It's been years since the Environmental Protection Agency announced the phase-out of chromated-copper-arsenate (CCA)treated lumber. Here's what you need to know.
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Nailing deck


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.

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

A Chemical Bath Starts the Process

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.

Some freshly pressure-treatedlumber holds so much moisturethat puddles form around theheads of driven fasteners.


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 pressuretreated lumber to avoid inhalation of dust.
  • Wash your hands or any exposed skin thoroughly after working with PT lumber.
Girl drilling deck holes

No Perfect Finish Solution

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.

Clear film finishes adhere only to the top layer of cells. When the wood sheds the sun-damaged cells, the film quickly sloughs off.
Oil-based, penetrating finishes combine a natural wood look with UV-protection and ease of application.

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


White Oak

Pros: Dense and strong, moisture- and decay-resistant, accepts stains readily
Cons: Unavailable as dimensional deck lumber, pricey



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


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


Thermally-modified wood

Pros: Lightweight, stable, rot- and insect-resistant, chemical-free
Cons: Limited availability, split-prone