In This Article
CCA-Treated Lumber
New Pressure-Treated Lumber
Alternative Lumber Treatments
Decay-Resistant Species
Plastic Lumber                       View all DECKS & PORCHES Articles

Most builders choose pressure-treated lumber for the structural framework because of its low cost and high durability. At this time there are few viable alternatives. However, composite and synthetic products are beginning to enter the market and are worth considering, especially for environmentally sensitive sites, such as wetlands or other applications where clients object to the use of treated lumber.

CCA-TREATED LUMBER (on older decks)

Up until 2004, almost all residential decks were framed with lumber pressure-treated with chromated-copper arsenate (CCA). CCA was phased out due to environmental and health concerns and replaced prima­rily by alkaline copper quat (ACQ) and copper azole (CA).

The treated lumber is typically Southern yellow pine in the eastern and central United States and hem-fir in the West. The more expensive and stronger Douglas fir is also used in the West, but it is more likely treated with a related chemical ACZA, which uses ammonia and heat to drive the chemicals into the wood. ACZA is also in the western states and Canada to treat hemlock, spruce, and western pine.

CCA Health Concerns. If you have an older wood deck, the framing and wood decking are most likely treated with CCA. CCA’s safety has long been questioned by health and environmental advocates because of its heavy concentration of arsenic, a known carcinogen.

Although most experts agree that leaching of arsenic from CCA lumber is minimal and poses no significant health risk to homeowners, CCA does pose risks to workers who handle the wet wood or burn scraps, and pollu­tion around treating plants has been well documented. By the mid-1990s, the material was no longer sold for use in residential applications or playgrounds.

Existing Structures. The EPA does not consider existing installations of CCA-treated lumber a health hazard. However, for homeowners who are concerned about poten­tial exposure, researchers at the USDA Forest Products Laboratory (FPL) recommend periodically treating the pressure-treated lumber with a water-repellant or a semitransparent pene­trating stain. If you are sanding or cutting into an aging deck, it’s a good idea to wear an N95 respirator to avoid breathing in the dust.


CCA-treated lumber was voluntarily phased out in 2004 due to health and environmental concerns over the arsenic and chromium. The new formulations addressed these issues, but caused perhaps a bigger problem. The new copper-rich treatments  were far more corrosive to steel and other metals, putting the metal fasteners and metal connectors that hold a deck together at risk of early failure.

New Treatment Chemicals. The main chemicals that replaced CCA starting in 2004 are the waterborne, copper-based compounds ACQ (alkaline copper quat), and copper azole (CA). Neither of these 2nd-generation treatments contains any chemicals considered hazardous by the EPA. A bigger concern is the fact that the high concentration of copper makes the lumber more corro­sive to metal fasteners and flashing materials. This can cause flashing leaks and even structural failures over time unless corrosion-resistant materials are used.

Health Precautions. Despite the lack of chromium, arsenic, or other hazardous chemicals, wood treated with ACQ and CA have the same handling instructions as the older CCA-treated wood. Workers are advised by the EPA to wear gloves or wash hands after contact, wear an N95 dust mask when cutting, and not to burn the scraps. Like the older CCA materials, wood with copper compounds is not recommended for direct contact with food or drinking water.

Ground Contact. While most CCA lumber was rated for ground contact, much of the treated lumber sold today should not touch or be buried in soil. Because of the higher costs of the new chemicals, manufacturers only use enough ACQ and copper azole for the expected application of the lumber. For example, 4x4s and 6x6s, typically used as posts are treated for ground contact, but decking and 2x lumber are not. Because chemical retention levels are difficult to interpret with the new formulations, the best approach is to look at the label on the end  of each board for the usage rating “above ground” or “ground contact”. Unfortunately, not everyone reads or follows these details, so there have been a number of reported failures due to using the wrong material for ground contact.

Increased Corrosion. Not long after CCA- and CA-treated lumber hit the market, concerns grew about the excessive corrosion of metal fasteners and connectors. Because of their higher concentrations of copper, ACQ and copper azole are significantly more corrosive to steel, aluminum, and galvanized coatings than CCA (see Galvanic Corrosion ). Also the chromium and arsenic in older CCA-treated lumber  helped inhibit corrosion. Many factors affect cor­rosion rates, but some studies have found ACQ-treated wood to corrode untreated steel and  galvanized coatings at two to five times the rate of CCA. Read more about Fasteners for Treated Lumber.

Micronized Treatments. In response to corrosion problems, many lumber treaters have tweaked their formulations and introduced a new generation of “micronized” treatments. Micronized ACQ and CA use less copper and smaller particles that do not require chemical solvents (also corrosive) to disperse into the wood. The smaller particles are less prone to leaching, providing good long-term protection and less corrosion of metals. In addition, manufacturers claim that they provide a more natural looking wood.

These 3rd-generation wood treatments are marketed as MCQ, MCA, and under a number of trade names and account for most of the treated yellow pine decking used in the US. ACZA and other dissolved formulas are still required for many species use in the West and Canada, including Douglas fir, hemlock, spruce, and western pine.

