What Size Lumber Do I Need for a Deck? Joists, Beams and Posts

There is no single answer, and anyone who gives you one without asking questions is guessing. Deck framing sizes come out of four variables — how far the member spans, how far apart the members sit, how much load they carry, and what species and grade you are building with. Change any one and the required size changes.

What follows is how the pieces relate to each other, so that when you look at a span table or talk to your building department, the numbers make sense rather than feeling arbitrary.

Start with the load, not the lumber

Residential decks are typically designed for a combined load of 50 pounds per square foot — 40 psf of live load, meaning people and furniture, plus 10 psf of dead load, meaning the weight of the deck itself. That number is the baseline behind every span table you will see.

It also changes in specific situations. A hot tub is a concentrated load that requires its own engineered framing, not a deck sized to the general table. Heavy snow regions design for more. A roof over the deck adds load, and a deck that carries a wall above it is a different structure entirely.

If your deck includes any of those, you are past the point where a table answers the question and into the territory where an engineer should size the members.

Joists: the piece most people ask about

Joists are the repeating horizontal members that support the decking. Their size depends on span and spacing.

Span is the clear distance between supports — from the ledger at the house to the beam, not the total deck width. This is the single most common mistake people make when reading a span table. A 16-foot-deep deck with a beam in the middle has two 8-foot joist spans, not one 16-foot span, and the difference is enormous.

Spacing is how far apart the joists sit, center to center. 16 inches on center is standard. 12 inches on center increases allowable span and stiffens the deck noticeably underfoot. 24 inches is used with some engineered products but is too loose for most solid decking.

As a rough sense of scale for typical framing species at 16-inch centers:

• 2x6 — short spans only, generally under about 9 feet
• 2x8 — commonly good to roughly 11 to 13 feet
• 2x10 — commonly good to roughly 13 to 16 feet
• 2x12 — the largest common dimension, for the longest spans

Those figures shift with species and grade, which is why the table matters more than the rule of thumb. Southern Yellow Pine and Douglas Fir carry further than Hem-Fir or SPF. A No. 2 grade board carries less than a Select Structural one.

Also worth knowing: passing the span table means the joist is strong enough, not that the deck feels solid. Decks framed right at the maximum allowable span are code-compliant and often bouncy. Going one size up, or tightening spacing, is the usual fix and costs very little relative to the whole project.

Decking thickness drives joist spacing

The decking you choose feeds back into the framing:

• 5/4 decking (about 1 inch actual) typically wants 16-inch centers
• 2x decking can span further, often 24 inches
• Hardwood decking in thinner profiles frequently requires 12 or 16-inch centers — Ipe in 1x4 or 1x6 is commonly specified at 16 inches or tighter
• Composite decking has manufacturer-specified maximum spacing that is not negotiable if you want the warranty, often 16 inches for perpendicular installation and 12 inches for diagonal

Diagonal decking patterns increase the effective span across each joist bay by roughly 1.4 times, so a diagonal layout usually requires closer joist spacing than the same deck run square.

Beams: where the joists land

The beam collects the load from all the joists and carries it to the posts. Its size depends on the beam span — the distance between posts — and the tributary width, which is how much deck area it supports.

Built-up beams from doubled or tripled 2x material are the most common residential approach: two 2x10s or three 2x8s fastened together per code-specified nailing or bolting patterns. Solid timbers and engineered beams are the alternatives when spans get longer.

The tradeoff to understand is that adding posts shortens the beam span and lets you use a smaller beam. Fewer posts means a bigger beam and more footings work per post. Neither is universally better — it depends on your site, your grade, and whether you want an open underside.

Beams also need proper bearing. A beam sitting on a post needs full bearing on the post with an appropriate connector, not toe-nailed to the side of it. This is one of the most common inspection failures.

Posts and footings

6x6 is the practical standard for deck posts. It provides real capacity, accepts standard post-to-beam connectors cleanly, and is what most jurisdictions expect to see. 4x4 posts are still permitted in some places for low decks with short spans, but their capacity is much lower and many building departments have tightened up on them.

Post height matters — a tall post is a slenderness problem as well as a load problem, and tall decks often need bracing that a low deck does not.

Everything above the ground is only as good as the footing under it. Footings must reach below the local frost line, be sized for the load coming down that post, and bear on undisturbed soil. Frost depth varies enormously by region, and this is a detail worth confirming locally rather than assuming.

The ledger connection

Where an attached deck meets the house is the single most safety-critical connection in the whole structure. Deck collapses overwhelmingly happen at the ledger, not in the field of the framing.

The ledger must be fastened to the house structure — the rim joist or framing — with correctly sized and spaced lag screws or through-bolts in a specified pattern. Nails are not acceptable. Flashing above the ledger keeps water out of the wall assembly, and its absence causes rot that hides for years before it announces itself.

This connection is specified in code in detail, and it is the first thing an inspector looks at. It is also the part of the job least worth improvising on.

What your supplier needs to quote framing

Once you know your sizes, a supplier can price the package quickly if you give them:

• Overall deck dimensions and height above grade
• Joist size, spacing and length
• Beam configuration and post count
• Species and grade, and whether you need treated, KDAT, or a naturally durable species
• Decking material and profile, since it affects joist spacing
• Hardware requirements — treated lumber needs compatible fasteners and connectors
• Delivery access, especially for long joists and 6x6 posts

Where to get the actual numbers

Use published span tables for your species and grade, cross-checked against your local amendments — jurisdictions modify the model codes, and yours may be stricter than what you find online. Our joist span charts, beam load tables and span calculator are a good starting point for scoping.

For anything unusual — hot tubs, roofs, long cantilevers, decks over living space, or heavily loaded members — have it sized by a licensed structural engineer. Lumberama supplies materials and does not provide structural engineering.

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