What the Markings on Treated Lumber Mean

Pick up a piece of pressure-treated lumber and you'll find more writing on it than on a plain board. There is usually a stapled plastic tag on the end, sometimes an ink stamp on the face, and often a field of small slits cut into the wood itself. Each of those is telling you something you need to know before you build, and most people ignore all of it.

The markings answer three questions: what chemical is in the wood, how much of it, and where the board is safe to use. Get those wrong and you can put above-ground lumber in the ground, where it rots years early, or overpay for heavy-duty stock a deck rail never needed. This guide reads the markings line by line so you can match the board to the job.

This is the treatment companion to the softwood grade stamp guide, which covers the strength grade printed on framing lumber. Treatment marks and grade stamps are two separate systems, and a treated board carries both.

The end tag, line by line

The stapled tag on the end of the board is the main label. It is printed by the treating plant and carries the details that matter. Here is what each part means.

• Exposure rating. Usually the biggest words on the tag: "Above Ground," "Ground Contact," or "Ground Contact - Heavy Duty." This is the single most important marking. It tells you whether the board can touch soil or must stay up in the air. More on this below.
• Retention number. A decimal like 0.15, 0.25, 0.40, or 0.60. It is the pounds of preservative left in each cubic foot of wood after treatment. Higher means more chemical and more protection.
• Preservative code. A short set of letters, such as ACQ, CA-C, or MCA, naming the chemical used. Different chemicals hit their protection level at different retention numbers, which is why the number alone means little without the code.
• Standard reference. Often "AWPA" with a use-category code like UC3B or UC4A. AWPA is the American Wood Protection Association, the body whose standards the whole system follows.
• Treating company and warnings. The plant that treated the wood, plus any handling notes, such as "for above ground use only" or a reminder to wear a mask when cutting.

Read the tag top to bottom and you have the full spec: what is in the wood, how much, and what it is rated to survive.

Exposure rating: the marking that matters most

Two ratings cover almost everything you will buy at a home center.

Above Ground lumber is treated for use where the wood stays off the soil and can dry out between rains: deck boards, railings, joists, fence rails, and trim. It carries a lighter dose of preservative because air and drying do part of the work. Put it in the ground and it will fail early.

Ground Contact lumber is treated to sit on or in the soil, or anywhere it stays damp and cannot dry, such as against concrete or in heavy shade. Posts, sill plates, deck framing near grade, and anything buried should be Ground Contact. Many builders now use Ground Contact everywhere on a deck because the small price bump buys a big safety margin.

A third tier, Heavy Duty Ground Contact, exists for permanent wood foundations, structural posts, and critical buried members. It is the same idea with a higher retention number.

If you remember one thing from the tag, remember this rating. It is the difference between a post that lasts decades and one that rots in five years.

The retention number, decoded

Retention is measured in pounds of preservative per cubic foot (pcf). The right number depends on the chemical and the exposure, which is why the tag pairs them. As a rough guide within the common copper-based treatments:

Retention (pcf)	Typical use
0.06 to 0.15	Above ground, dry exposure
0.21 to 0.25	General ground contact
0.31 to 0.40	Ground contact, damp or critical
0.60 and up	Heavy duty, foundations, buried structural

The exact break points shift a little between preservatives, so treat these as the shape of the scale rather than fixed law. What holds across all of them is the direction: a bigger retention number means more chemical in the wood and a board rated for wetter, harsher service.

The preservative code

The letters name the chemical. The common ones today are:

• ACQ (alkaline copper quaternary). A long-standing water-based treatment, copper plus a co-biocide. Reliable and widely stocked.
• CA-B and CA-C (copper azole). Uses less copper than ACQ for a similar result, so it is less corrosive to some fasteners and a touch lighter in color.
• MCA (micronized copper azole). A newer form where the copper is ground fine and suspended, which leaves a cleaner surface and less residue.

You may still see CCA (chromated copper arsenate) on industrial stock like utility poles and marine piles. It was pulled from residential lumber in 2004 because it contains arsenic, so you should not see it on deck or fence material. For interior wood kept dry, borate treatments (SBX or DOT) show up instead, aimed at termites and rot rather than weather.

Whatever the code, one rule follows from it: treated lumber eats ordinary fasteners. Use hot-dipped galvanized or stainless steel screws, bolts, and connectors, because the copper in modern treatments corrodes plain steel fast.

Those slits in the wood: incising

Look at the face of a treated post and you may see rows of small knife slits, as if someone ran it under a bed of blades. That is incising, and it is not damage. Some species, like Douglas fir and hem-fir, are dense and hard for preservative to soak into. Incising cuts thousands of tiny channels so the chemical can reach past the surface.

Southern yellow pine, the most common treated species in the eastern half of the country, soaks up treatment readily and usually needs no incising, so its face stays smooth. So the presence or absence of those slits is really a clue about the species under the treatment, which in turn tells you something about how the board will hold preservative over time.

Why the markings change your material takeoff

The tag does not change the size of the board, but it changes what you buy and how much it costs. Ground Contact stock runs more than Above Ground, Heavy Duty more again, and the price gap widens on long posts and thick timbers.

That is where your quantity math matters. Work out how much treated lumber the job actually needs before you stand at the rack comparing tags, so you are pricing the right volume at each rating. Run your post and framing list through the board foot calculator to get the volume, then price it against the exposure rating each piece truly requires. And because treated dimensional lumber is sold by the same nominal sizes as untreated, the nominal versus actual size guide still applies: a treated 2x6 measures the same 1.5 by 5.5 inches as a dry one.

Reading a tag in practice

Say you find a tag that reads: Ground Contact, 0.15 CA-C, UC4A, AWPA. Put together, that is a copper-azole treated board at a ground-contact retention, rated for general soil contact under the AWPA use-category system. Good for a deck post or a fence post set in the ground.

Now say the tag reads: Above Ground, 0.06 MCA, UC3B. That is a lightly treated, micronized-copper board meant to stay off the soil. Fine for a deck board or a rail cap, wrong for anything buried.

Same size, same species maybe, but two different jobs. The markings are the only way to tell them apart once they are on the truck, so read the tag before you load, not after you build.

Common questions

What do the numbers on treated lumber mean? The decimal, like 0.15 or 0.40, is the retention: pounds of preservative per cubic foot of wood. A higher number means more chemical and a board rated for wetter, harsher exposure.

What does "Ground Contact" mean on treated lumber? It means the board is treated to sit on or in soil, or anywhere it stays damp and cannot dry out. "Above Ground" lumber carries less preservative and must stay off the soil, or it rots early.

What is the tag on the end of treated wood? A label from the treating plant showing the exposure rating, retention number, preservative type, and the AWPA standard the treatment meets. It is how you know what the board is rated for.

What do ACQ, CA-C, and MCA mean? They are preservative codes. ACQ is alkaline copper quaternary, CA-C is copper azole, and MCA is micronized copper azole. All are modern copper-based, water-borne treatments, and all corrode plain steel, so use galvanized or stainless fasteners.

Why does some treated lumber have small slits cut into it? That is incising, cut to help preservative soak into dense species like Douglas fir and hem-fir. Southern yellow pine treats easily and usually skips it, so a smooth face often means pine underneath.