Publications Article

Preservative Treated Wood

© 1996 Stephen Smulski, Ph.D.
Wood Science Specialists Inc.
Shutesbury, Massachusetts
413 259-1661 fax-1610

Cellulose and lignin, the stuff of which wood is made, are the two most abundant organic compounds on earth. Without the fungi and insects that biodegrade them, the landscape would be literally littered with downed and dead trees. Problem is, these relentless recyclers don't distinguish between wood lying on the forest floor and wood supporting the first floor.

Man has been helping wood-destroying organisms make the distinction ever since Egyptians first smeared wood funerary objects with cedar oil. The wood preserving industry didn't begin in earnest though, until the late 1800s when America's railroads, faced with a shortage of naturally-durable woods for crossties, started saturating non-durable woods with creosote. Today preservatives protect everything from crossties, poles, posts, and piles to plywood, lumber, and millwork to shingles and shakes.

Wood destroyers
Aptly termed the "slow fire," wood decays or rots because it is being eaten by primitive plants known as decay fungi. In the early stages of decay, wood is discolored and often softened. As the slow fire advances, wood may become white and spongy, or brown and crumbly, depending on the fungus. By the time fan-shaped surface growths called mycelia, or fungal fruits (mushrooms) are seen, wood has suffered a loss of serviceability, and more importantly, a loss of strength.

Carpenter ants, termites, and dozens of beetles attack wood. Some, like carpenter ants, don't actually eat wood. They gnaw tunnels in wood simply as a place to live. Termites on the other hand, not only nest in it, but eat wood as well. Crustaceans called marine borers burrow into ships, piers, and other wooden saltwater structures. Even bacteria can degrade wood under the right circumstances.

Preventing deterioration
Wood can be protected indefinitely by eliminating any one of the four requirements that must be met before fungi and insects can attack: oxygen, temperature, moisture, and food.

It's hard to do much about oxygen. About the only time it's excluded is when stockpiled logs are sprinkled with water or floated in ponds. Temperature is tough too, since most living things thrive in the 40 to 90 (F range. Even at freezing temperatures, some fungi don't die, they just hibernate. Organisms infecting green wood though, are killed by the heat of kiln drying.

The most effective and common "method" of preventing deterioration is to keep wood dry. Wood's moisture content must be continuously at least 20% before it's attractive to fungi. Across most of the United States the moisture content of wood indoors ranges from about 4 to 16% over the course of a year, so decay isn't a problem. Interior wood wetted by leaks, sill plates on damp concrete, and framing over humid basements or dirt crawl spaces is another story.

In exterior or other uses where wood can't be kept dry, the traditional solution to delaying decay has been to use the heartwood of naturally rot-resistant woods like Western redcedar, redwood, baldcypress, and white oak. Since the main entree at the fungal feast is the wood itself, Nature has kept these and other woods off the menu by depositing in their heartwood unpalatable poisons called extractives. The sapwood of all types of wood lacks any natural decay protection. Even highly rot-resistant woods will give only 15 to 20 or so years of service in decay-promoting environments.

Supplies of naturally durable woods are too small to meet today's demand at an ecologically and economically acceptable price. In imitation of Nature's genius, the wood preserving industry fills that demand by impregnating woods lacking decay-resistance with a pesticide (preservative) that can extend service life by 30 to 50 years or longer.

Treating wood with preservatives
Both nonpressure and pressure processes are used to introduce preservatives into wood. Brushing and spraying are usually limited to field treatment of wood during construction, or remedial treatment of wood in place. Millwork makers routinely dip window sash and other exterior trim in a combination water repellant/preservative. Lasting from days to weeks, soaking of poles, posts, and piles is really nothing more than extended dipping.

The amount of protection gained with these nonpressure methods is unpredictable. The most effective processes are those in which preservative is driven into wood under pressure. Ergo, the monicker "pressure treated wood."

Though variations abound, two basic processes are the full-cell or Bethell process, and the empty-cell or Rueping/Lowry process. With both, wood cell walls are saturated with preservative. After treatment, cell cavities are filled with preservative in the Bethell process, but nearly empty in the Rueping/Lowry process.

Green wood is generally dried to around 20% moisture content before treatment. Otherwise, water saturating its cells inhibits absorption of preservative. By necessity, large timbers are incised with small slits as they pass between spiked rollers to promote penetration.

In the full-cell process wood is placed inside a pressure vessel and a vacuum drawn to remove air from its hollow cells. After being flooded with preservative, the vessel is pressurized to drive the solution deep into the wood. Pressure is later released, excess liquid pumped to a holding tank, and treated wood removed for airdrying.

