“What do inspectors look for on a deck?” Most of the answer sits in one section of the International Residential Code: R507, Exterior Decks, with the guard rules in R312 and the stair rules in R311.7. Where a town enforces the IRC, those are the sections a deck is checked against.
This article goes through them in the order you build: footings, posts and beams, joists, the ledger, then guards and stairs. It quotes the 2021 IRC. Your town may enforce a different edition or its own amendments, and Do You Need a Permit? covers finding out which edition applies and which small decks the model code exempts from a permit. A deck that needs no permit still has to be built to code.
The two references
The code itself is the first. The second is the American Wood Council’s Prescriptive Residential Wood Deck Construction Guide, known as DCA 6, a guide with drawings for single-level wood decks attached to a house, which the council publishes on its website. The edition its site links is based on the 2015 IRC. It says its methods meet or exceed the IRC’s minimums, that where it differs from the IRC the IRC applies, and that all construction must be approved by the authority having jurisdiction. It is worth a copy in the truck: it draws what the code describes in words.
When the inspector comes
The IRC calls for a foundation inspection after poles or piers are set or trenches are excavated and any forms are in place, before concrete is placed (R109.1.1); a frame inspection (R109.1.4); and a final inspection after the work is complete and before occupancy (R109.1.6). DCA 6 says it plainly: decks shall not be used or occupied until final inspection and approval is obtained. In practice that means booking the footing inspection while the holes are open. Pour first, and the inspector has nothing left to see.
Loads and materials
A deck is designed for the live load in Table R301.5, 40 pounds per square foot for decks, or the ground snow load for your area, whichever is greater (R507.1). Lumber must be No. 2 grade or better, preservative-treated or an approved naturally durable species, with cuts and drilled holes in treated lumber field-treated, and any treated wood in contact with the ground labeled for that use (R507.2.1).
Fasteners and connectors must meet the code’s corrosion rules (R507.2.3), and the ledger’s lag screws and bolts must be hot-dipped galvanized or stainless steel (R507.9.1.3). DCA 6 adds that fasteners and connectors within 300 feet of a salt water shoreline must be stainless steel, grade 304 or 316. Flashing must be corrosion-resistant metal at least 0.019 inch thick or an approved nonmetallic material (R507.2.4), and DCA 6 warns that aluminum should not be used in direct contact with lumber treated with copper-based preservatives such as ACQ, copper azole or ACZA.
Footings
Decks sit on concrete footings or another approved system sized for the loads (R507.3). Table R507.3.1 sets the minimum size from the deck area each footing carries and the soil’s bearing value. Two examples from it, at 40 pounds per square foot on soil rated 1,500 psf: a post carrying 60 square feet of deck needs a footing at least 17 inches square or 19 inches round, 6 inches thick; at 100 square feet, 22 inches square or 25 round, 8 inches thick.
Footings go at least 12 inches below undisturbed ground (R507.3.2). When the deck is attached to a house on frost-protected footings, the deck’s footings must be protected from frost too: below the frost line your jurisdiction lists in Table R301.2, on solid rock, or by another approved method (R507.3.3). The code exempts two kinds of freestanding deck from footings altogether: one with its joists supported on grade over their whole length, and one of no more than 200 square feet whose joists bear directly on precast concrete pier blocks at grade, with the walking surface no more than 20 inches above grade within 36 inches of the edge (R507.3).
Posts and beams
Post size comes from Table R507.4, by species, the deck area the post carries and its height. At a 40 psf live load, a Southern pine 4x4 carrying 60 square feet may stand 11 feet; a Southern pine 6x6 may go to 14 feet at every listed area. DCA 6 is stricter: its minimum post is a 6x6. Posts on concrete footings need lateral restraint from manufactured connectors or at least 12 inches of embedment in soil or concrete piers (R507.4.1).
Beam spans come from Tables R507.5(1) to R507.5(4). Built-up beam plies are fastened with two rows of 10d nails at 16 inches on center along each edge, and a beam may cantilever up to one-fourth of its actual span at each end (R507.5). Beam ends need at least 1½ inches of bearing on wood or metal and 3 inches on concrete or masonry (R507.5.1).
Joists
Table R507.6 gives the spans. At a 40 psf live load, Southern pine 2x8s at 16 inches on center may span 11 feet 10 inches, and 2x10s at 16 inches 14 feet; at 12 inches, 2x10s reach 16 feet 2 inches. The decking limits the spacing (Table R507.7): 1¼-inch wood boards laid square to joists over three or more joists allow 16 inches, but only 12 inches on a diagonal, and composite boards follow the span on their label (R507.2.2.1). Joists framing into the side of a beam or ledger need approved joist hangers (R507.6.1), and joist ends need restraint against rolling, with hangers or blocking at least 60 percent of the joist depth (R507.6.2).
The ledger and the house connection
An attached deck hangs from the house at the ledger, and the code spells that connection out in detail. It says a deck supported by an exterior wall must be positively anchored to the house and designed for vertical and lateral loads, and that the attachment “shall not be accomplished by the use of toenails or nails subject to withdrawal.” Where a positive connection to the house cannot be verified during inspection, the deck must be self-supporting (R507.8).
DCA 6 adds the attachments never to build. Ledgers may not attach to or through brick, masonry or stone veneer, to hollow masonry, or to cantilevered floor overhangs or bay windows; those need a freestanding deck. Lead anchors are prohibited. The siding comes off before the ledger goes on, and the ledger connection gets corrosion-resistant flashing. And if the band joist’s capacity cannot be verified, it calls for a non-ledger deck or a full plan submission.
Why so much attention? The North American Deck and Railing Association estimates there are more than 60 million decks in the U.S. and that 30 million of them are past their useful life and need repair or replacement. It adds that inspecting a deck takes expertise because structural materials deteriorate gradually from water, climate and corrosion of structural fasteners and connectors. Ledger fasteners rusting behind wet siding are that kind of deterioration, which is why the code asks for corrosion-resistant fasteners and flashing at the ledger.
Guards and stairs
Guards are required where the walking surface is more than 30 inches above the floor or grade below at any point within 36 inches of the edge (R312.1.1). They must be at least 36 inches high (R312.1.2), may not pass a 4-inch sphere (R312.1.3), and must carry a 200-pound concentrated load (Table R301.5) through a continuous load path to the joists (R507.10.1). A 4x4 post carrying guard loads may not be notched at its connection (R507.10.2).
Stairs need a clear width of at least 36 inches above the handrail height (R311.7.1), risers no taller than 7¾ inches and treads at least 10 inches deep, with no more than 3/8 inch between the largest and smallest in a flight (R311.7.5.1, R311.7.5.2), a handrail 34 to 38 inches high on any flight with four or more risers (R311.7.8, R311.7.8.1), and a floor or landing at the top and bottom (R311.7.6). What all of this does to a price is covered in Pricing Decks.
Before you call for the inspection
The decks page has more for deck builders.
General information, not legal or tax advice. Rules change and vary by state; check with the agency named above or a professional for your situation.