To build a 12x12 deck attached to your house, you need about 29 deck boards in 12-ft lengths, 10 joists, a ledger, a rim joist, a two-ply beam, 3 posts on 3 footings, around 550 deck screws and roughly 16 bags of concrete — plus joist hangers, hurricane ties and flashing. Stairs, railings and stain come on top if your deck needs them.
That’s the short answer. Below is the full shopping list, worked out with our calculators for one common layout, and the reasoning behind each line so you can adjust it for your own deck.
The deck we’re pricing
Every material list depends on a few decisions, so here are the assumptions behind the numbers:
- Size: 12 ft along the house × 12 ft out from it (144 sq ft).
- Attached to the house with a ledger, with one beam on posts near the outer edge. The joists cantilever 12” past the beam.
- Joists at 16” on center, running out from the house. Deck boards run parallel to the house, across the joists.
- Deck boards 5-1/2” wide (a standard “6-inch” board), with a 1/8” gap for treated wood or 3/16” for composite.
- Height: about 30” above the ground, which matters for stairs and railings.
Change any of these and the list changes. The calculators linked in each section let you rerun the numbers for your own deck.
Deck boards
A 12x12 deck takes 26 rows of boards across the 12 ft width. Each row is a single 12-ft board, so there are no butt seams to plan. Adding a 10% allowance for defects and mistakes gives:
| Decking | Boards to buy | Linear feet | Last row |
|---|---|---|---|
| Pressure-treated 5/4×6, 1/8” gap | 29 × 12 ft | 348 ft | ripped to about 3-3/8” |
| Composite, 3/16” gap | 29 × 12 ft | 348 ft | ripped to about 1-13/16” |
That composite last row is a sliver — awkward to cut and fasten. Before you order, try closing the gaps slightly, letting the boards overhang the rim a little more, or use the deck board spacing calculator to find a gap that fits whole boards. The 12x12 deck page shows the full cut plan and a drawing of this layout.
Why not just divide the area? A square-foot shortcut tells you to buy “158 sq ft of decking,” which you can’t take to a lumber counter. Counting rows tells you exactly how many boards of which length you need.
Framing lumber
The deck framing calculator gives this take-off for the deck above:
| Part | Quantity | Stock length | Notes |
|---|---|---|---|
| Joists | 10 | 12 ft | Cut to about 11’-10-1/2” between ledger and rim |
| Rim joist | 1 | 12 ft | Caps the outer ends of the joists |
| Ledger | 1 | 12 ft | Fastened to the house rim joist |
| Beam | 2 boards | 12 ft | Nailed together into a two-ply beam |
| Posts | 3 | to suit height | At 0, 6 and 12 ft along the beam |
Joist size. With the beam 1 ft in from the edge, each joist spans about 11 ft from the ledger to the beam. The 2021 IRC joist span table, as reproduced in Anne Arundel County’s deck guide, allows southern pine 2×8s at 16” on center up to 11’-10”, so 2×8 joists work for this layout. Other species and grades span less, and if you drop the cantilever and put the beam at the very edge, the 12 ft span is beyond a 2×8 at 16”. The joist spacing guide has the full table.
Beam size. The beam’s size comes from the IRC beam span table (R507.5), based on the joist span and the post spacing — here 6 ft between posts. Keeping posts close together keeps the beam small. Check the table your county uses before you buy.
Posts. Post size depends on height (IRC R507.4); 6×6 posts are the common choice. For a deck about 30” high, 8-ft posts leave plenty to cut down.
Building freestanding instead? Replace the ledger with a second beam near the house. The calculator then counts 2 beams, 6 posts and 6 footings instead of 1, 3 and 3. The ledger vs freestanding guide compares the two.
Hardware and fasteners
| Item | Quantity | What it’s for |
|---|---|---|
| Deck screws | about 550 | Two per board at every joist (520) plus 5% spares |
| or hidden clips | about 260–275 | One per board at each joist; edge boards are face-screwed |
| Joist hangers | 10 | Hang each joist from the ledger |
| Hurricane ties | 10 | Tie each joist down to the beam |
| Post bases | 3 | Anchor the posts to the footings |
| Post-to-beam connectors | 3 | Hold the beam on the posts |
| Ledger fasteners | roughly 10 lags or 5–6 bolts | 1/2” lag screws or through-bolts |
| Ledger flashing | 12 ft + overlap | Keeps water out of the house wall |
The two-screws-per-joist rule comes from the IRC’s decking fastener requirements (R507.7), reproduced in the Anne Arundel County guide. Composite and PVC boards follow their manufacturer’s fastening instructions instead. The deck screw calculator turns the count into boxes.
For the ledger, the 2021 IRC table in the same guide calls for 1/2” lag screws at 15” on center, or 1/2” bolts at 29” on center, when the joist span is 10’-1” to 12’. Over a 12 ft ledger that’s roughly 10 lags or 5–6 bolts, staggered top to bottom; the exact count depends on the required edge and end distances. Some jurisdictions also require lateral load connectors between the deck and the house.
Footings and concrete
The three posts don’t carry equal loads. Each one supports the deck area around it — its “tributary area”:
- Middle post: 6 ft of beam × about 6-1/2 ft of joist (half the span plus the cantilever) ≈ 39 sq ft
- Corner posts: 3 ft × 6-1/2 ft ≈ 20 sq ft each
The 2021 IRC footing table in the Anne Arundel County guide (50 psf load, 2,000 psf soil) calls for at least a 15” round footing at 40 sq ft and an 11” round footing at 20 sq ft. So the middle post gets a 15” hole and the corners can use 12” holes.