No initial vacuum is drawn with the empty-cell process. As the flooded vessel is pressurized, air inside wood cells is compressed. After pressure-release, the expanding air, aided by a small applied vacuum, kicks preservative out of the cell cavities leaving them practically empty.

Generally, lumber up to 1 in. thick is completely penetrated by preservative. Depending on the type of wood, 2 in.-thick lumber may or may not be fully penetrated. Only the shell of timbers 4 in. and thicker will be penetrated; the core will not. Also, while the sapwood of virtually all woods is easily penetrated, the heartwood of most resists penetration. For all intents and purposes, only the sapwood of treated wood has protection against decay greater than what Nature already handed the heartwood. What's important is that a zone of untreated wood may be exposed during machining, especially in large members.

Depth and uniformity of penetration, and amount of preservative retained by wood determine success of treatment. Measured in pounds of preservative per cubic foot of wood (pcf), retention varies with the preservative, product, and wood. Retention standards set by the American Wood Preservers Association are enforced through chemical analysis of treated wood by various third-party agencies who perform periodic inspections of wood-treating companies. Look for an agency logo in the treater's quality mark to ensure that wood has been treated to the right retention. And don't be fooled by lumber stamped "Treated To Refusal." This misleading label is used for difficult-to-treat woods that are only superficially penetrated.

Of countless compounds suggested and tested as preservatives only a handful have the safety, effectiveness, permanence, and economy that make them commercially important. In addition to creosote, preservatives fall into two classes, oil-borne and water-borne, depending on whether they are carried into wood in an organic liquid or water.

Creosote
In use since the 1850s, creosote is highly effective against fungi, insects, and marine borers. Injected into crossties, marine piles, and bridge and highway timbers in a full-cell process, creosote may exude from products into surroundings. Organic in origin, it eventually biodegrades. Utility and building poles, freshwater piles, fence posts, and industrial wood block flooring are treated in an empty-cell process that yields a clean, nonbleeding surface.

Dark brown to black, wood freshly infused with creosote gives off potentially harmful vapors that eventually disappear. Plants nearby may be affected. Gloves must be worn when handling creosote-treated timbers. Creosote products cannot be painted; coal tar pitch, urethane, epoxy, and shellac are acceptable sealants. Creosote crossties last about 30 years; utility poles may survive 60.

Oil-borne preservatives
Carried in organic solvents such as liquified isobutane, oil-borne family members include pentachlorophenol, iodo propynyl butyl carbamate (IPBC), copper and zinc naphthenate, and tributyl tinoxide (TBTO).

Potent against terrestrial pests, pentachlorophenol or penta extends wood's service life by 20 to 40 years. In use since the 1930s, most is borne in heavy oil to treat utility and building poles, fence posts, and highway timbers. Tinted light to dark brown, penta products glue and finish reasonably well after the noxious oil carrier evaporates.

As penta may migrate to form surface deposits, gloves are a must. Urethane, latex enamel, shellac, and varnish are effective sealants. Penta can move into surrounding soil, but because it binds tightly to soil, groundwater contamination is unlikely. Penta slowly breaks down into biodegradable compounds.

Until about 1985, exterior millwork was dipped in penta in light oil. With its safer solvents, IPBC has since replaced penta completely in this application. Because of nagging safety and environmental concerns, pundits predict penta will ultimately fade from the preservation picture.

Only licensed pesticide applicators may use EPA-regulated creosote and penta (and water-borne arsenicals) to treat wood. For farmers, builders and others who need to do their own treating, over-the-counter oil-borne preservatives are available. Copper naphthenate is the active ingredient in the green colored do-it-yourself preservatives; zinc napthenate packs the punch in clear products. While most exterior paints and stains already contain IPBC or TBTO to fend off mildew and other fungi, TBTO is sold in one-ounce bottles to add to those that don't.

With an eye to increasing safety, formulators are working to make these preservatives emulsifiable in water. Always wear eye protection, avoid skin contact, and follow label directions when using over-the-counter products. Preservative penetration and retention with self-treating are minimal. Unless preservative is regularly renewed, only a few years of extra service life are gained.

Water-borne preservatives
The treated lumber and plywood do-it-yourselfers and builders buy is protected with any one of a virtual alphabet soup of preservatives carried in water: chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA), acid copper chromate (ACC), chromated zinc chloride (CZC), ammoniacal copper zinc arsenate (ACZA), and fluor chrome arsenate phenol (FCAP).

Because of similar chemistry, these preservatives share a lot in common. Chromium helps hold the other components tightly to wood to prevent leaching in service. With the exception of CZC and FCAP, which see but minor use, all are nonleachable. Zinc and copper fight fungi, while arsenic guards against attack by termites and copper-resistant fungi. Ammonia in ACA and ACZA helps carry copper, arsenic, and zinc deeper into penetration-resistant heartwood. Douglas-fir and other western woods are commonly treated with ACA and ACZA. Southern yellow pine is usually impregnated with CCA. Wood treated with any one of these compounds has pretty much the same characteristics.

