To ensure structural soundness and long service life for exterior wood structures, all wood must be protected from attack by insects or microorganisms, and decay from fungi. This is especially important in hot and humid climates or wherever wood comes into contact with ground or water. While field applied surface stains and paints offer some protection, elements such as decks are best protected by pressure treatment.
In pressure treatment, chemical preservatives are forced deep into the cellular structure of the wood in a closed cylinder under pressure. This process enables the preserved wood to maintain a chemical barrier against insects and decay for long periods of time. Several manufacturers even guarantee their treated wood against such damage for 40 years or longer.
There are three basic types of wood preservatives: water-borne, oil borne and creosote. Only the water-borne chemicals are generally used in pressure -treated wood products intended for residential uses. Chromated Copper Arsenate (CCA) is the most commonly used water-borne preservative.
Oil-borne preservatives, the most common being pentachlorophenol, are primarily used for above ground structural components such as laminated roof beams. Wood treated with creosote is now only used for commercial applications, such as timbers for railroad ties, highway bridges and guardrail posts, and wood used in marine structures - bulkheads, docks, and seawalls.
Questions have been raised as to whether direct exposure to products commonly used for pressure treatment cause cancer. To date, no laboratory or experimental investigations of the product have confirmed that treated wood causes cancer in humans. Epidemiology studies of wood-treatment plant workers and carpenters have also indicated there is no increased risk of cancer as a result of exposure to preservative-treated wood.
Questions have also been raised by several organizations as to whether indirect exposure, such as chemicals leaching from products used in vegetable garden trellises, planter boxes or edging could be hazardous. But while some studies have show that some surface residue may migrate to the soil, the levels are not elevated substantially above what naturally occurs in any soils. Further, no direct leaching from the product normally occurred.
Incidental contact of treated wood with drinking water, as with piling, docks, piers, or bridges, is acceptable. However, treated wood should not be used where it is likely to become a component of food or mixed directly with foodstuffs. Accordingly, wood should not be used as kitchen countertops of food cutting boards.
While the wood treatment industry claims there is no reason to avoid using wood treated with a water-borne preservative in the playground environment, or to avoid touching or walking barefoot on pressure-treated backyard decks, it is generally recommended that a water-repellent or wood sealer be applied periodically to reduce cracking and splitting and thus the likelihood of children getting splinters. On-the-other-hand, creosote and oil-borne treated products should not be used in playground equipment or wherever direct contact is likely.
Some studies by environmental have reported elevated levels of arsenic under treated decks, however, it is unclear as to whether these levels are the result of construction residue due to improper control or cleanup of sawdust after deck completion.
But, to be on the safe-side, choose wood products that are visibly clean and free of surface residue. Eye protection, dust mask and gloves should be used whenever sawing or cutting any type of building material, including wood products, treated or untreated. Practicing good personal hygiene and a thorough site cleanup at the completion of any construction project are also important.
Pressure-treated wood has not been listed as hazardous waste. In most areas, it is acceptable to send treated wood to a landfill space but many industrial users are now selecting recycling of treated wood as their disposal option. In many cases, the wood can be reused in its original form or used in secondary applications such as fence posts, landscaping and other projects. Treated wood should not be burned in fireplaces, stoves, or other non-permitted units because toxic residue may be produced as part of the smoke or ashes.
PRESERVING TREATED WOOD
Even though lumber may be kiln-dried before treatment to approximately 19% moisture content, using water-borne preservatives increases the moisture content – possible to as much as 75%. This causes the wood to initially swell then shrink as it is exposed to the air and the wood comes into equilibrium with the environment. If this shrinkage is too rapid, as is often the case with full sun exposure, the wood is likely to crack and distort.
A water-repellent sealer should be applied at the time of construction, or as otherwise recommended by the wood or sealer manufacturer, to help slow this initial shrinkage and prevent further damage from wetting-drying cycles. Some treated-wood products are now available with water repellency built in through the addition of water repellents to the preservative solution.
Too much moisture in the wood, however, may prevent the stain or paint from penetrating the wood sufficiently. It is best to test the wood by painting or staining a scrap piece to see if it applies properly. If not, wait until it reaches moisture equilibrium. While some latex paint products are available for finishing treated wood , semi-transparent, oil-based stains work best. A water repellent should be applied annually. If you do not have time to wait for the wood to equal equilibrium you can purchase wood marked "KDAT" (Kiln-dried after treatment). This wood should be ready for an immediate sealer treatment.
But all treated wood should be cleaned and re-sealed yearly to maintain optimum appearance.
Unprotected lumber will also begin to change color and darken as a result of the wood's reaction to ultraviolet (UV) rays of the sun. Sealants with UV inhibitors will protect the finished paint film from deterioration; those with a UV "absorber" are more effective in minimizing changes to the natural wood pigment itself, an important consideration when using clear finishes.
Because of the exposed wood, treated lumber is commonly used for deck construction, one home improvement project often undertaken by the homeowner him/herself. In addition to the basic framing sizing and layout requirements for the deck, one other very important aspect requiring specific attention is construction of the railings.
All elevated deck structures should have railings around the deck and along the stairs. The specific height at which a rail is required will vary based on local codes but there are several common standards that are often followed. For example, the minimum height at which a guardrail is required ranges between 15 – 30 inches off the ground. The rail itself should be a minimum of 36 – 42 inches above the deck floor.
Not only are there standard requirements on the presence and height of the rail, but design features and load bearing capacities must meet minimum criteria. The guardrails can not be constructed with any pattern that would have a "ladder effect". This limitation is intended to prevent children from climbing up a ladder-type rail and then falling over. Consequently, guardrails now typically have a vertical spindle design. But even then, specific construction features must be considered. The spacing between vertical spindles should not be more than four inches; specifically, they should be spaced so that a sphere 4 inches in diameter can not pass through any part of the rail.
The fastening of the guard rail itself must also comply with minimum standards. The rail is not just decorative but must be capable of resisting a force of 200 pounds. Simple nailing of the support posts is usually not sufficient to withstand normal forces that might be applied by someone leading against it. Consequently, bolting of the posts and other key connecting points is often necessary to ensure the ability to resist reasonable forces. In all cases, check on local requirements before beginning any new projects.
CHOOSING TREATED WOOD FOR YOUR PROJECT
To be certain that that the material for your outdoor project is of a quality to meet its intended use, follow these guidelines when purchasing or specifying treated lumber:
Wood products treated with waterborne preservatives should conform to approved treating industry standards. Such approval is indicated by either an ink stamp (in addition to the lumber grade designation) or more commonly by a plastic tag stapled to the end of the lumber. The labeling will include the logo of an accredited inspection agency, the standard utilized in the treating process, the preservative retention level and intended end-use/application.
For waterborne preservatives, the following levels of preservative retention applies: 0.25 (lbs./ft³) – this is the minimum level for low exposure above ground contact elements; .40 – for ground contact elements such as fence posts and deck supports; and, 0.60 – for permanent ground burial such as wood foundations.