2021 IRC Chapter 5
This lesson covers two things that look unrelated but both come up constantly in GCHS rehab and infill work: concrete floors on grade, and wood decks. Slabs are a materials-and-moisture check. Decks are a load path check, from the ledger bolted to the house all the way down to the footing, and a deck built by an unpermitted handyman crew is one of the most common life-safety problems GCHS inspectors find on an older property. The deck material below restates the 2021 IRC's prescriptive deck provisions (IRC R507) together with the American Wood Council's DCA 6 prescriptive deck guide, which is written to walk through the same provisions with worked tables and details.
A concrete slab-on-ground floor must be at least 3-1/2 inches thick (IRC R506.1), placed on a subgrade that has had vegetation and organic material removed, since decomposing organic matter under a slab leaves voids that lead to cracking and settlement. Where the slab is placed below grade, R506.2.2 adds a 4-inch base course of clean graded sand, gravel, crushed stone, or similar material passing a 2-inch sieve, to provide uniform support and a capillary break. That base course can be omitted where the slab sits on well-drained Group I sand-gravel soils under the Unified Soil Classification System, since those soils already drain freely on their own.
Unless the slab falls under an exception, R506.2.3 requires a vapor retarder of at least 10 mil conforming to ASTM E1745 Class A, with joints lapped not less than 6 inches, placed between the concrete and the base course, or the prepared subgrade where there is no base course. Placement matters: the retarder has to sit immediately under the slab, not below the gravel, or the gravel stays free to hold and transmit ground moisture straight into the concrete above it. The exceptions are garages, utility buildings, and other unheated accessory structures; unheated storage rooms under 70 square feet and carports; driveways, walks, patios, and other flatwork not likely to be enclosed and heated later; and cases where the building official approves it based on local site conditions. None of those spaces put ground moisture in contact with a finished, occupied interior.
IRC R507 covers wood-framed decks built with the prescriptive joist, beam, post, and footing sizing the section provides. R507.1 has decks designed for the live load in Table R301.5, which is 40 pounds per square foot for decks, or for the ground snow load in Table R301.2, whichever is greater. The 40 psf figure matches habitable rooms other than sleeping rooms, on the assumption that a deck sees furniture, gatherings, and occasional crowding rather than a lighter, bedroom-only load. Once a deck's loading, span, or configuration falls outside what the prescriptive tables cover, R507.1 sends the design to Section R301, which is exactly the gap DCA 6 exists to help a builder or inspector recognize before it becomes a problem discovered after the deck is already up.
A deck ledger must be a minimum nominal 2x8 of pressure-preservative-treated Southern pine, incised pressure-preservative-treated hem-fir, or approved, naturally durable No. 2 grade or better lumber, and it may not support concentrated loads from beams or girders (R507.9.1.1). The band joist it attaches to must bear fully on the primary structure (R507.9.1.2). Ledger fasteners are 1/2-inch lag screws or 1/2-inch bolts, hot-dipped galvanized or stainless steel, spaced per Table R507.9.1.3(1) by joist span, load, fastener type, and sheathing thickness (R507.9.1.3); the lag screw tip must fully extend past the inside face of the band joist. Fasteners and connectors within 300 feet of a salt water shoreline must be stainless steel (Table R507.2.3), which matters along the Gulf Coast. A deck ledger may not be supported on stone or masonry veneer, so the veneer has to be removed down to the framing, or the deck has to be supported another way, such as a freestanding deck. Where decks have cantilevered framing, R507.8 requires the connection to resist uplift from the full live load on the cantilevered portion, and where a positive connection to the house cannot be verified at inspection, the deck must be self-supporting. Flashing is required at the ledger to keep water from reaching the house band joist (R507.2.4 and Table R507.9.1.3(1) note b); it is corrosion-resistant metal not less than 0.019 inch thick or approved nonmetallic material, since a ledger is one of the most common places a house frame rots from the outside in.
A deck that only leans on its ledger bolts for lateral support can pull away from the house under a lateral load, a mechanism serious enough that several deck collapses have been traced to it. R507.9.2 requires lateral loads to be transferred to the ground or to a structure that can carry them there. Where the connection follows Figure R507.9.2(1), that means hold-down tension devices in at least two locations per deck, within 24 inches of each end of the deck, each with an allowable capacity of at least 1,500 pounds. Where it follows Figure R507.9.2(2), it means at least four devices at 750 pounds each. Typical details, drawn in the figures, tie a deck joist through the house rim joist into a house floor joist. This is a connection an inspector has to trace through the framing, not just look for on the exterior face, since the visible hardware on the deck side is only half of it.
