Freestanding Deck Sizing: Footings, Joists and Beams

A freestanding low deck is a simple frame: footings, posts, beams, joists and boards. Each part carries the one above it, so you size them in that order, from the top down. This guide walks through each part with the same tables our plan engine uses, and explains why our decks never go higher than 28 in.

• Why do our low decks stop at 28 inches?
• What load is a low deck sized for?
• How far can deck joists span?
• How far apart should deck joists be?
• How big should the deck beams be?
• How do the joists attach to the beams?
• What posts and post bases does a low deck need?
• How big and how deep should deck footings be?
• How do you size the stairs?
• What hardware and lumber should you use?
• How do you plan your low deck?

Why do our low decks stop at 28 inches?

Height decides how much deck you have to build. The AWC deck guide, DCA 6, says all decks more than 30 in above grade need a guard. A guard is a full railing system with strong posts, and it is the hardest part of a deck to get right.

So our plans stay below that line. You can pick a walking surface 16, 20, 24 or 28 in above grade. The 2 in margin below 30 in leaves room for uneven ground. If your yard slopes, measure the height at the lowest corner, because that is where the deck sits highest.

Why freestanding?

Our low decks never attach to the house. There is no ledger board, no siding to remove and no flashing to get wrong. Every deck stands on its own posts and footings.

That choice has one catch. DCA 6 is written for decks that lean on the house for sideways stability. Its commentary says a freestanding deck is "both vertically and laterally independent," and the guide gives no prescriptive bracing for that case. Before you build, ask your local building department what it wants for bracing a freestanding deck.

What load is a low deck sized for?

Every DCA 6 table assumes 40 pounds per square foot (psf) of live load and 10 psf of dead load. Live load is people, chairs and grills. Dead load is the weight of the deck itself. Our plans use the same numbers, so every size in this guide comes from those tables.

DCA 6 also sets limits on what it covers. It does not cover decks carrying large concentrated loads such as hot tubs. It also does not apply where snow loads exceed 40 psf. If either is true for you, the deck needs a design from a professional, not a prescriptive plan.

How far can deck joists span?

A joist span runs from the face of one support to the face of the other. DCA 6 Table 2 lists the longest span for each joist size, spacing and wood species. Our plans read the row for the weakest species group: redwood, western cedars, ponderosa pine and red pine.

These spans assume No. 2 grade lumber in wet service, like a deck outdoors. Southern pine and the Douglas fir group span a bit more. We use the weakest group anyway, so the same plan works with any pressure-treated species your store stocks, and with cedar.

The table also shows why deeper joists are worth it. Going from a 2x8 to a 2x10 at 16 in apart adds more than 2 ft of span. That can save you a whole row of beams, posts and footings.

How far apart should deck joists be?

Joist spacing depends on the boards on top. Our plans use 5/4 wood decking with joists no more than 16 in apart. Some plans use 12 in spacing, which lets a smaller joist cover the same span.

If you switch to composite decking, read the label. Composite boards carry their own span rating, and some need closer joists than wood. Follow the board maker, not a general rule.

DCA 6 adds two decking rules worth keeping. Each length of decking must rest on at least 3 joists. And the boards go about 1/8 in apart, so water drains and the wood can swell.

How big should the deck beams be?

The beams carry the joists to the posts. A beam's allowed span depends on two things: its own size and the length of the joists it carries. Longer joists put more deck on the beam, so the beam's span between posts gets shorter.

DCA 6 Table 3A gives beam spans for built-up beams made of two or three 2x boards. Our plans read the row for the Douglas fir-larch, hem-fir, spruce-pine-fir and redwood group:

Read it like this: with 12 ft joists, a 2-2x10 beam needs a post every 5 ft 8 in. Add a third ply and the posts can be 8 ft 3 in apart. Fewer posts means fewer holes to dig.

A beam may also run past its end post. DCA 6 allows that overhang up to one quarter of the beam span. Our plans stay inside that limit.

How do the joists attach to the beams?

In our plans, each platform has two beams, one at the front and one at the back. The joists hang between them in joist hangers, with their tops flush with the tops of the beams. That keeps the deck low, which matters when you have only 28 in to work with.

