Overview
Pressure-treated (PT) lumber is softwood, usually southern yellow pine, forced full of preservative under pressure so it resists rot, fungal decay, and wood-boring insects. It is what you reach for anywhere wood touches moisture, sits on or near the ground, or contacts concrete. Untreated framing lumber in those spots rots out in a few seasons; PT survives for decades when you spec it right.
Two things decide whether a PT install lasts or fails, and most people only think about one of them. The first is the retention level: how much preservative is driven into the wood, which sets what exposure the board is rated for. Decking off the ground is one rating; a post buried in soil is another. The second is the fasteners. Modern treatments are copper-based, and copper eats regular steel. Use the wrong screws and the wood outlives the hardware holding it together.
The chemistry today is waterborne copper systems with names like ACQ, copper azole, and micronized copper. They replaced the old arsenic-based CCA for residential use years ago. Get the retention and the fasteners right and PT is the cheapest durable lumber you can buy. Get either wrong and you are rebuilding.
Retention Levels and Exposure Ratings
Retention is measured in pounds of preservative per cubic foot of wood. More preservative means a higher use category and a tougher exposure rating. You do not have to memorize the numbers; you have to read which category a board carries and match it to the job.
There are three you will actually buy. Above Ground (UC3) is for wood that stays off the dirt and drains freely: deck boards, railings, fascia. Ground Contact (UC4A) is for wood touching soil, concrete, or anywhere drainage is poor and the board stays damp. Plenty of pros, me included, run Ground Contact everywhere on a project because the upcharge is small and the failure mode is expensive. Then there is heavy-duty (UC4B) and the critical structural and permanent wood foundation grade (UC4C), the highest retentions, for poles, pilings, and load-bearing wood that goes in the ground and must never fail.
Every piece carries an end tag stapled to it. Read it. The tag states the use category and exposure. The single most common PT mistake is grabbing Above Ground lumber, burying it or setting it on grade, and watching it rot in a few years. The wood looks identical. The tag is the only thing that tells you the truth.
The Treatment Chemistry
Modern PT uses waterborne preservatives, and they are all copper-based. The common ones are ACQ (alkaline copper quaternary), CA or copper azole, and MCA, the micronized copper formulations that leave the wood a lighter green-to-brown and bleed less copper. These replaced CCA, chromated copper arsenate, which contained arsenic and got pulled from residential use. CCA still exists, but only for industrial uses like utility poles and marine pilings.
Copper is what does the work, killing fungus and insects, and copper is also why these treatments are hard on metal. The same element that protects the wood corrodes the steel you fasten it with, which is the whole reason fastener selection matters so much on PT. Higher retention means more copper in the wood, so ground-contact lumber is more aggressive on fasteners than above-ground.
One thing people forget: PT ships wet. The treatment is waterborne, and the wood comes off the truck heavy and soaked, sometimes still dripping. That water has to leave, and as it does the board shrinks, checks, and can cup or twist. You plan around that, both in how you fasten it and how soon you seal it.
Fasteners: The Part People Get Wrong
The copper in modern PT corrodes regular and electro-galvanized steel fast. A bright deck screw or a cheap electro-galv nail in ground-contact lumber can be rusted to nothing in a handful of years. The wood is fine; the fasteners are gone, and that is how a deck comes down. Failures on treated lumber are usually rusted-out hardware, not rotted wood.
Use hot-dipped galvanized (HDG) that meets ASTM A153, stainless steel, or fasteners and hardware specifically rated for ACQ and modern treated lumber. The manufacturer's coating callout is on the box; if it does not say it is approved for treated lumber, do not use it. Near saltwater or in coastal air, skip the galvanized and go stainless; the chloride plus copper combination chews through zinc coatings too.
The connectors matter as much as the screws. A joist hanger, post base, or hurricane tie has to match: G185 hot-dipped galvanized or stainless, never the bright or G90 stuff. Mixing metals makes it worse; a stainless screw through a galvanized hanger sets up galvanic corrosion. Match the whole system to the lumber and to the environment, then do not mix.
Working With PT Lumber
Because PT ships soaked, the biggest jobsite reality is movement. As the board dries it shrinks across its width, checks along the grain, and sometimes cups or bows. On decking, that means gapping right: tight boards that look perfect on install day open into uneven gaps as they dry. If the lumber is genuinely wet, you can butt boards closer because they will shrink apart; if it has already dried on the rack, gap them. Fasten well and the board has less room to twist.
Field cuts are the other thing people skip. Every saw cut exposes untreated wood inside the board, and the pressure treatment does not reach the core at full strength. Brush every cut end and every bored hole with a copper-based field-cut preservative, especially anything that will sit in contact with the ground. Skipping it is how a post starts rotting from the cut end up.
Let decking dry before you seal or stain it; sealing in trapped moisture defeats the finish. And handle the dust safely: wear a mask when you cut, do not breathe the sawdust, and never burn PT scraps. The treatment chemicals do not belong in your lungs or in a fire.
How to Choose / Common Mistakes
The decision is short. Match the retention rating to the exposure, and when you are not certain, go Ground Contact (UC4A); the price bump is small and it covers you. Use fasteners and connectors that are HDG to ASTM A153, stainless, or labeled for treated lumber, and go stainless near the coast. Read the end tag before the board goes in. Treat every field cut. Gap and fasten for the shrinkage that is coming as it dries.
Two failures account for most dead PT structures. The first is Above Ground lumber set in the ground or on grade, which rots from the bottom because it never carried enough preservative for soil contact. The second is the wrong fasteners, where the copper corrodes bright or electro-galv steel until the hardware lets go. Both are free to avoid and expensive to fix.
PT is not always the right wood. For visible decking surfaces, naturally durable species like cedar or a composite board often beat PT on looks and upkeep, which is a separate call covered in our decking materials comparison. PT earns its keep on structure, posts, ledgers, joists, and anything in or near the ground, where durability per dollar is what counts.
FAQ
Can I use pressure-treated lumber for a deck surface? Yes, and plenty of decks are all PT. Use Above Ground (UC3) rated boards or better for the deck surface and railings, and Ground Contact for posts and anything touching the ground. Let the boards dry before you seal or stain them. If you want better looks and lower upkeep, cedar or composite are the alternatives, covered in our decking materials comparison.
Why do my deck screws keep rusting? Because they are the wrong screws. The copper in modern PT corrodes regular and electro-galvanized steel. Switch to hot-dipped galvanized rated to ASTM A153, stainless, or screws specifically labeled for treated lumber, and go stainless if you are near saltwater.
Do I really need to treat the cut ends? Yes, on anything exposed to weather or ground contact. A saw cut opens up untreated wood inside the board, and that is where rot starts. Brush cut ends and bored holes with a copper-based field-cut preservative.
What is the difference between Above Ground and Ground Contact? Retention, meaning how much preservative is in the wood. Ground Contact (UC4A) carries more and is rated for soil, concrete, and damp, poorly drained spots; Above Ground (UC3) is for wood that stays off the dirt and drains. When in doubt, use Ground Contact.