Wood Touching Concrete — Pressure-Treated Requirements — IRC 2018

What IRC 2018 § R317.1 requires

IRC 2018 R317.1 requires pressure-treated or naturally durable wood for all wood in contact with concrete, masonry, or the ground. This includes sill plates on concrete foundations, sleepers on slabs, wood embedded in concrete, and posts bearing on concrete. The protection level varies by use — ground contact requires a higher preservative retention than above-grade contact.

Section R317.1 of the IRC 2018 lists specific conditions where wood protection against decay is required. The relevant situations for wood in contact with concrete are:

• Wood joists, girders, subfloor, or other structural wood closer than 18 inches from exposed ground in crawl spaces — must be pressure-treated or naturally durable.
• Wood in contact with concrete or masonry that is exposed to the weather or is in contact with concrete that is in contact with the ground — must be pressure-treated.
• Sill plates on concrete or masonry foundations regardless of interior or exterior exposure — must be pressure-treated or naturally durable.
• Wood sleepers on concrete slabs on or below grade — must be pressure-treated.
• Posts, poles, and columns embedded in concrete in contact with the ground — must be rated for ground contact (higher retention level).

The code specifies that pressure-treated wood must meet the requirements of AWPA (American Wood Protection Association) standards. For above-ground use (sill plates not in ground contact), UC3B or UC4A retention levels are typically appropriate. For ground contact (posts, piers, embedded elements), UC4B or UC4C is required. The treatment retention level must be stamped on the lumber or verified by the wood's marking label.

Naturally durable species permitted by the code include heartwood of redwood, western red cedar, and black locust — but these exceptions are rarely used in modern construction in most of the country. Pressure-treated lumber is by far the most common compliance method.

A critical provision: pressure-treated wood in contact with concrete must not use formulations that are corrosive to steel fasteners. ACQ (alkaline copper quaternary), CA (copper azole), and other copper-based treatments require corrosion-resistant fasteners — hot-dipped galvanized or stainless steel nails, bolts, and hardware. Standard bright steel fasteners will corrode rapidly in contact with copper-based preservatives.

Why This Rule Exists

Concrete is alkaline and absorbs and retains moisture. Wood in contact with concrete absorbs this moisture, creating persistently damp conditions that promote fungal decay. Without preservative treatment, sill plates and sleepers typically begin showing significant decay within 10 to 20 years in most climates, eventually compromising the structural connection between the wood framing above and the concrete foundation below. Pressure treatment prevents this fungal deterioration by infusing the wood with a biocide that inhibits the organisms responsible for wood rot.

What the Inspector Checks at Rough and Final

At the framing inspection and during foundation work, inspectors check:

• Sill plate material — pressure-treated lumber is identified by its green tint and AWPA grade stamp printed on the face of each board.
• Retention level — the stamp must show the retention level and use category appropriate to the application.
• Sleepers on slabs — any wood strip used as a nailing base on a concrete slab must be pressure-treated.
• Posts in concrete — any structural post embedded in or bearing on a concrete pier must be pressure-treated or use an approved post base that lifts the wood off the concrete.
• Fastener compatibility — galvanized or stainless steel hardware is required with copper-based preservative treatments.

What Contractors Need to Know

Never use bright steel (uncoated) framing nails, joist hangers, or anchor bolts with ACQ or CA pressure-treated lumber. The copper in these preservatives creates a galvanic cell with bare steel that causes rapid corrosion — within 5 to 10 years, the fasteners will corrode to the point of failure. Simpson Strong-Tie, MiTek, and other hardware manufacturers publish corrosion-resistance ratings for each product to match specific preservative types.

Cut ends of pressure-treated lumber — saw cuts made in the field — expose untreated wood fiber. AWPA field treatment using an approved preservative end-cut treatment (brush-on copper naphthenate or other listed product) is required at all field cuts where the treated zone is exposed.

Wood bearing on a properly isolated post base (such as a Simpson ABU or ABA post base that keeps wood off the concrete) does not require pressure treatment at that connection if the post is above grade and not in a high-moisture environment. However, posts in wet areas or in contact with splash zone concrete should still be pressure-treated as a best practice.

Pressure-treated lumber requires corrosion-resistant fasteners because the preservative treatments, particularly ACQ and CA, are highly corrosive to standard steel. Standard bright nails and common zinc-plated hardware corrode rapidly in contact with ACQ or CA-treated wood, causing connection failures within a few years. Hot-dipped galvanized nails at a minimum zinc weight of 1.85 oz per square foot, designated G185, or stainless steel fasteners are required. Electro-galvanized fasteners are not equivalent to hot-dipped galvanized, as the zinc coating is much thinner. Hardware connectors including post bases, hold-downs, and joist hangers must also be rated for use with pressure-treated wood and must be specified accordingly.

The treatment retention level must match the use category. For sill plates on concrete slabs with potential moisture contact, UC3B treatment is required. For foundation wood in contact with soil, UC4A or UC4B treatment is required. Verify that the treatment level on the end tag matches the required designation for the specific application. Lumber treated to UC3B retention will fail prematurely if used in a UC4A or UC4B application. The AWPA UC designation is printed on the end tag or grade stamp of every piece of treated lumber. Check this before installation and do not rely on the retail store label alone, as treatment levels are sometimes mixed in bulk inventory.