While there is general agreement that the micronized formulas are significantly  less corrosive, concerns have been raised, mostly by competitors, that micronized treatments do not provide the same level of protection as ACQ and CA. The issues remain unresolved and there are many in the industry who would like every piece of wood 2×8 or larger, in addition to posts, to be treated for ground contact, providing an extra level of protections.

Large Timbers. Douglas fir, hemlock, spruce and other western species are typically “in­cised” with surface cuts for better penetration of treat­ment chemicals. Even with incising, however, it is difficult  to get chemicals deep into the heartwood of larger timbers. This leaves the center of beams vulnerable to rot, particularly in 4x and larger material.

With these species, it is important to treat all holes and cuts made on site with a liquid preservative. It is especially important to treat cut ends of posts and beams. Most experts recommend a solution of  minimum 1% copper naphthenate, or 2% for ground contact. If possible, don’t put cut ends in contact with the ground.

Decking Materials Pros Cons Costs
yellow pine,
 Inexpensive, strong, and readily available. Yellow pine tends to cup, twist, and check over time.  Wood preservatives corrosive to metal fasteners and connectors.  1 Economical for most projects. Wear gloves when handling and N95 dust masks when cutting. Avoid burning scraps. Embedded posts must be rated for ground contact. Do not put cut ends in the ground. With hem-fir, treat all field cuts and holes with copper naphthenate solution. Use stainless-steel or G-185 galvanized hardware.
Very strong,
Surface must be incised for good penetration. ACZA is very corrosive to metal fasteners.  2 Good choice for where high strength is required. Same worker precautions as above. Treat all field cuts and holes with copper naphthenate solution. Use stainless-steel or G-185 galvanized hardware.
Redwood, cedar Natural decay resistance, attractive appearance. Decay resistance best in heart wood and in old-growth timber; less available.  3-5 Good choice for premium projects where natural materials are preferred. Use structural grades in accordance with design loads.
Plastic structural lumber Resists rotting, checking, and splitting. Works like wood. Heavy. Too flexible for long spans. May seem bouncy due to high elasticity.  2-3 Recommended near ponds, wetlands, salt water, other wet locations. Use with caution in structural applications as few industry standards exist for these products.


There was never any credible evidence that the old CCA-treated lumber posed any health risk to end users (although pollution at the treating facilities was widespread). Although the new formulations are less toxic, they are also more prone to leaching into the surrounding soil. Due to concerns about chemical leaching and corrosion of metal fasteners, and growing interest in green building, manufacturers continue to develop an array of innovative products.

Borate Treatment. Wood preservatives based on borate compounds have been used for decades abroad and are slowly becoming available in the United States. Borates are non­corrosive to metals and harmless to pets and humans, but they are very effective against insects and decay. Borate’s main limitation is its tendency to leach out of wood that is buried in soil or exposed to regular wetting, making it unsuitable for decks or other exterior applications exposed to the weather.

One manufacturer has developed a borate-treated lumber, called EnviroSafe Plus (ES+Wood), that uses a polymer binding to seal the borate into the wood . The wood is warrantied  for use as decking, deck framing, and other exposed applications not in contact with the ground.  However, the product is not yet widely available.

Carbon-Based Treatments. Another approach to the corrosion issue is to get rid of the copper altogether by using other commercial pesticides and fungicides that do not contain any corrosive metals. Both Ecolife and Wolmanized L3 Outdoor Wood use this approach. To distinguish these from the widely used copper-based preservatives, and perhaps play up the “green” angle, both products are marketing as “carbon-based,” since the chemicals used are “organic chemicals,” which are based on carbon.  So carbon-based sounds nice, but it doesn’t mean non-toxic, environmentally friendly, or much else, since all pesticides are carbon-based.

Wood treated with these products lacks the familiar green tinge, an advantage for some uses. Some contractors and homeowners, however, prefer treated wood with the familiar greenish color.

TimberSIL is a unique product for both framing lumber and decking. It uses a patented process to fuse microscopic glass particles into the wood fibers giving the wood added strength, dimensional stability, and fire resistance, in addition to resistance to decay and insects. It cuts and fastens like ordinary Southern Yellow Pine, but cut ends in contact with the ground need special treatment with an epoxy-based product and predrilling is needed for decking screws near an end cut. The material is completely non-toxic and non-corrosive to metal and carries a 40-year warranty. Reports from the field have been mixed, however, on it’s long-term durability and it does not code-approved by was of an ICC evaluation report.


For a price, redwood and cedar are available in structural grades. How rot-resistant the untreated wood is depends on the amount of extractives in the wood, which is great­est in the heartwood cut from dense, old-growth trees. To purchase all-heart, structural-grade redwood or cedar, expect to spend two to three times more than for pressure-treated lumber. It is also difficult to find away from the West Coast. Left untreated, even the heartwood of these species is not recommended for ground contact.


Structural lumber products made of recycled plastics are starting to make their way into the marketplace and are turning up as railroad ties, dock components, and park walkways. Since there are few standards available for these products, designers and installers will need to rely on man­ufacturer data for structural characteristics.

One product, TriMax, is made of recycled plas­tic and fiberglass and has compression and horizontal-shear strength similar to treated yellow pine. Like most plastic products, however, the material is more flexible than wood due to a low modulus of elasticity. This means spans must be small and the structure may have a bouncy feel. TriMax is sold for both framing and decking.



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