Frequently Asked Questions (FAQs)

Preserved wood basics

Pressure treatment is a process that forces preservatives into the wood. Wood is placed inside a closed cylinder, the cylinder is filled with preservatives and then vacuum and pressure are applied to force the preservatives into the wood. The preservatives help protect the wood from attack by termites and fungal decay.

Wood is a plentiful and economical building material which comes from a renewable and sustainable resource. However, untreated wood is subject to attack by insects, micro-organisms and decay fungi. To ensure structural soundness and long service life, wood must be protected from its natural predators.

This is especially important in hot and humid climates or wherever wood comes into contact with the ground or water, since wood is subject to fungal decay and attack. In short, preserved wood provides users a longer-lasting product by extending the service life and the useful life of structures, indoors or outdoors.

Most preservative manufacturers offer a limited lifetime warranty on their preservative products against fungal decay and termite attack. However, it is safe to say most preserved wood products will last at least 25 to 40 years or for the lifetime of the intended application. There are cases where preserved wood utility poles have been in service for over 100 years.


There are two categories of preservatives, based on the carrier used to infuse the preservatives into the wood cells: waterborne preservatives and oil-type preservatives. Common waterborne preservatives include:

  • Alkaline Copper Quat (ACQ)
  • Ammoniacal Copper Zinc Arsenate (ACZA)
  • Copper Azole (CA)
  • Chromated Copper Arsenate (CCA)
  • Propiconazole Tebuconazole Imidacloprid (PTI)
  • DCOI/Imidacloprid/Stabilizer (EL2)
  • Inorganic Boron (SBX-DOT)
  • Micronized Copper Azole (MCA)


Common oil-type preservatives are:

  • Copper Naphthenate (CuN)
  • Creosote & Creosote Petroleum Solutions
  • Pentachlorophenol (Penta)
  • 3-IODO-2-Propynyl Butyl Carbamate (IPBC)


It depends on the end-use application. Waterborne solutions are better in consumer applications where there may be human contact. Oil-type solutions like creosote, penta or copper naphthenate are suitable for mostly industrial applications.

For the preservative you choose for Western preserved wood, make sure it meets the appropriate American Wood Protection Association (AWPA) standard and has been inspected by a qualified, third-party, ALSC accredited agency.

Wood treated with waterborne preservatives may be used anywhere termites and fungal decay attack could occur. Building codes require preserved wood to be used in many residential and non-residential building applications.

Other applications include docks, marinas, bridges, highway sound barriers, cooling towers, agricultural poles and stakes, decks, fences, outdoor lighting fixtures, trellises, railings, roller coasters, playground equipment and landscape architecture.

Oil-type preservatives are used for industrial applications such as poles and posts for agriculture; laminated beams; lumber and timbers for bridges; decking and cribbing for highway construction; lumber, timbers and piling for marine applications; utility poles and crossarms; and for treating railroad ties.

Penta is widely used for oil-type treatment of utility poles and crossarms. Also, vaulted ceilings over sports arenas, swimming pools, churches, and shopping centers are supported by glu-laminated beams treated with penta.

Wood treated with creosote is primarily used for railroad ties, trestles, utility poles and piling. It is also widely used for timbers in highway construction and bridges, as well as for marine use such as bulkheads, docks and seawalls.

Wood treated with copper naphthenate is used as utility poles, bridge and highway timbers, posts, lumber and glu-laminated beams.

Borates are an excellent preservative for treating wood, however the end uses of borate-treated wood are limited. The standard states that it must be “continually protected from liquid water” which means it is not appropriate for exterior applications such as decking or ground contact.

But, it is an excellent choice for such building applications as interior framing or sill plates. Borates are also being used as a dual treatment with creosote or copper naphthenate in exterior applications to provide additional protection to the wood.

The preserved wood industry voluntarily removed CCA-treated lumber for consumer (residential) applications in 2003. The few consumer applications remaining  include CCA-treated plywood, shakes and shingles.

CCA is still widely used for industrial and commercial applications such as highway and marine construction, utility poles and piling.

