How to Plan Deck Framing

A deck's frame has five layers, top to bottom: decking, joists, beam, posts and footings. An attached deck also has a ledger board fixed to the house. Each layer carries the one above, so plan them in that order. Changing your mind about the decking at the end can change every other size.

This guide explains the order and the rules behind it. It isn't a structural design. The span tables for joists and beams, post sizes, footing sizes and ledger fastening all come from your local code or a prescriptive guide your building department accepts. The American Wood Council's DCA 6 is a widely used one for single-level attached wood decks.

1. Check what rules apply

Start with your building department: which edition of the code it uses, whether it accepts DCA 6, and what it needs for a permit. DCA 6 itself says it applies to single-level residential wood decks attached to the house. It doesn't apply where snow loads exceed 40 psf, and hot tubs are outside its scope. The current DCA 6 is based on the 2015 IRC. Newer codes have changed some tables (decking spans, for example, below), so the local code wins.

2. Choose the decking first

The decking board sets the maximum joist spacing. The 2021 IRC (R507.6) says joist spacing is limited by the decking, per Table R507.7. For 1¼ in thick wood decking, the 2021 table allows:

• Boards square to the joists: 16 in on centre where boards span three or more joists, 12 in where a board spans only two.
• Boards at 45°: 12 in where boards span three or more joists, 8 in where a board spans only two.

Composite and PVC boards follow the maker's instructions (R507.2). Trex, TimberTech and Fiberon all allow 16 in on centre for standard residential boards laid square, and 12 in at 45°. Trex notes that at 45° the allowed span is 4 inches less than its square-laid figure. TimberTech allows up to 24 in only for specific products rated for it. If you want a diagonal or herringbone pattern, decide now: it usually means 12 in joists and extra blocking.

3. Size the joists from their span

The joist span is the clear distance from the ledger (or a beam) to the next support. Once you know spacing and span, read the joist size from a span table. Here is DCA 6's Table 2, allowable span only, for 40 psf live load and 10 psf dead load, No. 2 grade lumber, wet service:

Two footnotes matter: DCA 6 limits joist length to 18'-0" prescriptively, and requires the ledger to be at least 2x8 nominal. The table also gives allowable cantilevers (overhangs past the beam), which are left out here because they depend on the joist span. Read them from the original.

4. Place the beam, posts and footings

The joists' outer ends sit on (or hang from) a beam, which rests on posts, which stand on footings. DCA 6 sizes the beam from the joist span and the post spacing (Tables 3A for dimension lumber and 3B for glulam), and the footings and post heights from Table 4. Longer joist spans put more load on the beam, so a shorter joist span with a second beam can let you use smaller lumber throughout. Work through a couple of layouts before you choose.

Footings are where local conditions matter most, including frost depth and soil. Ask your building department what it requires.

5. Plan the ledger connection

On an attached deck the ledger carries one end of every joist. DCA 6 gives fastener spacing for ledger attachment by joist span in its ledger table (Table 5) and has a section on joist hangers. Your local code will spell out the fastening, hangers and flashing it requires where the ledger meets the house.

6. Add the details: blocking, stairs and guards

Blocking between joists keeps them straight and gives seams somewhere to land. TimberTech, for example, calls for blocking every 4–6 ft for its composite boards. Plan double joists or blocking under every butt joint and picture-frame border. Stairs and guards have their own tables and height rules. DCA 6 covers both, and local codes often add to them.

7. Do the material take-off

With the layout fixed, count the materials. For a 16 × 12 ft deck with joists at 16 in on centre, thedeck material calculator gives 13 joists, 29 boards of 16 ft and676 screws, two at every joist, as R507.7 and DCA 6 both specify for wood decking. Its shopping list adds the ledger, rim joist and hangers. Beams, posts and footings come from the tables above.

Worked example: framing a 16 × 12 ft attached deck

The deck runs 16 ft along the house and 12 ft out. It will have standard composite boards laid square to the joists, and Southern Pine framing. The beam will sit at the outer edge, so the joists span roughly the full 12 ft from ledger to beam.

1. Decking → spacing. Standard residential composite boards laid square allow 16 in on centre (Trex, TimberTech, Fiberon). Diagonal boards would force 12 in.
2. Spacing + span → joist size. From DCA 6 Table 2, a Southern Pine 2x8 at 16 in spans 11'-10", just short of 12 ft. A 2x10 at 16 in spans 14'-0", which works. A 2x8 at 12 in spans 13'-1", which also works.
3. Compare the two options. At 16 in you need 2x10s; at 12 in you can use 2x8s but need more of them. Counted with the deck material calculator: 13 joists at 16 in versus 17 at 12 in. Price both. The 12 in option also leaves room to switch to diagonal boards.
4. Joists → beam, posts, footings. With the joist span fixed at 12 ft, look up the beam size for your post spacing in DCA 6 Table 3A, then the footing size in Table 4. Fewer posts mean a bigger beam.
5. Ledger. At least 2x8 nominal per DCA 6, fastened to the house per its ledger table and your local code.

Every value here comes from the DCA 6 tables quoted above. Your local code's tables, species and grade decide the real answer.

Draw a framing plan

Before buying anything, draw the deck from above to scale. DCA 6 has a deck framing plan section showing what to include. Mark the house wall and ledger, each joist at its spacing measured from one end, the beam line, every post, and openings for stairs. Then add the board direction and where butt joints will land. The plan is what most building departments ask for with a permit application, and it shows mistakes, such as a post landing on a buried pipe or a stair where a joist should be, while they still cost nothing to fix.

Planning mistakes to avoid

• Choosing decking last. Switching to a diagonal layout or a different board after framing can mean adding joists.
• Using old decking-span figures. The 2021 IRC split Table R507.7 into single-span and multiple-span columns. Check the edition you build under.
• Reading spans at the wrong spacing. The same 2x8 spans 13'-1" at 12 in but 9'-8" at 24 in (Southern Pine). Read the right column.
• Ignoring species and grade. The span table assumes No. 2 grade. Redwood and cedars span less than Southern Pine at the same size.
• Treating DCA 6 as universal. It covers single-level attached decks with snow loads up to 40 psf and no hot tubs. Anything else needs another route.

For the decking itself, see deck board spacing. If the deck sits under a new roof, the rafter length calculator androof pitch calculator cover the roof framing.