IRC Deck Construction and Inspection: What the B1 Exam Tests

Where the Deck Rules Live, and Which Edition You Sit

Decks are in IRC Chapter 5, Section R507. R507.1 requires wood-framed decks to be designed for the live load in Table R301.5 or the ground snow load in Table R301.2(1), whichever is greater.

The numbering moved between editions. In the 2018 IRC, R507.3.2 held the frost rule and pointed to R403.1.4.1. In the 2021 IRC, R507.3.2 is the 12-inch minimum depth and frost protection sits in R507.3.3. This article follows the 2021 IRC, flagging the 2018 difference where it changes an answer.

Materials: R507.2 and the Treated Wood Rules in R317

R507.2.1 allows wood that is No. 2 grade or better, treated per Section R317, or approved naturally durable lumber, with termite protection under R318 where required. Field cuts are treated per R317.1.1.

• Grade. No. 2 or better is the floor. Unmarked lumber fails R507.2.1 before a span is measured.
• Treated is not ground contact. R317.1.2 requires treated wood suitable for ground contact where wood touches soil or sits in concrete exposed to weather.
• Posts on piers. R317.1.4, Exception 3 exempts posts on concrete piers or metal pedestals projecting 1 inch above a concrete floor or 6 inches above exposed earth.
• Fasteners. Table R507.2.3 allows hot-dipped galvanized per ASTM A153, mechanically galvanized or stainless, and requires stainless within 300 feet of a salt water shoreline.

Plastic composite decking, treads and guards fall under R507.2.2: ASTM D7032, a label stating allowable load and span, and a flame spread index not exceeding 200 per ASTM E84.

Footings and Frost Depth

R507.3 requires concrete footings sized from Table R507.3.1 by tributary area and the soil bearing value in Table R401.4.1. Footing dimensions must allow complete bearing of the post, and a brick or CMU pier needs a 2-inch projection on all sides.

In the 2021 IRC, R507.3.2 sets a minimum depth of 12 inches below undisturbed ground, and R507.3.3 governs frost protection: below the frost line in Table R301.2, on solid rock, or by another approved method. R403.1.4 repeats the 12-inch rule and R403.1.4.1 forbids bearing on frozen soil unless the frozen condition is permanent. The 2018 IRC put frost in R507.3.2, with an exception for free-standing decks of 200 square feet or less on pier blocks and no more than 20 inches above grade.

The Ledger Is Where Decks Come Off the House

R507.8 decides most deck inspections: an attached deck must be positively anchored to the primary structure for vertical and lateral loads, and not with toenails or nails subject to withdrawal. Where positive connection cannot be verified, the deck must be self-supporting. That is the code giving an inspector an answer when the ledger is buried behind siding.

• Ledger material (R507.9.1.1). Minimum 2-inch by 8-inch nominal, treated Southern pine, incised treated hem-fir, or naturally durable No. 2 or better. No concentrated loads from girders, and no ledger on stone or masonry veneer.
• Band joist (R507.9.1.2). Minimum 2-inch nominal solid sawn spruce-pine-fir or better, or an engineered rim board under R502.1.7, bearing fully on structure.
• Fastener spacing (Table R507.9.1.3(1)). Read by joist span, not deck size. At 40 psf live load, a 6-foot span allows 1/2-inch lag screws at 30 inches on center; an 18-foot span drops to 10 inches.
• Fastener placement (Table R507.9.1.3(2)). 2 inches from the top edge, 3/4 inch from the bottom edge, 2 inches from the ends, rows 1-5/8 inches apart and staggered, at most 5 inches apart.
• Flashing (R507.2.4, R703.4). Corrosion-resistant metal not less than 0.019 inch nominal, installed so water cannot reach the band joist. A bare ledger top is a rot call.

Joists, Beams and Cantilevers

R507.6 sets joist spans from Table R507.6 by species, size and spacing, with the wet service factor. Spacing is capped by the decking: Table R507.7 allows 1-1/4-inch wood at 12 inches on center single span and 16 inches multiple span, and 2-inch wood at 24 inches, tighter on the diagonal.

• Joist cantilever. Limited to one-fourth of the joist span or the maximum in Table R507.6, whichever is less.
• Beam cantilever. R507.5 permits a cantilever at each end of up to one-fourth of the actual beam span, with spans in Tables R507.5(1) to (4). Plies get two rows of 10d nails at 16 inches on center along each edge.
• Bearing. R507.5.1 and R507.6.1 require at least 1-1/2 inches on wood or metal and 3 inches on concrete or masonry, over the full member width.
• Rotation at joist ends. R507.6.2 requires lateral restraint: hangers or blocking at least 60 percent of the joist depth, or a rim joist with three 10d nails at each joist.

Lateral Load: The Failure the Code Is Trying to Prevent

Lag screws resist gravity, not the horizontal pull of a deck acting as a diaphragm. Under R507.9.2 and Figure R507.9.2(1), hold-down tension devices go in at least two locations per deck, within 24 inches of each end, rated at 1,500 pounds allowable stress design capacity; under Figure R507.9.2(2), four devices at 750 pounds. An unattached deck is allowed if it is not more than 30 inches above grade.

Stairs and Guards

Deck stairs bring R311.7 and R312 into play, and the exam tests them as pure measurements.

Open risers carry the same 4-inch sphere limit where the opening is more than 30 inches above the floor or grade below (R311.7.5.1). Commercial work switches to IBC Section 1015: the same 30-inch trigger (1015.2), a 42-inch guard height under 1015.3 with a 36-inch exception for some residential occupancies, and the sphere rule in 1015.4.

What the Inspector Documents, and How the B1 Asks

Deck inspections come in stages: footings before concrete, framing before decking hides the ledger, and a final look at stairs, guards and lateral devices. Record the ledger fastener type and spacing, the flashing detail, and the guard opening measurement.

Exam questions reuse three shapes: a dimension and a request for the minimum, a site finding and a request for the section, or a span lookup. The test is open book, so tab the deck tables with the footing tables in the IRC foundations guide. Guard numbers repeat in the means of egress guide, and flashing details in the IRC roof guide. Try a free ICC practice question, then see the full bank on the pricing page.