What treatment does
Pressure treatment forces preservative deep into the wood, so the lumber resists rot, fungi and insects. Untreated lumber has none of this and decays in a few years outdoors when it is wet. Treated lumber costs more per board foot, so use it only where it is needed. The lumber cost calculator helps compare quotes.
Use categories
American Wood Protection Association (AWPA) categories describe where treated wood can go. The tag tells you which one the board meets.
| Category | Meaning | Typical uses |
|---|---|---|
| UC2 | Above ground, damp | Sill plates |
| UC3A | Above ground, coated, quick runoff | Porch flooring, protected trim |
| UC3B | Above ground, uncoated, slow runoff | Deck boards, railings, fence pickets, joists and beams |
| UC4A | Ground contact, general use | Posts, ledgers, landscape walls, planters |
| UC4B | Ground contact, heavy duty | Permanent wood foundations |
Anything that touches the ground or sits in soil needs UC4A or better. Deck joists are above ground but exposed to weather, so UC3B is the minimum. Do not use a board rated for above ground in the soil.
Use untreated lumber for
- Interior framing, walls, floors and ceilings that stay dry.
- Furniture, shelving and trim indoors.
- Anything you will seal and paint where it is out of the weather and never wet. If in doubt, treated is safer.
Fasteners and handling
- Hardware. Treated wood is more corrosive to plain steel than untreated wood. Use fasteners and connectors made for it, such as hot-dip galvanized or stainless steel. Check the maker's instructions.
- Cutting. Wear gloves and a dust mask, and treat cut ends with end-cut preservative where the maker says to.
- Disposal. Do not burn treated wood. Dispose of scraps as the local rules require.
Sizes are the same
A treated 2x4 is the same 1-1/2 by 3-1/2 inches as an untreated one. Treated boards are often wetter when new, so they shrink as they dry. See the 2x4 lumber guide for the grade stamp, and standard 2x4 lengths for what is stocked.
Frequently asked questions
Can I use untreated lumber outside?
For a short time, or where it stays dry and sealed. Any wood that gets wet or touches the ground should be treated.
What is UC4A lumber?
Pressure-treated wood rated for ground contact in general use, such as posts and ledgers.
What screws should I use with treated lumber?
Hot-dip galvanized or stainless steel fasteners made for treated wood, following the maker's instructions.
Sources
- AWPA: use categories for residential treated wood (PDF)
- Journal of Light Construction: Selecting hardware for use with treated wood