Building a Timber Deck: Sizing Every Member by What It Carries

Read the deck upside down before you dig

A deck is a stack of handoffs. The board takes the foot, the joist takes the board, the beam takes the joist, the post takes the beam, and the footing hands everything to the ground. Every dispute about member size on a job site traces back to somebody sizing one of those members without knowing the number arriving from above. So the first hour on a new deck belongs to arithmetic, not to a shovel — walk the stack downward on paper, write the tributary width against each member, and only then decide where the holes go.

Tributary area is the whole trick. A joist spanning between a ledger and a beam gives half its load to each; a beam under joists landing from both sides carries half the span on either hand. Get in the habit of writing that number on the plan next to each member — beam carries six feet of tributary over a fourteen foot span — because it is the single figure a building official will ask for and the single figure most home-built decks never established. Everything downstream, including how deep and how wide the concrete goes, is that number multiplied out.

Loads themselves are set by your local adopted code, not by a national default, and the residential deck live load in one jurisdiction is not the live load two states over. Snow country adds a ground snow load that can dwarf occupancy. Elevated decks near assembly-style use get treated differently again. Name the adopted code edition on your drawing and size to it; publishing a single number here would be wrong somewhere, and wrong somewhere on a deck means a collapse during a party.

The footing: where the number from above lands

Concrete does not care how pretty the framing is. A footing sized for a modest joist layout under a deck that later got a hot tub, a pergola and an outdoor kitchen is a footing that will settle out of level and drag the whole frame with it. Size the pad for the beam reaction it actually receives, and size the depth for frost — frost depth is set by the local authority having jurisdiction and varies enormously, from essentially nil in warm coastal ground to well past four feet inland at latitude. If you are unsure, the building department will tell you over the phone in thirty seconds. Do not guess.

Bearing capacity of the soil is the other half. Presumptive values in code tables cover common soils, but fill, expansive clay and anything within the zone of an old excavation want a geotechnical opinion rather than a table lookup. A hole augured through eight inches of topsoil into undisturbed subgrade behaves; the same hole stopped short in backfill will hand you a post that has dropped a full inch by the second spring. Dig until the auger is fighting you, not until the tape reads the number you hoped for.

Water is what actually kills footings. Bell the base if your code permits it, keep the top of the pier above finished grade so the post base is never sitting in a puddle, and use a standoff post base that lifts end grain clear of the concrete. A post cut flush to the pier top wicks water into its end grain and rots from the inside where nobody can see it; the visible face still looks fine the year it fails. Where the pier top will be seen, form it — a tube cut square and struck flush reads as intentional, a hand-dug flare reads as a repair.

Post and beam: the compression and bending pair

Posts carry axial load, and for typical residential heights the timber is rarely governed by crushing — it is governed by slenderness and by the connections at both ends. A tall post on an exposed deck is a column with a lateral job as well: it resists the sway that people walking, and wind, put into an elevated frame. Undersized posts do not crush, they lean, and the lean shows up as a squeak long before it shows up as a crack.

Bolting the beam to the post is where inspectors focus, and rightly. A beam merely toe-nailed to the top of a post relies on fasteners loaded in withdrawal, the weakest way to use them. Notch the post and through-bolt the beam into the notch, or use a rated post cap, so the load path is bearing rather than fastener shear. Bolt holes want the fastener manufacturer's specified clearance, snug but not oversized, with washers under both head and nut so the hardware bears on wood rather than sinking into it.

Beams themselves are sized by span and tributary width together, and the two trade off. Doubling the beam span roughly quadruples the bending demand, so shortening the span by adding one more post-and-footing pair is almost always cheaper in labour and material than jumping two beam sizes. Where you build up a beam from plies, fasten to the connector schedule and orient the plies crown up as a set; a built-up beam assembled with mismatched crowns has a permanent kink you will chase all the way to the deck surface.

Cantilever earns its own paragraph because it is the most frequently abused detail on residential decks. A beam or joist may overhang its support by a limited proportion of its backspan — the proportion is set by the code you are building to, and it is not a number to eyeball. Overhang beyond it and the backspan lifts: the far end of the joist tries to rise out of its hanger, and the deck develops a springy edge that owners describe as bouncy and engineers describe as the beginning of the end.