DCA 6 sets a few rules for this layout:

• Hanger depth: each hanger must be at least 60% of the beam's depth.
• Beam depth: where you use hangers, the beam must be at least as deep as the joists.
• Hanger strength: a hanger must hold at least 500 lb for a 2x8, 600 lb for a 2x10 and 700 lb for a 2x12.
• One side only: joists must not frame into the same beam from opposite sides.

That last rule is why a large deck in our plans is built as separate platforms. Each platform has its own pair of beams, so no beam ever carries joists from both sides.

What posts and post bases does a low deck need?

DCA 6 is clear on post size: all deck posts must be 6x6 or larger. That holds even for the short posts of a low deck.

Our plans set each post on a post base bolted to the concrete. The Simpson Strong-Tie ABU66Z is one example: it holds the post 1 in above the footing and is rated for uplift. The gap keeps the end grain out of standing water.

The beam sits on top of the post in a post cap. DCA 6 allows a notched post or a post cap, and it requires a post cap for every 3-ply beam. It does not allow a beam bolted to the side of a post without a notch.

Why post bases do not brace the deck

A post base holds the post down, but it does not hold it upright. Simpson's own data says post bases do not stop a post from rotating about the base. They are not recommended where the top of the post is not supported. That is one more reason to ask your building department about bracing a freestanding deck.

How big and how deep should deck footings be?

Footing size depends on how much deck each post holds up. DCA 6 calls that the post's tributary area. For a post in the middle of a beam, it is about half the joist length times the beam span between posts. A corner post holds roughly half as much.

DCA 6 Appendix B, Table B3, turns that area into a footing size. Our plans use the column for soil that bears 1,500 psf, the weakest soil in the table:

Two of our own choices sit on top of that table. Our smallest footing is 12 in round, even when the table allows less. And a post under the middle of a continuous beam uses 1.25 times its area, as Table B3 asks.

How deep should the footings go?

DCA 6 says footings must bear on undisturbed soil at least 12 in below the undisturbed ground, or below the frost line, whichever is deeper. It tells you to contact your local building department for the frost line. Some local codes ask a freestanding deck to reach frost depth, and some do not. Ask before you dig.

DCA 6 lists a few places never to put a footing. Keep footings off septic systems, leach fields, utility lines and enclosed meters. Contact local utilities (call 811) before you dig. And if a footing sits closer than 5 ft to the house foundation, DCA 6 says it must bear at the same depth as the house footing.

How do you size the stairs?

Even a low deck needs a step or two. DCA 6 says stairs must be at least 36 in wide. Our plans keep each riser at most 7-3/4 in tall and each tread at least 10 in deep.

The stringers carry the treads. DCA 6 says stringers must be at least 2x12. If you use only cut stringers, you need at least three, placed no more than 18 in apart. Our plans use cut 2x12 stringers at that spacing.

When do deck stairs need a handrail?

DCA 6 asks a handrail on at least one side of any stair with 4 or more risers. The handrail sits 34 to 38 in above the tread nosing, and it must be easy to grip. A 28 in deck needs four risers to stay at 7-3/4 in or less, so it gets a handrail.

The bottom of the stair needs a solid base too. DCA 6 says stringers must not bear on a concrete pad or patio that is not founded at least as deep as the deck footings.

What hardware and lumber should you use?

The metal matters as much as the wood. To resist corrosion, DCA 6 calls for hot-dip galvanized or stainless steel screws, bolts and nails with treated wood; silicon bronze and copper also qualify. Our plans use hot-dip galvanized or stainless throughout. Joist hangers, post bases and other connectors must be galvanized or stainless. Near salt water, within 300 ft of the shoreline, DCA 6 asks for stainless steel grade 304 or 316.

The wood needs the right treatment for where it sits. DCA 6 says any lumber in contact with the ground must be treated for ground contact. It also says to treat every cut end with a field preservative such as copper naphthenate.

How do you plan your low deck?

You now know how a freestanding low deck is sized: joists from the span table, beams for that joist span, 6x6 posts on post bases, and a footing for each post's share of the deck. Keep it at 28 in or lower, and check bracing and frost depth with your building department.

Our free low deck preview turns your yard size into a 3D model sized with the rules in this guide, before you pay anything. You can also compare deck layouts on the low deck plans page. If you want shade over the deck area too, see our guide to pergola post, beam and rafter sizes.