How to build a deck
A deck is built in a fixed order, and three of those steps are irreversible once the next one starts: the footings, the ledger flashing, and the frame being square. Everything else can be adjusted.
The sequence
- Layout and string lines. Establish the deck corners and the height. Check the diagonals — they must be equal within ¼″.
- Footings. Dig, inspect, pour. This is the first irreversible step.
- Ledger. Attach, flash, and cap. The second irreversible step, because the flashing goes behind the siding.
- Posts and beam. Set posts in the footing brackets, bolt the beam on top.
- Joists. Hang from the ledger, bear on the beam, add rim joists and blocking.
- Stairs. Usually built before decking so the stringers can be attached to the frame, not to the finished surface.
- Decking. Lay from the house out, or from a chalk line if the board pattern requires it.
- Railing and guards. Last, because posts are easier to set before the decking around them is finished.
Step 1: layout, and the diagonal check
Mark the two corners against the house, then pull strings to the two outer corners. Measure both diagonals: on a 12 × 16 deck, both should read 20′ 0″. A difference of ½″ is visible in the finished decking; 1″ will show as a tapering gap at the far end.
The 3-4-5 method does the same thing on a single corner: measure 3 ft along one line and 4 ft along the other, and the distance between those points must be exactly 5 ft for a right angle.
Step 2: footings, and the inspection before concrete
Hole depth is set by local frost depth, not by a rule of thumb. Diameter is usually 12″ for a 4×4 post and 10–12″ for a deck that is not carrying a roof. The bottom of the hole must be undisturbed soil — loose fill at the bottom is a footing sitting on nothing.
Most jurisdictions require the hole to be inspected before it is filled. If your jurisdiction does not require it, take a photo with a tape measure visible in the hole. It costs nothing and it is the only record you will have.
Set the post anchor in the wet concrete, plumb, and brace it. The anchor position is far easier to get right now than to correct after the concrete sets.
Step 3: the ledger, which is the detail that fails
The ledger carries half the deck. Three things have to be right:
- It bears on framing, not siding. Remove the siding where the ledger sits, or use standoffs, so the ledger contacts the band joist or the studs.
- Fasteners hit the framing. Mark the stud and joist locations on the wall before drilling, and lay out the fastener pattern to land on them.
- Flashing goes under the siding and over the ledger. The sequence is: flashing tape on the wall, Z-flashing behind the ledger and over its top edge, then a cap. Water has to be able to run over the top of the ledger and out the front.
See how to flash a ledger for the layer order, which is where most of these go wrong.
Step 4 and 5: the frame
Posts sit in the anchor and are bolted to the beam, which sits on top of them rather than bolted to their sides. Joists hang from the ledger in hangers and bear on the beam with a rim joist across the ends.
Blocking goes in at the mid-span of the joists and anywhere a guard post lands. It is what keeps the joists from rotating and ties the frame into a diaphragm.
If the deck is more than 30″ above grade, the frame needs lateral bracing — diagonal steel or wood from the corner of the frame to a post or to the house. A deck without it resists gravity and nothing else.
Step 6 and 7: stairs and decking
Cut stringers from a single template so all of them are identical, and check the bottom riser against the actual grade — the most common defect on a new deck is a first riser that differs from the rest by more than 3/8″.
Decking goes down with a gap of ⅛″ to 3/16″ for wood, and per the manufacturer for composite. Use a spacer jig; a bundle of nails or a purpose-made clip is faster and more consistent than measuring each gap.
Step 8: railing
Set posts first, then rails, then balusters. Posts must be anchored to the frame, not just to the decking, and blocking inside the rim is what the fasteners actually bite into. Check every opening with a 4″ test sphere before you call it finished.
What this guide will not size
- It does not size members. Joist, beam and footing sizes come from span tables or an engineered plan.
- It does not replace the permit drawings. A permit needs a plan, not a sequence.
- It does not cover elevated decks, which need engineered lateral bracing and often deeper footings.
- It does not cover ledger attachment to brick, stone or stucco, which usually means a freestanding deck instead.
Questions people actually ask
What order do you build a deck in?
Footings, ledger and flashing, posts and beam, joists and blocking, stairs, decking, then railing. The footings and the ledger flashing are irreversible once the next step starts.
How long does it take to build a deck?
Two people typically take 4 to 7 days for a 12 × 16 ground-level deck, plus a day for the footings and a cure period before framing. An elevated deck or one with a complex railing takes 8 to 12.
Do I need to be home for the footing inspection?
Yes, and you need to call for it before pouring concrete. Some jurisdictions require it, some do not — but photograph the depth with a tape measure regardless.
How do I make sure a deck is square?
Measure both diagonals. On a 12 × 16 deck they should both be 20 feet and match within a quarter inch. If they differ, adjust the outer corners before setting posts.