, especially for a raised or attached deck. Check with your local authority before you build, and for the footing and ledger rules.
More guides
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.