Most pressure-treated lumber is softwood: Southern Yellow Pine leads, with Douglas-fir, Hem-Fir, and SPF also used by region.
Types Of Wood Used For Pressure-Treated Lumber
Here’s the short list you’ll see at the yard across North America. The notes explain how each wood behaves in the treating plant and where it shines on a project. The notes below reflect treating practice across regions and the woods yards keep stock for deck, fence, and work.
| Species Or Group | Treatability & Notes | Typical Outdoor Uses |
|---|---|---|
| Southern Yellow Pine (SYP) | Easily treated; deep, uniform penetration. Common in the South and East; wide retail range. Often sold in many thicknesses and lengths. | Deck framing and decking, fence posts, sill plates, landscaping timbers, playground builds. |
| Douglas-fir | Refractory to water-based preservatives; usually incised (small slits) before treatment to improve penetration. Strong and stiff; common in the West. | Framing where high strength is handy; posts and beams; marine and utility work when treated to higher retentions. |
| Hem-Fir (Hemlock–Fir blends) | Similar to Douglas-fir for treatability; typically incised. Good availability in the Pacific Northwest and mountain states. | Deck framing, fascia, and trim that will be coated; general above-ground uses. |
| SPF (Spruce-Pine-Fir) | Mixed spruce and pines; often incised. Lighter weight; common in Canada and the North. | Above-ground parts, light deck framing, rails and balusters when treated for the right exposure. |
| Ponderosa, Lodgepole, Red & Jack Pines | Used regionally; treatability ranges by species and sapwood amount; incising common. | Posts, rails, ranch and farm structures; utility poles at higher use categories. |
| Western Red Cedar, Redwood | Usually sold untreated thanks to natural durability; when treated, penetration can be limited without incising. | Above-ground trim and siding, fence pickets, decorative parts; many builds use these without pressure treatment. |
Why Southern Yellow Pine Leads
SYP takes preservatives well, which means the treatment reaches heartwood and sapwood with fewer steps. That deep penetration delivers long service when matched to the right use category. The Southern Forest Products Association notes this ease of treatment and wide product mix across deck, fence, and framing sizes. It machines cleanly, takes fasteners well, and comes in broad stock at home centers and pro yards. Treaters also offer many profiles in SYP, so matching replacements years later tends to be easier.
Where Douglas-Fir And Hem-Fir Fit
In the West, mills lean on Douglas-fir and Hem-Fir. Both need incising before treatment to help the fluid reach target zones. Those tiny slits look like a grid on the surface. They can trim strength a bit, so engineers and code tables account for it. You’ll often see incised DF/HF in posts, beams, and framing where stiffness and span strength matter.
SPF And Other Regionals
SPF, Ponderosa, Lodgepole, Jack Pine, and a few other softwoods show up handily in northern stores. These species can be treated for many above-ground jobs and, at the right retention, for contact with soil. Many of them need incising, so expect the familiar rows of tiny cuts. Cedar and redwood rarely go through a pressure cylinder for decks because they already resist decay above ground; buyers pick them for looks and natural color.
Preservatives Used On Pressure-Treated Wood
Preservatives guard against decay fungi and wood-boring insects. The mix you see today is mostly copper-based for outdoor use, and borate for interior framing in termite zones. Current consumer lumber is dominated by ACQ, CA, and MCA; CCA lives on in industrial poles, pilings, and similar work.
Common Preservative Systems
- ACQ (Alkaline Copper Quaternary): Copper plus a co-biocide. Well-known greenish tone. Listed for decks, posts, and general exterior work.
- CA (Copper Azole): Copper with azoles as co-biocides. Often has a brown tint when dyed. Widely used for residential decking and posts.
- MCA/MCQ (Micronized Copper): Copper in fine particles, paired with co-biocides. Popular in many home-center boards.
- Borate: For interior framing in damp or termite areas; not for wet exterior exposure.
- CCA (Chromated Copper Arsenate): No longer used for most home projects since 2004; still allowed for poles, pilings, and some industrial uses.
For chemistry basics and where each system is allowed, see the NPIC treated-wood fact sheet.
What The End Tag Tells You
Every piece carries an end tag or ink stamp. Look for the preservative name or code, the “AWPA U1” mark, the Use Category (like UC3B or UC4A), and a retention number. That combo tells you where the board can live and for how harsh an exposure. The AWPA homeowner page breaks down each line on the tag in plain language.
Fasteners, Corrosion, And Finish
Copper-based preservatives can be tough on bare steel. Pick hardware that holds up, match coatings across parts, and give the wood a good topcoat once it dries to a stable moisture level.
Fastener Choices That Last
- Best pick: 300-series or 316 stainless screws and connectors in coastal or wet zones.
- Good pick: Hot-dip galvanized parts that meet ASTM A153 or A653 Class G185; keep nails, screws, and hangers in the same coating family.
- Avoid: Uncoated steel and thin electro-galvanized parts for exterior pressure-treated wood.
Hardware makers publish corrosion notes for treated wood and list coating classes that match their hangers and anchors. When in doubt, pick stainless for long, wet exposure.
Stain, Sealers, And Drying
Most treated boards arrive wet from the cylinder. Let them dry before film-forming finishes. If the tag says KDAT (kiln-dried after treatment), you can coat sooner. Penetrating stains with water repellents work well and keep checking in check. Always follow the finish label for new treated wood and recoat timing.