Wood treated with waterborne preservatives may be used anywhere termites and fungal decay attack could occur. Building codes require preserved wood to be used in many residential and non-residential building applications. For example, it may be used for permanent wood foundations, floor joist, sill plates, plywood sub-flooring, floor trusses, and high moisture (kitchen, laundry, bathroom, etc.) framing.

Wood treated with waterborne preservatives emits no vapors or fumes and may be used inside residences as long as all sawdust and construction debris are cleaned up and disposed after construction. Wood treated with oil-type preservatives should not be used in interior applications, except for treated laminated beams that are exposed to high humidity such as above-ground swimming pools.

Retentions are measured in pounds of preservative per cubic foot (pcf) and are shown on the end tags of preserved wood products. In the past, retentions were consistent for most waterborne preservatives, but that is no longer the case.

Because of the variety of preservatives used today, products for the same use may have different retentions based on the preservative used. These retentions are based on testing that has confirmed the effectiveness for the each specific use condition. This balances the appropriate amount of preservative needed for protection with the need to safeguard the environment where the products are used.

Safe use of preserved wood

Yes, common wood treatments such as ACQ or CA-C may be used for garden applications such as raised beds or boxes. Very small amounts of the preservatives may migrate out of the wood over time, but they will not create unsafe or unhealthy conditions. To alleviate any concerns about preservatives migrating into the plants, place plastic sheeting between the preserved wood and the garden soil. See a research report by Oregon State University on the use of preserved wood for garden boxes.

ACQ and CA-C can be used to construct outdoor picnic tables. However, we recommend that the table be covered to prevent food from contacting the surface of the table. Do not put food in direct contact with the preserved wood.

Absolutely! Preserved wood has been approved for use in docks, marinas, pilings and bulkheads and is widely used for these applications.

Preserved wood vs. alternative materials

Most likely it will. Plastic composite manufacturer product claims are not based on performance history, as most plastic products have only been in service for less than a decade.

In 1996, the U.S. Forest Service and Bureau of Land Management constructed a massive boardwalk system through wetlands created by a series of beaver dams in an abandoned channel of the Salmon River near Mt. Hood in Oregon. Three different types of preservatives were used for the preserved wood boardwalk decking and plastic composites were used for the handrail. The boardwalk is still in service today, but the plastic composite handrail failed after one harsh winter.

Preserved wood has a great environmental story to tell when compared to products such as plastics, plastic composite decking, galvanized steel and concrete. The Treated Wood Council commissioned several life cycle assessment (LCA) studies to evaluate preserved wood and alternative products. Metrics such as greenhouse gasses, fossil fuel use, acid rain, water use, smog, eutrophication and overall ecological impact were analyzed. In almost every measure, preserved wood scored very favorably compared to the alternative materials. Review the summaries of the LCA studies in the Environmental Benefits section.

Preserved wood in aquatic uses

Preserved wood is widely used to construct piers, docks, buildings, decks and walkways in or over aquatic and wetland environments. Of course, protecting the water quality and the diversity of life found in lakes, streams, estuaries, bays, and wetland environments is the industry’s No. 1 goal.

WWPI recommends when specifying preserved wood for these types of applications, request it conforms to WWPI's Best Management Practices (BMP) program. The BMPs are recommended guidelines for the production and installation of preserved wood products destined for use in aquatic and wetland environments.

The guidelines were developed through a consensus process, based on the core philosophy of preservative minimization. Both environmental and economic concerns support the goal of placing enough preservative into a product to provide the needed level of protection while minimizing use of the preservative above the required standard minimum in order to reduce the amount potentially available for movement into the environment.

WWPI does offer new online Environmental Assessment Model for planning the use of preserved wood in aquatic environments. Knowledge of preservative loss rates from properly treated wood, when coupled with site-specific environmental data (such as water current speeds and background levels of metals and organics) allow the industry to use relatively simple computer models to predict the environmental response to any project you might design. For more information on the model, go to the Aquatics section or review materials in the Online Technical Library.