CCA is hands down the workhorse of the water-borne stable. Since use began in the 1930s, three basic formulations, Types A, B, and C, which vary by amount of chromium, copper and arsenic, have evolved. All things considered, the newer Type C or oxide form, is preferred. Before and during a modified full-cell treating process CCA is water-soluble. During the first day or two of airdrying after treatment, CCA is rendered insoluble in water in a process called "fixation." During fixation chromium reacts chemically with the wood, permanently bonding itself, copper, and arsenic to its cell walls. For this reason CCA does not leach from wood in service. Because it's applied as a water solution, no vapors are ever emitted.

In addition to resisting attack by fungi and termites, CCA thwarts marine borers. Because of its effectiveness and safety, CCA is gaining ground in creosote and penta pole markets. Agricultural products include fencing, plant stakes and arbors, and greenhouse flats. Surprisingly, CCA-treated wood is not completely immune to insects who, like carpenter ants, do not ingest the wood. Also, CCA products are susceptible to surface mold and stain if solid-piled when wet. No one knows how long CCA-treated wood will really last. Treaters guarantee at least 40 years, and consider 100 possible. After being in the ground for 50 years, treated test stakes in Mississippi and Florida still show no decay. Untreated controls lasted fewer than 4.

The hallmark of CCA-treated wood is its blue-green tinge. Products have a clean surface and can be used where contact with bare skin is frequent, as in decks and benches. Watch for the odd piece with a white, gritty surface residue that forms infrequently when preservative precipitates out of solution during treating in a phenomenon known as sludging. Leave it with the seller. If on-site, wash residue off with water, or install wood residue-side down.

Only visibly surface-clean CCA-treated wood should be used for playground equipment and picnic tables. Wood treated under AWPA's special standard, "C17-95 Playground Equipment Treated With Inorganic Preservatives," presents the cleanest surfaces possible. To further lessen potential for skin contact, apply a water repellant, or better yet an oil-based stain or paint. Round edges to prevent splintering; knots and hardware should be flush with surfaces. In its 1987 report to the State Legislature, the California Department of Health Services advised that "With the possible exception of creosote, none of the wood preservatives pose an acute or chronic toxic hazard to children playing on treated wood."

CCA products finish reasonably well. But because wood is saturated with water during treating, and seldom kiln dried afterward, it is often still wet during construction. Wait at least a week or until its surface is thoroughly dry before brushing on water repellants, stains, or paints. Oil-based semi-transparent stains are the best performers on CCA-treated wood. If the construction cries for color, follow an oil-based primer with acrylic latex topcoats.

At the very least, a water repellant with mildewcide should always be applied to minimize the warping and checking that victimize wood exposed to the elements. Repellants retard absorption of water, causing rain to bead on wood's surface where it can harmlessly evaporate. Without repellant, repeated rapid swelling and shrinking will carry surface checks into the untreated core of larger members. Water trapped in fissures allows fungi to destroy wood from the inside out. Renew repellants every couple of years. Usually oil-based stains can be applied over repellants without trouble. With water repellants so critical, the latest development in pressure treating is to impregnate wood with preservative and water repellant simultaneously.

Mold, mildew, dirt, and friable fibers on weathered wood may interfere with finishes. Scrub surfaces with trisodium phosphate or a solution of water and household bleach (chlorinated), then rinse with water, being sure to follow the manufacture's directions. Wear goggles and gloves, and cover nearby plants and shrubs. Better yet, use commercial products based on disodium peroxydicarbonate, an oxygenated bleach. It's safer and won't burn plants.

Bark side up?
Long before treated wood arrived, it was advised that deckboards (as well as stair treads, handrails, siding, and trim) be laid bark side up to avoid a roughened surface condition that happens most commonly on the pith side of flatsawn softwood lumber. Loosened grain or shelling, as it's called, occurs when the edges and tips of the darker late wood layer of the growth rings separate from the wood's surface and curl upward. Most prevalent in uneven-grained softwoods like southern yellow pine and Douglas-fir, shelling can occur as wood is repeatedly and rapidly wetted and dried in service.

What confounds the original wisdom of "bark side up" is that virtually all CCA-treated deckboards are flatsawn southern yellow pine sold water-saturated. After being fastened in place their moisture content drops, eventually equalizing at the locale's year-round average for wood out-of-doors. Laid bark side up, water-saturated, flatsawn deckboards will cup in the direction opposite the curvature of their growth rings as they dry. The deckboards become concave upward as a result, because their edges lift higher than their center. This isn't too serious a problem because correct fastening can partially restrain wood, keeping cupping to a minimum. For standard 5/4 x 6 in. radius edge decking, a 10d hot-dipped galvanized ring or spiral shank nail driven at a slight angle about 1 in. from each edge will do. A smaller nail may pop. Better yet, use the zinc-coated, case-hardened screws developed for this application. One drawback with "bark side up" is that water will pond between the fasteners of cupped deckboards. This too isn't a problem as long as a water repellant has been applied.