Pressure-treated lumber requires corrosion-resistant fasteners because the preservative treatments, particularly ACQ and CA, are highly corrosive to standard steel. Standard bright nails and common zinc-plated hardware corrode rapidly in contact with ACQ or CA-treated wood, causing connection failures within a few years. Hot-dipped galvanized nails at a minimum zinc weight of 1.85 oz per square foot, designated G185, or stainless steel fasteners are required. Electro-galvanized fasteners are not equivalent to hot-dipped galvanized, as the zinc coating is much thinner. Hardware connectors including post bases, hold-downs, and joist hangers must also be rated for use with pressure-treated wood and must be specified accordingly.

The treatment retention level must match the use category. For sill plates on concrete slabs with potential moisture contact, UC3B treatment is required. For foundation wood in contact with soil, UC4A or UC4B treatment is required. Verify that the treatment level on the end tag matches the required designation for the specific application. Lumber treated to UC3B retention will fail prematurely if used in a UC4A or UC4B application. The AWPA UC designation is printed on the end tag or grade stamp of every piece of treated lumber. Check this before installation and do not rely on the retail store label alone, as treatment levels are sometimes mixed in bulk inventory.

Pressure-treated lumber requires corrosion-resistant fasteners because the preservative treatments, particularly ACQ and CA, are highly corrosive to standard steel. Standard bright nails and common zinc-plated hardware corrode rapidly in contact with ACQ or CA-treated wood, causing connection failures within a few years. Hot-dipped galvanized nails at a minimum zinc weight of 1.85 oz per square foot, designated G185, or stainless steel fasteners are required. Electro-galvanized fasteners are not equivalent to hot-dipped galvanized, as the zinc coating is much thinner. Hardware connectors including post bases, hold-downs, and joist hangers must also be rated for use with pressure-treated wood and must be specified accordingly.

The treatment retention level must match the use category. For sill plates on concrete slabs with potential moisture contact, UC3B treatment is required. For foundation wood in contact with soil, UC4A or UC4B treatment is required. Verify that the treatment level on the end tag matches the required designation for the specific application. Lumber treated to UC3B retention will fail prematurely if used in a UC4A or UC4B application. The AWPA UC designation is printed on the end tag or grade stamp of every piece of treated lumber. Check this before installation and do not rely on the retail store label alone, as treatment levels are sometimes mixed in bulk inventory.

What Homeowners Get Wrong

Homeowners who build their own sheds, deck frames, or interior basement sleeper systems often use untreated dimensional lumber on or near concrete because they do not understand the decay mechanism. Interior concrete slabs — even in a finished basement — are not "dry." They wick moisture from the soil below and can maintain high moisture content in direct contact zones indefinitely. Untreated sleepers on a basement slab will decay.

Another common mistake is treating all pressure-treated lumber as equivalent. Above-ground-rated (AWPA UC3B) and ground-contact-rated (UC4B/UC4C) are different products with different preservative retention levels. Using above-ground material for ground-contact applications provides insufficient protection and does not comply with R317.1.

In crawl space applications, the vapor retarder on the crawl space floor must be continuous and lapped against the foundation walls and piers to prevent moisture migration from the ground into the crawl space air. An inadequate or torn vapor retarder in the crawl space creates elevated moisture conditions that can wet the sill plates and floor framing above from below, causing decay even in pressure-treated sill plates if the wetting is continuous enough. Verify vapor retarder continuity at the rough framing inspection and again before any insulation is installed in the floor cavity above.

State and Local Amendments

IRC 2018 R317.1 is adopted across TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO. In the South's humid subtropical climate, wood decay is a significant concern, and many building departments inspect sill plate treatment closely. Coastal areas with high ground moisture may require higher retention levels by local amendment. Some Texas counties have traditionally required pressure-treated plates on slabs even in interior applications, which exceeds the baseline IRC requirement but reflects local experience with slab moisture.

IRC 2021 did not substantively change R317.1 requirements. The 2021 edition updated the AWPA standard references to more recent editions of those standards, but the practical requirements for sill plates, sleepers, and ground-contact wood remain the same as in 2018.

When to Hire a Licensed Contractor

Pressure-treated lumber selection and fastener compatibility require knowledge that goes beyond simply buying "green wood" at the lumber yard. A licensed framing contractor or general contractor will specify the correct treatment type, retention level, and compatible fasteners for each application. For any renovation involving existing sill plates or sleepers on concrete that show signs of decay, a licensed contractor should assess the extent of the damage and replace affected members before covering them. A structural engineer may be needed if decay has compromised the load path.

Common Violations Found at Inspection

• Untreated sill plate on concrete foundation — by far the most common R317.1 violation.
• Untreated sleepers or nailers on concrete slab floor — common in basement finishing work.
• Pressure-treated material used, but with wrong retention level — above-grade material (UC2 or UC3A) used in a ground-contact (UC4B) application.
• Standard bright zinc-coated or uncoated steel fasteners used with ACQ or CA pressure-treated lumber — will corrode rapidly.
• Field cuts at sill plate ends not treated with an approved end-cut preservative, exposing untreated wood fiber.
• Wood post bearing directly on concrete pier without a code-compliant post base or pressure-treated designation.
• Pressure-treated stamp absent or illegible — inspector cannot verify the treatment type and level.

Pressure-treated wood in contact with aluminum flashing or trim will corrode the aluminum. When PT lumber is used for sill plates adjacent to aluminum window frames or aluminum flashing at the base of the wall, install a barrier between the PT wood and the aluminum component. EPDM tape or a compatible flashing tape provides the separation needed to prevent galvanic corrosion at the contact point.