Pressure Treated Wood Directly on Concrete: What Code Allows

Yes, pressure treated lumber can legally sit on concrete. IRC R317.1 says sills and sleepers on a slab must be preservative-treated or separated from it by an impervious moisture barrier. Legal is not the same as long-lasting, though: a board that stays wet will still decay, and wood rated only for above-ground use will fail if it is buried or embedded. This guide covers the code wording, the treatment rating to buy, what to put between wood and slab, and fasteners.

Can Pressure Treated Wood Go Directly on Concrete?

Most advice online says never let treated wood touch concrete. The code does not say that. Every house framed on a slab or foundation wall has a pressure treated sill plate sitting on concrete, normally with a foam gasket under it, and inspectors pass it every day. What the code requires is a choice: use treated wood, or use a barrier. Either one satisfies it.

The real rule of thumb is about water, not contact. Concrete is porous and pulls moisture up from the ground and holds rain on its surface. Treated wood resists decay, but it does not repel water. Where a board can dry out between rains, direct contact is fine. Where it stays wet, the preservative buys time and nothing more.

What the Code Actually Says About Wood and Concrete

The residential code wording comes from IRC R317.1 and R317.1.2. Local codes adopt different editions and add amendments, so confirm with your building department before a permitted project.

Situation	What the code requires	What that means in practice
Sills and sleepers on a slab in direct contact with the ground	Treated or naturally durable wood, unless separated from the slab by an impervious moisture barrier	Treated lumber alone passes; untreated lumber needs a barrier
Wood embedded in concrete that touches the ground, or in weather-exposed concrete supporting a permanent structure	Pressure-preservative-treated wood suitable for ground contact	Above-ground lumber is not acceptable here
Girder ends entering exterior concrete or masonry walls	Treated wood, or at least 1/2 inch clearance on tops, sides and ends	Leave an air pocket around the beam end
Joists or floor bottoms within 18 inches of exposed ground in a crawl space	Treated or naturally durable wood	Applies to the framing above the soil, not only the concrete

Code point worth knowing: the sleeper rule is either/or. Treated wood satisfies it with no barrier, and a barrier satisfies it with untreated wood. Source: IRC R317.1, item 3.

Why Wet Concrete Still Rots Treated Wood

Decay fungi need water more than they need a food source. USDA Forest Products Laboratory research puts the minimum wood moisture content for fungal growth at about 20 percent, with serious decay once the wood is above its fiber saturation point of roughly 25 to 30 percent. A board lying flat on damp concrete can sit above those numbers for months.

Three things make the concrete edge the weak spot:

• Capillary wicking. The slab pulls ground moisture up and hands it to the wood face that touches it.
• Trapped water. A flat board against a flat slab leaves no gap, so rain and condensation have nowhere to drain.
• Unprotected cut ends. The preservative only reaches part of the way into the lumber, so a fresh cut exposes untreated wood right where moisture is highest.

Concrete itself does not chemically attack treated lumber. The damage is the wet environment, and the hardware tends to fail before the wood does, which is covered in the fastener section below.

Which Treatment Rating Do You Need?

Every treated board carries an end tag or stamp showing its American Wood Protection Association use category. The category matters more than the brand. For a deeper breakdown of the ratings, see our guide to ground contact pressure treated lumber.

Use category	Intended exposure	Use it for
UC3B	Exposed above ground, weather-wetted, with good drainage	Deck boards and rails that stay off the soil and the slab
UC4A	General ground contact	Sleepers on an outdoor slab, fence posts, landscape timbers
UC4B	Heavy-duty ground contact, critical or hard-to-replace parts	Structural posts and anything set in or against concrete in wet soil

For sleepers on a patio or garage slab, buy lumber tagged for ground contact. The code text only says treated, but an outdoor slab is a wet surface, and the higher-rated board costs little more than the above-ground board. Never use an above-ground tag for anything embedded in concrete or buried.

What to Put Between Pressure Treated Wood and Concrete

Pick the break based on how wet the slab will be, not on which product is cheapest:

Option	What it does well	Where it falls short	Best use
Foam sill gasket	Breaks capillary contact; seals small gaps; cheap	Compresses over time; not waterproof	Sleepers and sill plates on a dry slab
6-mil polyethylene sheet	Blocks liquid and vapor from the slab surface	Outdoors it can trap rainwater on top of the sheet	Indoor sleeper floors over a basement slab
Pavers, deck blocks or post bases	Lifts the wood off the slab so it can dry	Adds height and cost	Outdoor decks and platforms
Composite or plastic shims	Will not rot or compress	Spot support only; needs spacing	Leveling sleepers and joists
Nothing (treated wood, direct)	Code compliant for sleepers	Nothing stops moisture; end cuts rot first	Dry, covered slabs only

A detail that trips people up: the 6-mil polyethylene the IRC requires (R506.2.3) goes under a slab, between the concrete and the gravel, with seams lapped at least 6 inches. The 2021 edition briefly asked for 10-mil, and the 2024 edition returned to 6-mil. That under-slab layer is a different job from a sheet laid on top of the slab beneath your sleepers.

