Pressure-Treated Plywood: Uses, Grades and Safety

Pressure-treated plywood is softwood plywood, usually a CDX-type sheathing grade, that has had a wood preservative forced deep into it under pressure. Copper-based treatments (ACQ, CA or micronized copper) protect it against rot, fungi and termites, so it lasts outdoors and in contact with damp ground. It is not waterproof and it is not furniture plywood: it is rough, wet when sold and full of chemicals.

Grades: above ground or ground contact

Use category	Where
UC3B, above ground	Outdoors, off the ground, where it can dry: siding, outdoor furniture frames, shed walls.
UC4A, ground contact	On or near the ground, or hard to replace: shed floors, skirting, raised beds, decks close to the ground.
UC4B, heavy duty	Permanent wet or structural ground contact, foundations.

Pressure-treated vs marine and exterior plywood

Exterior and marine plywood have waterproof glue but no protection against rot: they must be sealed. Pressure-treated plywood resists rot but is rough and should not be used for anything you touch or eat from. For outdoor furniture, use exterior or marine plywood and paint it; for floors and structures near the ground, use pressure-treated.

Advantages

Resists rot, fungi and termites for decades
Rated for ground contact in the right grade
Strong and cheap for outdoor structures
Same 4 × 8 sizes as ordinary plywood

Drawbacks

Not for indoor furniture, food contact or burning
Often wet when bought: it shrinks and warps as it dries
Copper corrodes plain steel screws and nails
Rough faces and voids: not a finish-grade plywood

What it is used for

Shed and dog house floors
Bases and skirts of outdoor structures
Subfloors over damp crawl spaces
Raised beds and outdoor storage boxes

Tips for working with Pressure-treated plywood

Use hot-dip galvanized or stainless fasteners rated for treated wood.
Wear gloves and a dust mask when cutting; sweep up the sawdust; never burn offcuts.
Let it dry for a few weeks before painting or staining.
Treat the cut edges with a brush-on preservative.

Frequently asked questions

Is pressure-treated plywood waterproof?

No. It resists rot, but it still absorbs water, swells and can delaminate if it stays wet. Paint or seal it, and slope surfaces so water runs off.

Can you use pressure-treated plywood indoors?

Building codes allow it where wood meets concrete, such as sill plates and basement framing, but not for furniture, cabinets, cutting boards or anything that touches food.

Can you paint pressure-treated plywood?

Yes, once it has dried out, usually a few weeks to a few months: water sprinkled on it should soak in rather than bead. Use an exterior primer and paint.

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