Joists: span, spacing, and the ledger that holds half of them

Joist size follows from span and spacing, and spacing follows from what the decking above needs — which makes joist layout the hinge in the whole stack. Standard spacing suits most solid timber boards run square to the joists. Diagonal decking increases the effective span of each board and tightens the required spacing accordingly. Composite and plastic-composite boards carry their own manufacturer-published maximum joist spacing, and that spacing is a condition of the product warranty; ASTM D7032 is the specification such products are rated under, and the manufacturer's installation instructions govern in the field.

Ledger attachment deserves more care than any other single connection on the deck, because roughly half the deck's load goes through it into the house. The failures that make the news are ledger failures. Fasten to the rim board or to solid blocking, never to sheathing or to brick veneer, and use a fastener schedule from the connector manufacturer's published tables rather than a pattern copied off a previous job. Lag screws and structural screws have different withdrawal behaviour, edge distances and required penetration; the schedule accounts for all three.

Flashing behind the ledger is not weatherproofing trim, it is structural preservation. Water that gets behind an unflashed ledger rots the rim board, and a rotted rim board holds nothing regardless of how good the fasteners were. Cut the siding back, install continuous flashing lapped correctly with the water-resistive barrier above and turned out over the ledger below, and do not rely on sealant to do the lapping's job. Order flashing to a measured length rather than piecing offcuts — a lapped joint mid-ledger is a funnel.

Lateral load connection is the companion requirement in most current codes: hardware tying deck framing back into the house floor framing, resisting the deck's tendency to pull away rather than fall down. It is separate from the ledger fasteners and is not satisfied by them. Confirm which detail your jurisdiction requires — hold-down devices at specified locations, or a full continuous load path — before the framing is closed in, because retrofitting it means pulling boards.

Boards: the layer everybody sees

Decking is the only member sized by comfort and appearance rather than by structure alone, but it still carries the point load of a foot between two joists. Run the boards, count them honestly with the gap you actually intend to hold, and order a surplus for the cuts — a board layout planned as about right always ends with a sliver rip at the house wall, which is the one row a client sees every day from the kitchen window. Where a run exceeds stock length, plan the butt joints as a deliberate pattern over doubled blocking rather than letting them fall where the stock runs out.

Gapping depends on the material and its moisture state at installation. Kiln-dried timber installed dry will shrink less than green treated stock, which arrives wet and closes its gaps as it dries — install green boards tight or near-tight and they will open on their own, while installing them pre-gapped gives you a slatted deck by August. Composite boards move with temperature rather than moisture, and their end gaps are specified by the manufacturer against install-day temperature; ignoring that spec is how a summer-installed composite deck buckles in its first heatwave.

Fastener metallurgy interacts with the board. Face screws want stainless or a coating rated for contact with the preservative in your treated timber — AWPA U1 covers the use category and hence the preservative retention, and the wrong metallurgy in ACQ-type treatment corrodes visibly within a season. Hidden clip systems shift the load path into the board edge and demand a groove profile and a spacing the clip maker specifies. Pre-drill hardwood ends without exception; the split you cause at a board end is permanent and telegraphs straight along the grain.

Crown, cup and the direction of the growth rings all get argued about on site. What matters more is consistency: pick a convention, hold it across the whole deck, and keep the boards' factory-sealed ends where you can. Cut ends of treated timber lose their protection and need field-applied preservative before they go down — a bundle of untreated cut ends in the middle of a deck is a hundred small rot sites nobody will find until the boards feel soft underfoot.

Where the stack gets checked

Sequence your inspections to the hierarchy. Holes open and depths measured before concrete. Post bases and beam connections visible before joists go on. Ledger flashing, fasteners and lateral connectors visible before decking. Once the boards are down, every one of those becomes a demolition job to verify, and an inspector who cannot see a connection will assume the worst about it.

Paperwork belongs with the frame. Connector part numbers, fastener schedules, the manufacturer's installation instructions for composite boards, the treatment tag off the lumber bundle — photograph them against the work. Warranty claims on decking and hardware turn on demonstrating the product was installed to its published instructions, and a photo of the ledger schedule before the boards went on has settled more disputes than any amount of recollection.

Finish work sits on top of the same hierarchy. Guard posts are not decorative; they transmit a specified horizontal load into the frame, and ICC-ES AC273 is the acceptance criteria such assemblies are evaluated against. A guard post bolted only through the rim, without blocking and a tension tie into a joist, will pass a shove and fail a crowd. Balusters, stair rise consistency and the finish schedule all follow once the load path underneath them is closed.