How to build a deck

Before anything else

Three things get decided before a shovel comes out, and changing any of them later is expensive.

Size, from the furniture rather than from a round number — the deck size guide works through how to lay that out. Material, because it determines joist spacing as well as cost; most composite requires 16 inches on centre and 12 on the diagonal. And attachment: ledger to the house, or freestanding, which some wall types decide for you.

Then the permit. Submit a site plan showing property lines and setbacks, and a framing plan showing joist and beam sizes, spacing, footing locations and stair details. The permit guide lists what most departments ask for.

Layout and footings

Set batter boards and string lines to mark the deck's outline, then square it by measuring the diagonals — they are equal when the rectangle is true. Mark each footing centre from the string lines, not from the house, because the house is rarely as square as it looks.

Dig to below the frost line for your area, which your building department publishes. Set tube forms, check they are plumb and at the right height, and add gravel at the base for drainage. Work out concrete volume with the footing calculator.

Then stop. The footing inspection happens before any concrete goes in, and it is the one inspection that cannot be done after the fact. Pour only after it passes, and set anchor bolts or post bases into the wet concrete rather than drilling later.

Ledger, beam and posts

If the deck is attached, the ledger goes on next. Cut the siding back, install the flashing with the house wrap lapping over its top flange, and fasten at the spacing the DCA 6 table gives for your joist span. This is the connection that matters most on the whole deck.

Set posts on their standoff bases, brace them plumb, and mark the beam height from the ledger with a level and a long straightedge — allowing for the slope you want, typically an eighth of an inch per foot away from the house. Cut the posts, fit the post caps and set the beam on top rather than bolting it to the sides.

Framing

1. Hang the two end joists first and set the rim joist at the outer edge, checking the frame for square by the diagonals again.
2. Mark joist positions on the ledger and the rim together so they cannot drift apart, at 16 inches on centre for most decking.
3. Fit the joists crown-up — sight along each board and put the slight bow upward, so the deck flattens under load instead of dishing.
4. Fill every nail hole in every hanger with the hanger nails the manufacturer specifies. Deck screws have no rated shear capacity.
5. Add blocking, one row per 8 ft of span, staggered so you can nail through the joists from alternate sides.
6. Run any lighting or electrical cable now. Retrofitting it under finished decking is the expensive part of any deck lighting job.

Framing inspection happens here — after the frame and ledger are complete, before the decking covers any of it.

Decking

Dry-lay the first three boards and measure the remaining width before fastening anything. Boards vary across a bundle, and the error compounds; catching it at board three costs nothing, catching it at board twenty costs a rip down the last one.

Use a consistent spacer rather than eyeballing the gap — wet pressure-treated lumber goes down at about a sixteenth of an inch because it shrinks, dry lumber and composite at three sixteenths, PVC at a quarter. The board gap calculator gives the right figure for your material and works out the last board's width.

Run boards long and cut the ends flush in one pass with a circular saw against a straightedge. It is faster than measuring each one and the line comes out perfectly straight.

Railing and stairs

Railing posts are the part that has to resist a real load — 200 pounds applied in any direction at the top rail. Bolt them through the rim joist with blocking behind, or use a rated post mount. A post screwed through the decking alone will look solid and fail the test.

Balusters go at a spacing that stops a 4 inch sphere passing through; the baluster calculator works out the count and the even spacing for each bay.

For the stairs, measure total rise from the finished deck surface to the finished landing, divide into equal risers under 7 3/4 inches, and remember to drop the stringer by one tread thickness before cutting or the bottom step comes out tall. Cut stringers are normally 2x12, because a 2x10 does not leave the required throat. The stair code guide covers the rest.

Final inspection comes once railing, stairs and guards are complete.