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Framing and structure

A deck frame is a chain of members that carries load from the board you stand on to the ground. Every member in that chain has one job, and every connection between them is where failures actually happen. This section explains the chain, does the arithmetic that can be done safely, and stops where a span table has to take over.

The load path, in order

Member	Carries	Governed by
Decking	You, across to the joists	Joist spacing and board thickness — arithmetic
Joists	Decking load, along to beam or ledger	Span table: species, grade, size, spacing, load
Beam	Joist reactions, along to posts	Span table: plies, species, post spacing
Posts	Beam reactions, down to footing	Height, load, and buckling — usually a table minimum size
Footing	Post load into soil	Soil bearing, frost depth — site-specific
Ledger	Half the deck, into the house band	Fastener schedule and flashing detail

Read the third column again. Three of six rows are arithmetic. Three are not, and they are the three that fail. That split is the whole reason this site counts joists instead of sizing them: a form that takes four inputs cannot answer a question with six variables, and a wrong answer here is a collapsed deck rather than a second trip to the yard.

What can be calculated honestly

Counts and spacing. How many joists at 16″ on centre across 14 ft, what the real spacing becomes after rounding, how many rows of blocking. Pure arithmetic — see the joist layout calculator.
Concrete volume. A cylinder minus the post is geometry, not engineering — see the footing calculator and the post hole calculator.
Linear feet of rim, beam and blocking. Perimeter arithmetic, once the layout is decided.
Cantilever limits as a ratio. The code states a fraction of the back span, and a fraction is arithmetic — see cantilever rules.

What cannot

Joist and beam spans. Five variables interact: species, grade, size, spacing and the deflection limit. Two decks that look identical on a drawing can legally need different joist depths. See how to read a span table.
Footing diameter and depth. Bearing capacity is a soil property, and frost depth is a local published number. Both are inputs we cannot see, so the tools ask you to supply them and compute only the volume.
Lateral load. A deck must resist racking, and how depends on whether it is attached to a house, freestanding, on posts or on a ledger. That is a design decision, not a formula.

The connections, which is where decks fail

Most deck collapses that get investigated trace to a connection rather than a member: a ledger pulled off a rim board, a joist hanger nailed with the wrong fastener, a post that rotted at the base because it sat in soil, or a guard attached to nothing structural. None of those are arithmetic problems, which is why the pages that follow spend most of their words on details rather than numbers.

Everything in this section

Deck ledger attachmentHow a deck ledger attaches to a house, why the connection fails, which fastener schedules apply, and the wall conditions where a ledger is not permitted at all. How to read a span tableWhat each column of a span table means, why five variables interact, and why this site will not publish a span number. Includes the steps to read one correctly. Post hole calculatorConcrete and gravel per post hole, with the post volume subtracted. Enter hole diameter, depth, gravel base and post size for bags at 60 lb or 80 lb. Deck blocks versus poured footingsPrecast deck blocks are cheap and fast and fail predictably. Where they are permitted, where frost rules them out, and how to build on them. Cantilever rulesHow far a deck joist can overhang its beam, why the limit is a fraction of the back span, and how cantilevering changes post layout and guard loads. Flashing detailsWhere water goes at a deck-to-house junction, the flashing sequence that works, why caulk is not flashing, and the material compatibility rules for treated lumber. Deck guard requirementsWhen a deck needs a guard, how high it must be, what load it has to take, and the infill rule that fails more inspections than any other. With IRC clause numbers. Deck stair codeThe IRC dimensions for exterior deck stairs: maximum riser, minimum tread, nosing projection, handrail height and the 2R+T comfort rule that code does not enforce. Deck load requirementsThe design loads a deck is built for, why 40 psf is not the same as a hot tub, and the lateral load requirement that most deck collapses trace back to. Do you need a permit for a deck?The three factors that decide whether a deck needs a permit, what the application usually requires, and why an unpermitted deck becomes a problem at sale. Joist spacing chartWhat joist spacing changes: decking stiffness, joist span, material cost and the feel underfoot. A chart of spans by size and spacing, with the trade-offs. Frost depth and deck footingsHow frost heave lifts a footing, how deep footings go in cold climates, and the two details that prevent heave without digging deeper. Deck frame anatomyLedger, joists, beam, posts, footings and blocking explained by the load each one carries, with the decisions that are arithmetic and the ones that need a table. How to build a deckThe build order for a ground-level attached deck, with the three steps that cannot be corrected later and the inspection that must happen before concrete. How to install deck boardsBoard orientation, gap size by material, fastener placement and the two patterns worth using. Includes why the gap depends on whether the board is wet or dry. How to set deck postsSetting a post in concrete without rot or heave: hole depth, gravel base, anchor type, plumbing and the bracing that keeps it true while the concrete cures. Deck stair layoutLaying out a stringer with a framing square: riser and tread marks, the first mark that accounts for tread thickness, and notch depth limits. Deck railing installationInstalling deck railing so it passes the 200 lb load test: post blocking, through-bolts, rail layout and the baluster sequence that keeps spacing even. How to flash a deck ledgerThe flashing sequence for a deck ledger, in order from the wall out, with the two details that keep water out of the band joist and the one that guarantees rot. Deck board patternsFour deck board layouts with the material, framing and joist-spacing consequences of each. Includes why diagonal laying needs 15% more board. Patio slope and drainageHow much fall a patio needs, where the water actually goes, and why a 1% slope that satisfies the code still leaves a puddle against the house. Wobbly deck railingA loose deck railing is almost always a post anchorage problem. Diagnosing whether it is the post, the blocking or the frame, with the fix for each. Deck footing failureWhy deck footings heave in winter or sink over time, how to tell which is happening, and the repairs that work versus the ones that only hide it.

Order of operations

Footings first, because frost depth and soil decide placement. Then posts, which are set by beam span. Then beam, which is set by joist span. Then joists, then decking. Railing and stairs are priced and built last but must be designed first — a guard post needs blocking in the frame, and a stair needs a stringer attachment point that the rim has to carry.

Questions people actually ask

Can I build a deck without a permit?

In most US jurisdictions, no — a deck above a certain height, or attached to a house, requires a permit regardless of size. The threshold and the drawing requirements are local. Assume you need one, then check.

How far can a deck joist cantilever?

The IRC limits a joist cantilever to one quarter of the back span. A joist spanning 12 ft back to its beam can overhang 3 ft. The clause moved between editions, so check the one your jurisdiction adopted, and check the joist manufacturer if you are using engineered lumber.

Do I need a beam if the deck is low?

A beam is what transfers joist load to posts. A very low deck on grade can sometimes bear joists on a treated sleeper or on precast blocks instead, but that changes the whole framing scheme and the detailing that keeps wood out of soil.

What size posts for a deck?

Post size is set by height and load; 4×4 is common for low decks and 6×6 for taller ones, and some codes set a minimum for guard posts regardless of load. Post sizing is a table question, not arithmetic — do not infer it from the beam it carries.

Why does the ledger need flashing above it?

Because the ledger is a horizontal board screwed to a vertical wall, which is a perfect water trap. Water running down the wall hits the top of the ledger, and without a flashing that laps over it, that water goes into the joint and into the rim board behind it.

Deck code checklistThe dimensions that get measured at inspection. Deck joist calculatorCounts and spacing, with sizing left to the table. Deck footing calculatorConcrete volume and bag counts per hole.