Common preserved wood application questions

Application of paints, stains or waterproofing can enhance the protection of the preserved wood. It is recommended that preserved wood be cleaned and coated annually with a water repellent or water repellent stain to maintain optimum appearance. Unprotected preserved lumber will begin to change color as a result of the wood’s reaction to ultraviolet (UV) rays of the sun. Coatings with pigments or UV inhibitors will slow the color change. Water repellent coatings will reduce moisture pickup and help to minimize checking and cracking. Always follow the manufacturer’s recommendations for application.

Even though the lumber may be kiln-dried before treatment, using waterborne preservatives restores moisture to the wood. Too much moisture in the wood may prevent the stain or paint from penetrating the wood sufficiently and result in a blotchy appearance or poor adhesion. It is best to test the wood by painting or staining a scrap piece to see if it is dry enough to apply properly. If not, wait until it is. To enhance the service life of the wood after staining, a clear water repellent should be applied annually.

Unfortunately, no products are “maintenance free,” regardless of whether they are preserved wood or other materials. For preserved wood, pressure treatment helps protect the wood from fungal decay and termite attack. But, moisture, sun and other outdoor conditions may cause the project to show signs of weathering. Natural weathering such as raised grain, splitting, checking, twisting, warping, shrinkage, or swelling occurs in wood, be it treated or untreated.

Application of a clear water repellent coating or a semi-transparent water repellent stain annually will help maintain the appearance of the preserved wood. Always follow the manufacturer’s instructions on the label of the finishing product.

Pressure washing with plain water is a good way to clean pressure-treated wood. Also, there are number of deck cleaning products available at your local building products store to brighten weathered exterior wood projects. These products remove ground-in dirt, mildew and algae to help give weathered wood a brighter, natural appearance.

Please note that surface molds on preserved lumber do not affect the decay-resistant properties of the wood preservative. A clear water repellent or stain with water repellent can be applied after cleaning and will protect your outdoor project from the effects of weathering.

Wood naturally shrinks and swells. As decking boards dry out, they typically shrink in width. Consequently, a small space between boards at installation my result in a larger space later.

Depending on the desired look, it is advisable to install deck boards spaced about the width of the screws or nails used. Wider spacing is appropriate for lumber that is extremely dry or has been dried after treatment, i.e., KDAT (Kiln-dried After Treatment).

The slits are incising marks. Douglas fir and Hem-Fir are referred to as refractory species, as the cells of the wood do not allow for easy penetration by the preservative. Therefore, all lateral sides of western species must be incised prior to treatment as an aid in securing deeper and more uniform penetration of preservative. Typically, incising is required for Douglas fir and Hem-Fir when used in ground contact applications.

Incising does impact the strength of the preserved wood product and must be adjusted when used in an engineered application. Review how to calculate the strength reductions in the publication PreserveTech - Incising

Wood cells in Southern Yellow Pine readily accept penetration of preservative during the treating process, so incising is not required for this species.

Western wood treated to AWPA standards will have an end label or ink stamp that references AWPA standards and bears the quality mark of an ALSC accredited agency next to the WWPI CheckMark logo.

Handling and disposal of preserved wood

In most cases, use common sense when handling preservative-treated wood. Wear gloves to protect your hands from splinters. When sawing treated wood, wear a dust mask to protect against breathing in the dust.

First, it is important to remember that pressure-treated wood is not a hazardous product and has not been listed as a hazardous waste.

It is completely proper and legal to send preserved wood to a landfill. However, due to the growing shortage of landfill space, many industrial users select recycling of preserved wood as their disposal option. There is a growing movement to “manage” the disposal of preserved wood. In many cases, the wood can be reused in its original use or used in secondary applications such as fence posts, landscaping and other projects.

In cases where the wood cannot be reused, creosote- and penta-treated wood are increasingly being utilized as a fuel in properly permitted industrial burners for the generation of steam energy to power manufacturing plants onsite or nearby, and to cogenerate electricity which may be fed into the electric utility power system.

No. Preserved wood should not be burned in fireplaces, stoves, or other non-permitted units because the preservative may be released as part of the smoke or ashes. Where appropriate, preserved wood can be burned in an approved commercial or industrial permitted cogeneration or incinerator facility in accordance with state and federal regulations.