Tricks for Treated Lumber
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Wood Magazine

Tricks for Treated Lumber

It's been seven years since the Environmental Protection Agency announced the phase-out of chromated-copper-arsenate (CCA)treated lumber. CCA's replacements, although safer, bring different challenges to your outdoor projects. Here's what you need to know.

Nailing deck
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4 screws
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Hot-dip galvanized fasteners have a
thicker zinc coating and therefore
provide more rust resistance than
fasteners galvanized by other
2 screws
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Stainless-steel fasteners are nearly
impervious to corrosion, but can cost
twice as much as hot-dip galvanized.
6 screws
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Some manufacturers use proprietary
anti-corrosion layers. They are often
color-matched to blend with the
wood. Many offer a lifetime guarantee
against corrosion.


CA, ACQ, MCQ, Who?

Alkaline copper quaternary (ACQ) and copper azole (CA) accepted CCA's abdication of the outdoor-lumber throne with a fairly smooth transfer of power. Both treatments provide similar, high levels of preservative power with arsenic-free chemicals.

In order to achieve the preservative power of CCA, ACQ and CA ramp up the amount of copper used. But because copper speeds corrosion in ferrous metals, steel fasteners must be chosen with care. (See photos.)

The newest kid on the block, micronized copper quat (often marked MCQ) substitutes dissolved copper with a finely ground copper particulate. While it doesn't yet have the long, proven track record of ACQ and CA, proponents claim MCQ achieves the same level of wood preservation and rot-resistance with a lowered level of fastener corrosion.


A Chemical Bath Starts the Process
Some freshly pressure-treated
lumber holds so much moisture
that puddles form around the
heads of driven fasteners.

A Chemical Bath Starts the Process

Wet and warped

The new breeds of PT lumber still start out as stacks of Southern yellow pine placed into large, sealed vats where air is evacuated and the preservative chemicals are introduced. The low pressure draws the chemicals into the sapwood. Because there is little chance for it to dry between its chemical bath and the time it hits the home-center shelves, PT lumber can have a moisture content percentage into the low-to-mid 20s. (Compare to kiln-dried hardwoods that average 6-8 percent.) Once left in your shop or a sunny spot, the drying--and the warping--begins.

The best approach to counter this: Let the wood acclimate outdoors in the shade for several weeks; then fasten it firmly in place with screws. One common mistake for beginning deck-builders is to leave a gap for drainage between the decking boards. This strategy isn't necessary for PT lumber; the shrinkage during drying will provide all the gap necessary.




Are PT projects safe?

There's no need to run out and demolish that weathered pressure-treated (PT) deck for fear of arsenic leaching from older CCA-treated lumber. The majority of the leaching seems to happen in the first year.

But if it's time to replace the deck anyway, do take some precautions as you?re working with both old and new PT lumber.

• Clean up and dispose of all debris through your municipality?s trash collection. Never burn PT lumber: The smoke and ashes can contain toxic chemicals.
• Along with gloves and safety glasses, wear a dust mask when working with pressure-treated lumber to avoid inhalation of dust.
• Wash your hands or any exposed skin thoroughly after working with PT lumber.


No Perfect Finish Solution
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Clear film finishes adhere only to the
top layer of cells. When the wood
sheds the sun-damaged cells, the
film quickly sloughs off.
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Oil-based, penetrating finishes
combine a natural wood look with
UV-protection and ease of

No Perfect Finish Solution

But can I finish it?

PT lumbers' transformation to a weathered gray color is almost as inevitable as the sun and rain, primarily because it is caused by the sun and rain. Because there's no perfect finishing solution, you must choose either a durable finish that hides the wood grain or a lightweight finish that requires frequent reapplication.

Any outdoor finish heavy on pigments, such as paint or solid-color stains, provides the best protection against damaging UV light. Without those sun-screening pigments, clear, film-forming finishes, such as varnish and polyurethane, quickly slough off as the wood's surface degrades, shown in photos. Skip these clear or low-pigment finishes unless you enjoy refinishing every 6-12 months after a thorough sanding.

For a good compromise between the two, choose a penetrating-oil finish with finely ground, UV-inhibiting pigments, such as Penofin 550 (Performance Coatings, 800/736-6346,; Ready Seal (Ready Products, 888/782-4648,; or Total Wood Preservative (Gemini Coatings, 800/262-5710, The near-microscopic trans-oxide pigments effectively block most UV rays. You'll have to re-coat about every two years, but because you don't have to remove the previous finish, application is easy compared to other options.


Alternative Outdoor Woods

Alternative Outdoor Woods

Strong pressure-treated wood resists decay and insect damage at a cost of about $1 per lineal foot. But these alternative outdoor materials bring their own advantages to the backyard table.


Pros: Lightweight, naturally decay-resistant
Cons: Requires replacement about every 10 years
Cost per lineal foot: $1-2

White Oak

Pros: Dense and strong, moisture- and decay-resistant, accepts stains readily
Cons: Unavailable as dimensional deck lumber, pricey
Cost per lineal foot: $2-3


Pros: Super-dense, strong, and stable; resists warping, cracking, decay, and denting; life-expectancy of 40-50 years with proper care
Cons: Heavy, pricey, and sometimes hard-to-find
Cost per lineal foot: $3-4

Wood/plastic composites

Pros: Rotproof, defect-free, and dimensionally stable; widely available in home centers
Cons: Not for structural applications, becomes hot to the touch in sunlight
Cost per lineal foot: $2-3

Thermally-Modified Wood

Pros: Lightweight, stable, rot- and insect-resistant, chemical-free
Cons: Limited availability, split-prone
Cost per lineal foot: $2



Wood Magazine