2024 IRC Deck Code Changes: The New Rules on Treated Wood

Few changes in the 2024 IRC have sparked more debate than the update to Section R507.2.1. On the surface, it looks minor – a handful of words were changed in the decay resistance requirement for deck wood. In practice, those words have inspectors, contractors, and code officials across the country actively disagreeing about what they mean.

If you build decks or supply materials to the residential construction market, understanding what changed and why it matters is essential before your next project.

What Changed in the 2024 IRC Deck Code for Wood Materials

To understand what changed, it helps to know what the 2021 IRC said first.

The 2021 IRC required all “wood materials” used in deck construction to be decay resistant – No. 2 grade or better, either preservative treated or naturally durable. The language was broad and applied to everything on the deck.

The 2024 IRC updated that language to reference “wood structural members” instead of “wood materials.” It also moved the decay protection requirements to Sections R304.1 and R304.1.2, which the new Section R507.2.1 now points to.

The “Wood Materials” vs “Wood Structural Members” Debate

That shift in language is what created the controversy. By referencing structural members specifically, the 2024 IRC deck code raised a question that nobody has fully answered yet: do decking boards, stair treads, and stringers still need to be decay resistant?

Most code officials say yes. Their reading is that Sections R304.1 and R304.1.2 – the sections R507.2.1 now references – include decks in their list of locations where decay resistant wood is required, which effectively keeps everything in scope. Others argue that the plain language of R507.2.1 narrowed the requirement to structural members only, and that decking boards fall outside that definition.

The debate is ongoing. The 2027 IRC cycle will almost certainly address it with clearer language. Until then, the practical guidance is straightforward: spec pressure treated wood for all deck components and confirm with your local building department if you have questions about how they’re interpreting the current code.

What Qualifies as Naturally Durable Wood Under the 2024 IRC

The 2024 IRC deck code introduced a formal definition of naturally durable wood for the first time – and it comes with restrictions that will affect what materials qualify on your next build.

Under the 2024 IRC, the only species that qualify as naturally durable are redwood, cedar, black locust, and black walnut. Ipe, teak, and other tropical hardwoods that have long been marketed for their natural decay resistance are not on the list and do not qualify under this definition.

Cedar and Redwood – Are They Still Code Compliant?

Cedar decking and redwood decking remain options under the 2024 IRC, but there is a significant catch. The code now requires that naturally durable wood contain no more than 10% sapwood on any face of the board.

Sapwood is the lighter-colored outer wood found near the bark. It is far less resistant to decay than the darker heartwood at the center of the tree. The problem is that sapwood is extremely common in standard-grade cedar and redwood – grades that have been used for residential decking for decades.

B-grade redwood, standard and better cedar, and most finger-jointed cedar products all typically contain well above 10% sapwood and no longer qualify as naturally durable under the 2024 IRC definition. In practice, sourcing heartwood-dominant cedar or redwood in consistent supply and at a reasonable price is a genuine challenge for most contractors. For the majority of residential deck builds, pressure treated lumber is the more reliable and cost-effective path to compliance.

One exception is worth noting: the 2024 IRC allows naturally durable species to skip the sapwood restriction in regions where local climate conditions have historically demonstrated that decay is not a significant risk. Always check with your local building official before relying on this exception.

Preservative Treated Lumber – What the 2024 IRC Requires

Pressure treated wood meeting AWPA U1 is the most straightforward way to satisfy the decay resistance requirement under the 2024 IRC deck code. But pressure treated lumber is not a single product – there are use categories and treatment levels that determine whether a piece of lumber is appropriate for a given application.

AWPA U1 is the American Wood Protection Association’s Use Category System. It defines the minimum preservative retention required for specific end uses and is the standard the 2024 IRC references for all preservative treated wood on decks. The label on every piece of pressure treated lumber should reference this standard.

Above Ground vs Ground Contact – Know the Difference

Using above-ground rated lumber in a ground contact application is one of the most common code violations inspectors catch on deck builds – and one of the most preventable. The treatment levels between the two categories are significantly different, and the wrong choice will result in premature rot at the most critical structural points of the deck.

Deck posts are where this mistake happens most frequently. Above-ground rated posts installed in concrete footings are a code violation and a long-term structural liability. The post will rot from the inside out well before any visible decay appears on the surface. If any part of the wood touches soil, is embedded in concrete, or sits within 6 inches of grade, it requires ground contact rated lumber.

Does Decking and Stair Stringers Still Need to Be Treated?

This is the question most contractors are asking right now, and the honest answer is that it depends on how your local jurisdiction is interpreting the 2024 IRC.

The revised Section R507.2.1 references “wood structural members” and points to R304.1 and R304.1.2 for decay protection requirements. Those sections list decks among the locations where decay resistant wood is required, which most inspectors read as maintaining the requirement for all deck wood including decking boards and stringers. The section also explicitly requires that sawn lumber for joists, beams, and posts be No. 2 grade or better.

Some jurisdictions are issuing local guidance to clarify their interpretation. Others are leaving it to inspector discretion. Until your local building department provides clear direction, specifying decay resistant wood for all deck components is the right call. A failed inspection or a structural callback will always cost more than the difference in material cost between treated and untreated decking.

Proper material selection doesn’t stop at the decking boards either – the ledger connection requires the same attention to decay resistance and moisture protection

How the 2024 IRC Deck Code Affects Your Material Choices

The treated wood rule changes have a direct impact on what to specify on every residential deck build.

Pressure treated lumber meeting AWPA U1 remains the clearest compliance choice. It is widely available, inspectors are familiar with it, and there is no ambiguity about whether it qualifies. The key is verifying the correct use category for each application – above-ground rated lumber for joists, beams, and decking, and ground contact rated for posts and any member near or in soil.

Cedar decking and redwood decking remain viable options for contractors who can source grades that meet the 10% sapwood limit. Verifying compliance requires checking grade certificates, not just a visual inspection of the boards. For most residential projects, the sourcing challenge makes pressure treated lumber the practical choice.

Composite decking products that meet ASTM D7032 are inherently decay resistant and sidestep the treated wood debate entirely. The trade-off is framing requirements – some composite products require 12-inch on-center joist spacing instead of the 16-inch standard used for wood decking. Specify the decking product before framing, not after.

How to Stay Compliant – What to Look for on the Lumber Label

Every piece of pressure treated lumber sold in the U.S. should carry a quality mark stamp that verifies its compliance. Before using any treated lumber on a deck build, confirm the following on the label.

The treatment standard should reference AWPA U1. If it does not, request documentation from the supplier before the material goes on the job site. The use category – UC3B for above-ground or UC4A/UC4B for ground contact – is the single most important thing to verify, since the wrong category in the wrong location is an automatic inspection failure.

The preservative type will typically be ACQ (Alkaline Copper Quaternary), CA-B (Copper Azole), or MCA (Micronized Copper Azole). All three are effective for deck construction, but copper-based preservatives corrode standard steel fasteners. Use hot-dipped galvanized or stainless steel hardware with all pressure treated lumber. The label should also reference an ALSC-accredited certifying agency, which confirms the treatment level was independently verified at the treating plant.

One final step that is commonly skipped: field cuts. Any time pressure treated lumber is cut, notched, or drilled, the treated surface is compromised at the cut point. The 2024 IRC requires those cut ends to be treated in the field with an AWPA M4-compliant end-cut preservative. Inspectors check for this, and it takes less than a minute per cut to stay compliant.