Buying Tips And Label Checks
Scan the tag first. Match the use category to your project, then scan species and grade. For ground contact, look for UC4A or higher. For decking above ground with good runoff, UC3A or UC3B is common. If you live in a wet climate or near the coast, bump to the tougher category when the design traps water. Ask the desk for KDAT if you need tight joints, rail kits, or trim that will be painted soon.
Incising: What Those Tiny Slits Mean
Incising helps refractory species like Douglas-fir and Hem-Fir reach target penetration. Those small cuts reduce some strength values, which code tables reflect. For most backyard projects, the difference is minor, but span charts and post sizing should always follow treated-lumber tables for the species and grade you’re using.
Color Isn’t A Guarantee
Green or brown tint comes from the treatment or dye. Color alone doesn’t tell you the use category or the retention. Always go by the tag. If tags are missing, pick another bundle.
Which Woods Get Pressure Treatment? Regional Picks And Projects
The yard’s mix mirrors the local mill stream. Local mills shape supply. In the South and much of the East, SYP dominates. In the Pacific states and the Rockies, Douglas-fir and Hem-Fir step in, often with incising. In the North and Canada, SPF shows up in many dimensions. Any of these can serve on a deck or fence when matched to the right use category, retention, and fasteners.
Deck Framing And Decking
For joists, beams, and ledgers above ground with good drainage, UC3 is typical. Where framing is within six inches of soil, near splash zones, or boxed in with poor airflow, jump to UC4A. Many builders use SYP for framing and pick a surface that suits the look: treated SYP, cedar, or a composite.
Posts And Ground Contact
Posts live hard lives. Soil contact, trapped moisture, and lawn sprinklers push decay risk. Pick UC4A for general ground contact and move to UC4B for heavy-duty jobs like permanent planters, retaining steps, or posts in poorly drained clay. When in doubt, size up the category and add gravel for drainage at the base.
Railings, Stairs, And Touch Points
Handrails and stair parts see sun, water, and hands. Treated SYP or incised DF/HF works well when sanded smooth and sealed. For a finer touch, many crews use treated framing with cedar or composite sleeves and caps above it.
Garden Beds And Play Structures
Modern residential preservatives like ACQ, CA, and MCA are registered by the EPA for home use. For edible gardens, many builders staple a liner on the inside face to keep soil off the wood and then top with a cap board. For swings and forts, stick with stainless or heavy hot-dip hardware and keep joints clear for drainage.
Use Categories That Guide Your Choice
AWPA use categories set the exposure level each piece can handle. Match the tag to the job and you’ll get better service life. Here’s a quick reference you can read at the aisle.
| Use Category | Exposure Summary | Common Projects |
|---|---|---|
| UC2 | Interior frames where dampness can occur; borate treated. | Bathroom plates, mudroom partitions in termite areas. |
| UC3A | Exterior, above ground, coated with quick runoff. | Deck joists with flashing, rails, fascia, trim that sheds water. |
| UC3B | Exterior, above ground, uncoated or poor runoff. | Decking boards, exposed stair parts, open fence rails. |
| UC4A | Ground contact, general use. | Fence posts, deck posts on footings near grade, garden borders. |
| UC4B | Ground contact, heavy duty. | Permanent planters, structural posts in wet soil, boardwalk sleepers. |
| UC4C | Ground contact, extreme duty. | Foundation systems, utility structures, freshwater pilings. |
| UC5A-C | Salt or brackish water, by region. | Pilings, piers, marine fender systems. |
Why The Category Matters
The category sets the minimum retention the treater must hit, and that directly shapes long-term performance outdoors. UC4 parts carry more preservative than UC3 parts. Pick the wrong category and decay risk goes up fast in damp places. Pick the right one and simple details like cap flashing, end-grain seal, and good airflow will carry your deck or fence a long time.
Field Care: Cuts, Holes, And Ends
Any cut exposes fresh end grain. Dab on a brush-on preservative that the tag allows for field treating. Seal end grain before hardware goes on, and keep holes tight to the shank size for screws and bolts. Where posts meet soil, coat the cut at the base and add gravel for drainage to reduce standing water.
Reading The Rack: Grades, KDAT, And Straightness
Grading still applies to treated wood. Look for No. 2 or better for deck framing and stairs, and select structural grades for long spans. KDAT boards stay straighter and take paint sooner, which helps on handrails, fascia, and trim. Sight every piece. Swap out badly bowed or twisted lengths before you leave the store.
Sustainability And Disposal Notes
Treated wood should never be burned in a home fire pit or stove. Scraps go to the landfill unless your area has a special program. When replacing older decks built before 2004, you may run into CCA lumber. Handle it with gloves, cut outdoors, and bag dust and debris. Check the EPA guidance for safe handling and disposal rules.
Bottom Line For Buyers
Pressure-treated lumber isn’t one wood; it’s a family of softwoods that accept preservatives to suit their job. SYP leads for ease of treatment and wide availability. Douglas-fir, Hem-Fir, and SPF fill in by region, often with incising to meet penetration targets. Read the end tag, match the AWPA category to your exposure, and pair the boards with stainless or heavy hot-dip hardware. Do that and your next deck, fence, or post will be better built from day one. Plan details with your local code desk today.