Indoor Slabs vs. Outdoor Slabs

Basements and garages

Indoors, the goal is to keep slab moisture out of a sleeper floor. Test the slab first: tape an 18-inch square of clear plastic sheet to the concrete, seal all four edges, and leave it for 16 to 24 hours (the plastic sheet method in ASTM D4263). Fogging or dark patches mean the slab is releasing moisture and needs a sealer or a proper barrier. If you are building a wood floor over concrete, our guide on how to lay a hardwood floor over concrete walks through the moisture tests and subfloor options in detail.

Patios, porches and decks

Outdoors, a sealed plastic sheet is often the wrong answer because rain gets in at the edges and sits on top of it. Use points of support instead: pavers, deck blocks or post bases so air moves under the frame. Check that the slab slopes away from the structure, and keep the lowest wood at least an inch above where water pools.

Embedding Pressure Treated Wood in Concrete

Posts set in a concrete footing or slab are the hardest case, because the wood is wet at the base and cannot dry. The code requires wood suitable for ground contact wherever it is embedded in concrete that touches the ground or is exposed to weather. Use UC4A at minimum and UC4B for structural posts.

Even then, a metal post base that holds the post about an inch above the concrete usually outlasts burying the end, because water drains away from the end grain. If you do set wood in concrete, slope the top of the concrete away from the post so rain does not pool at the collar.

Concrete formwork is a separate case. Form boards and stakes are temporary and removed after the pour, so standard framing lumber is normal. Treated wood only matters for pieces that stay in place.

Fasteners and Anchors for Treated Wood on Concrete

Copper-based preservatives corrode steel faster than the older CCA treatment did. Simpson Strong-Tie states in its preservative-treated wood FAQ that hot-dip galvanized or stainless steel fasteners are recommended, that exterior wet locations need upgraded ZMAX or hot-dip galvanized coatings at minimum, and that galvanized and stainless components should not be mixed because dissimilar metals corrode faster.

Location	Fastener choice
Dry, covered, interior	Standard G90 galvanized connectors are acceptable for common copper-based treatments
Exterior, wet	ZMAX (G185) or hot-dip galvanized as a minimum
Ground contact or high retention	Stainless steel, copper or silicon bronze
Do not combine	Galvanized and stainless in the same connection

Anchor bolts into concrete are handled differently. Simpson notes that anchor bolts 1/2 inch and larger in dry mudsill applications are exempt under IRC R317.3.1, but that is for a sill plate that stays dry. For sleepers on an outdoor slab, use stainless or hot-dip galvanized anchors instead.

Treat the Cut Ends

Every cut, drilled hole and notch exposes untreated wood. The American Wood Council recommends field treating cut ends with a preservative, and AWPA M4 specifies a copper naphthenate solution for that job. Brush it on until the surface is wet, follow the can, and do it before the board goes down, because the end that sits closest to the slab is the end that rots first. Finishing is a separate question: see when pressure treated lumber can be stained before you seal anything.

Common Mistakes

• Using an above-ground tag on sleepers or posts that touch concrete or soil.
• Laying plastic sheet on an outdoor slab, which traps rain instead of draining it.
• Skipping end-cut preservative because the lumber looks green all the way through.
• Mixing galvanized screws with stainless brackets.
• Setting a flat board tight against the slab with no gap or shim for airflow.
• Starting a sleeper floor before checking whether the slab is still releasing moisture.

The Bottom Line

Treated lumber on concrete is allowed. Whether it lasts depends on the tag, the water, and the hardware. Buy ground-contact lumber for anything that touches a slab, put a foam gasket or shims under it so it can breathe, brush the cut ends, and use stainless or heavy hot-dip galvanized hardware. If you are unsure about your local code, ask your building department; this guide does not replace it or an engineer for structural work.

Keep reading

• Can You Use Pressure Treated Wood for Floor Joists?
• Pressure-Treated Wood vs Regular Wood: What Actually Changes
• Ground Contact Pressure Treated Wood: What to Bury, What to Skip

Frequently Asked Questions

Can pressure treated wood touch concrete?

Yes. The IRC allows treated sills and sleepers to rest on a slab, and treated sill plates sit on concrete foundations in most houses. The risk is moisture, not contact, so keep the wood from staying wet by adding a gasket, shims or air space.

Do you need a barrier between pressure treated wood and concrete?

Not under the code if the wood is treated. IRC R317.1 requires either treated wood or an impervious moisture barrier under sleepers and sills on a slab. A barrier or foam gasket is still a smart upgrade because it keeps the wood drier and the board lasts longer.

Can you embed pressure treated lumber in concrete?

Only if it is rated for ground contact. The code requires wood suitable for ground contact where it is embedded in concrete that touches the ground or is exposed to weather. Use UC4A at minimum, UC4B for structural posts, and never above-ground rated lumber.

What fasteners should you use for pressure treated wood on concrete?

Use hot-dip galvanized or stainless steel. Simpson Strong-Tie recommends ZMAX or hot-dip galvanized at minimum in exterior wet locations, and stainless, copper or silicon bronze for ground contact. Do not mix galvanized and stainless parts in one connection.

Is foam sill gasket enough for sleepers on a patio slab?

It helps, but it is only a capillary break. A foam gasket works on a dry or covered slab. On an open patio that collects rain, lift the frame on pavers or deck blocks so air can move underneath and water can drain away.