Deck footings are sized from the tributary area and soil bearing value in Table R507.3.1. They must be placed at least 12 inches below the undisturbed ground surface (R507.3.2), and where the deck is attached to a frost-protected structure they must also extend below the frost line, bear on solid rock, or be protected another approved way (R507.3.3). R507.4 sizes single-level deck posts from Table R507.4 by species, post size, load, and tributary area, with height measured from the underside of the beam to the top of the footing. At 40 psf live load, a Douglas fir, hem-fir, or spruce-pine-fir 4x4 is limited to 8 feet at 100 square feet of tributary area, 7 feet at 120, and 5 feet 3 inches at 160, so a 4x4 gets shorter as the area grows; a 4x6 or larger allows more height. Beam ends need at least 1-1/2 inches of bearing on wood or metal and 3 inches on concrete or masonry, and each ply of a beam over an intermediate post needs full bearing on the post (R507.5.1). Notched post-to-beam connections follow Figure R507.5.1(2), whose dimensions are in the drawing and are not restated here, and the beam still needs a positive connection, bolts with washers or a manufactured connector sized for the post and beam, not a toenail (R507.5.2). Many builders and inspectors prefer an unnotched post with a cap or saddle connector instead. A 4x4 post that supports guard loads at the top of a guard may not be notched at the connection to the supporting structure (R507.10.2). Beam spans come from Tables R507.5(1) through R507.5(4) by species, size, and effective joist span (DCA 6 restates these tables in an easier lookup format).
Deck joists bearing on top of a single-ply beam or ledger must be attached by a mechanical connector; joists framing into the side of a beam or ledger must be supported by approved joist hangers; and joists on a multiple-ply beam or ledger are fastened per Table R602.3(1) (R507.6.1). Connectors follow the manufacturer's specification with a zinc coating or hot-dipped galvanizing (Table R507.2.3), and joist ends need lateral restraint, such as hangers or blocking at least 60 percent of the joist depth (R507.6.2). Wood decking is fastened to each supporting joist with at least two 8d threaded nails or two No. 8 wood screws (R507.7), and Table R507.7 limits joist spacing by decking thickness. Any deck walking surface more than 30 inches above grade, at any point within 36 inches horizontally of its edge, needs a guard at least 36 inches high, with openings sized so a 4-inch sphere cannot pass through (R312.1.1 through R312.1.3), under the same IRC R312 guard provisions covered in more depth in Lesson 13 alongside stairways and handrails; a deck framing inspection is a natural point to flag a missing or undersized guard even before the life-safety lesson gets to it in detail.
In the field
A GCHS rehab crew is asked to shore up an older deck rather than replace it outright. The ledger bolts look adequate and the joists are sound, but there is no hardware anywhere connecting the deck framing back through the house rim joist, nothing but the ledger bolts themselves resisting the deck pulling away from the house. That is the lateral load connection missing, and it is invisible from a casual walk-around because there is nothing to see on the exterior face; it has to be checked by opening the rim joist area from inside, at the floor framing the deck ties into, or by asking whether it was ever installed in the first place on a deck built before that requirement was common practice.
Worked example. A deck post carries a beam with a tributary area (the deck surface the post carries) of 120 square feet, at an unbraced height of 7 feet from the underside of the beam to the top of the footing. At 40 psf live load, Table R507.4 gives a Douglas fir, hem-fir, or spruce-pine-fir 4x4 a limit of 7 feet at 120 square feet, so a 7-foot post is exactly at the limit, and a builder planning to go taller, or carry more area, fails the 4x4. At 140 square feet the 4x4 limit drops to 6 feet 2 inches, but a 4x6 is allowed 8 feet 7 inches. Interpolation between tributary areas is permitted and extrapolation is not, which is exactly the kind of boundary case the tables settle instead of a guess.
1. What is the minimum thickness of a concrete slab-on-ground floor under IRC R506.1?
2. A slab is placed below grade over ordinary soil that is not a well-drained Group I sand-gravel mix. R506.2.2 requires a base course beneath it of at least:
3. Where is the required vapor retarder placed relative to the slab and base course, per R506.2.3?
4. What live load do decks need to be designed for under R507.1, unless the ground snow load is greater?
5. Under R507.9.1.3, which fasteners may attach a deck ledger to the house band joist?
6. Under R507.9.1.1, a deck ledger may not be supported on which of the following?
7. Why must a deck ledger be flashed (R507.2.4 and Table R507.9.1.3(1) note b)?
8. Where the lateral load connection follows Figure R507.9.2(1), R507.9.2 calls for hold-down tension devices in two locations, each with an allowable capacity of at least:
9. Why is the deck lateral load connection easy to miss during a casual walk-around inspection?
10. Under R507.10.2, a 4-inch by 4-inch wood post that supports guard loads applied to the top of the guard:
11. Per Table R507.4 at 40 psf live load, compared with a 4x4, a 6x6 deck post at the same tributary area allows:
12. Where deck joists frame into the side of a ledger or beam, R507.6.1 requires them to be supported by:
13. A deck's walking surface is more than 30 inches above grade within 36 inches of its edge. Under R312.1, the required guard along the open side must be at least:
14. Where a vapor retarder would otherwise be required under a slab, which of the following is a typical exception?