Poured 36” deep, that’s about 9.2 cu ft of concrete including 10% extra — 16 × 80 lb bags or 21 × 60 lb bags. The deck footing calculator reruns this for your depth.
Depth is the number to confirm locally. The IRC requires footings at least 12” below undisturbed ground, and below the frost line where your building department sets one — Anne Arundel County, for example, requires at least 30” for attached decks. The footings and frost depth guide explains how to read the tables.
Stairs
A deck 30” above the ground needs a short set of steps. With a 7-3/4” maximum riser, two-board treads (11-3/16”) and a 4 ft wide stair, the deck stair calculator gives:
- 4 risers of 7-1/2” and 3 treads, with a total run of 2’-9-5/8”
- 5 stringers at 12” spacing, each about 4’-5-7/8” long — cut from 8-ft boards
- 6 tread boards 4 ft long (you can often cut these from offcuts or two extra 12-ft deck boards)
Measure the rise where the bottom step will land: ground that slopes away from the house changes the riser height.
Railings
Under the IRC, a deck needs a guard where the walking surface is more than 30” above the ground below (IRC R312.1, Simpson Strong-Tie code reference). At exactly 30” this deck is at the threshold, so check where your grade really sits — and many people add a railing anyway.
If you do, the open sides are the two 12-ft sides plus the front, minus a 4 ft stair opening: 32 linear feet. With 4×4 posts no more than 6 ft apart and 2×2 balusters keeping every gap under 4”, the deck railing calculator counts about 10 posts and 62 balusters, plus top and bottom rails. Railings add stair handrail requirements too; see the railing height and baluster spacing guide.
Stain or sealer (wood decks)
Two coats on the 144 sq ft deck surface at about 200 sq ft per gallon, plus 10%, comes to roughly 1.6 gallons — buy 2 gallons. Stairs and railings add a lot of surface, so include them in the deck stain calculator. Composite and PVC decks don’t need staining.
The complete 12x12 deck material list
| Category | What to buy |
|---|---|
| Deck boards | 29 × 12 ft (5/4×6 treated or composite) |
| Joists | 10 × 2×8 × 12 ft (southern pine at 16” o.c.) |
| Rim joist | 1 × 2×8 × 12 ft |
| Ledger | 1 × 2×8 × 12 ft |
| Beam | 2 boards × 12 ft (size from IRC Table R507.5) |
| Posts | 3 × 6×6, length to suit height |
| Concrete | 16 × 80 lb bags (36” deep; adjust for frost depth) |
| Deck screws | about 550 (or ~270 hidden clips + edge screws) |
| Connectors | 10 joist hangers, 10 hurricane ties, 3 post bases, 3 post caps |
| Ledger | ~10 × 1/2” lags or 5–6 × 1/2” bolts, plus flashing |
| Stairs (30” rise) | 5 stringers from 8-ft boards, 6 tread boards × 4 ft |
| Railing (optional) | about 10 posts, 62 balusters, top and bottom rails for 32 ft |
| Stain (wood) | about 2 gallons |
What does it cost?
Prices vary a lot by region and material, so treat these as ballpark figures from US cost guides:
- Decking boards only: Ergeon’s 2026 materials ranges of $2–3 per sq ft for pressure-treated pine and $5–13 for composite work out to roughly $290–430 or $720–1,870 for 144 sq ft, before waste (Ergeon).
- All materials: HomeGuide puts new-deck materials at $14–28 per sq ft, or about $2,000–4,000 for a 12x12 deck (HomeGuide).
- Built by a contractor: Ergeon’s installed ranges of $25–40 per sq ft (treated wood) and $35–55 (composite) put a 12x12 deck at roughly $3,600–5,760 or $5,040–7,920, before railings, extra height or demolition.
For your own prices, the deck cost calculator prices the boards by length, and the deck cost breakdown covers every other line.
Before you order
- Check your local code. Joist, beam, post and footing sizes all come from span tables your county adopts, and frost depth is set locally. Ask your building department for its deck handout and whether you need a permit.
- Check what your supplier stocks. If 12-ft boards are out, the deck board calculator works out the best mix of other lengths and where the seams go.
- Buy a little extra framing. One spare joist and a few extra boards cost far less than a second trip mid-build.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
About 29 boards in 12-ft lengths with standard 5-1/2” boards: 26 rows across the deck plus a 10% allowance. The exact count depends on your board width and gap, so check the last row — with a 3/16” gap it’s a thin rip you may want to avoid by adjusting the gap.
How many joists does a 12x12 deck need?
Ten joists at 16” on center across the 12 ft width, including the two outer joists. In southern pine, 2×8s can span the roughly 11 ft from ledger to beam at that spacing under the 2021 IRC table; other species may need 2×10s or closer spacing.
How many posts and footings for a 12x12 deck?
Three posts and three footings for a deck attached to the house, spaced 6 ft apart along the beam. A freestanding 12x12 deck needs a second beam, so six posts and six footings.
How many bags of concrete for a 12x12 deck?
About 16 bags of 80 lb concrete for three footings poured 36” deep (one 15” diameter, two 12”), including 10% extra. Deeper frost lines or bigger footings need more — rerun it in the footing calculator with your local depth.
How much does it cost to build a 12x12 deck?
Using US cost guides, the boards alone run about $290–430 in pressure-treated pine or $720–1,870 in composite, all materials about $2,000–4,000, and a contractor-built deck roughly $3,600–7,900 depending on material. Railings, stairs, height and local labor rates move the total.