Plan for shrinkage when spacing deckboards. The rule of thumb is to gap deckboards using a 16d nail, or by about 5/32 in. But because CCA-treated wood is usually wet during construction, you can end up with bigger gaps than bargained for. A flatsawn southern yellow pine deckboard dried to 20% moisture content before treating is 5 1/2 in. wide. Its post-treating width will be about 5 5/8 in. Used in New England, it will shrink to about 5 7/16 in. as it dries to the region's year-round 16% outdoor average moisture content. A 3/16 in. gap will open even if wet deckboards are butted as they're laid!

Is it still "bark side up"? From my experience a good bet is to buy treated lumber a couple of weeks ahead of time to permit it to partially dry before building, lay deckboards best-face up, and generously apply a water repellant as soon as their surfaces are dry.

Copper in CCA, ACA, and ACC is corrosive to uncoated metal. In above-grade construction use stainless steel or hot-dipped or hot-tumbled galvanized fasteners. Joist hangers, framing anchors, and other hardware should be corrosion resistant too. Type 304 and 316 stainless steel, Type H silicone bronze, ETP copper, and monel fasteners are required for below-grade applications like wood foundations.

Southern yellow pine is prone to splitting. When nailing within 2 in. of the end, or close to the edge of lumber, drill a pilot hole. It may also be necessary to do so when using screws. Splitting is reduced with blunt nails that punch through wood fibers, rather than cleave them apart as sharp ones do. Splits caused by careless fastening create watertraps irresistible to fungi.

Where nails are impossible or objectionable, use lag screws or through-bolts. Put a washer under lag screw heads and under both bolt heads and nuts to distribute stress and prevent crushing of the wood. Tighten only until snug. Overtightening to make sure they're "good and tight" is a common mistake because wet or dry, wood under the washer is easily crushed. Since most CCA-treated lumber will shrink after it's in place, lag screws and through-bolts need to be retightened a few weeks later.

CCA-treated wood glues well. Phenol-resorcinol-, resorcinol-, and melamine-formaldehyde adhesives are used in making laminated timbers from treated dimension lumber. On-site, use only construction adhesives specifically formulated for treated wood.

Aside from increasing abrasion to tools slightly, impregnation with CCA doesn't affect wood's machining qualities. Extend saw sharpness by using carbide-tipped blades. As should be done when machining any wood product, treated or untreated, wear goggles, dust mask, and ear protection. Before assembly treat site-cut surfaces liberally with an over-the-counter preservative. Don't overlook pilot holes, mortises and tenons, bevels, and site-sawn stair stringers. Consider using pre-cut railings, post caps, balusters, and stair stringers. Always put the "factory end" of treated posts towards the ground; cap the site-cut surface with flashing or an overhanging railing. Use a continuous railing to avoid the watertrap formed when a joint between segments falls atop a post.

Studies released in 1988 and 1989 by the U. S. Forest Products Laboratory showed that the bending strength of No. 2 southern pine 2 x 6s treated with CCA was less than that of untreated controls. Stiffness was unaffected. Oxidation of wood by chromium is part of the problem, but most of the strength-reduction happened because wood was kiln dried after treatment at temperatures over 190 (F. The biggest losses occurred in pieces sawn near the pith of the tree.

Should builders be worried? Not really. Less than 10 percent of all CCA-treated wood is kiln dried after treatment, and supplied only on request. (KDAT appears in the quality mark.) Also, AWPA set a post-treatment drying temperature limit of 190 (F in January 1989. What's more, the strength losses happened in the pieces that tested strongest before treatment. Pieces at the lower end of the strength distribution on which code-accepted design values are based were unaffected. And lumber stiffness, not strength, is typically the deciding factor in specifying joist sizes. The FPL did recommend that grade rules writing agencies may want to consider developing a special grade for treated wood to address concerns with pith-associated lumber.

CCA products are used mainly in exterior situations where decay hazard is high, and whenever wood is used in ground contact or against concrete and masonry in ground contact. Lumber for above ground use is treated to 0.25 pcf retention. Ground contact use requires 0.40 pcf. Wood foundations are treated to 0.60 pcf. Though rarely required, CCA-treated wood can be used indoors without being sealed as long as all machining dust is cleaned up.

To help builders use treated wood wisely, the treating industry publishes an EPA-approved Consumer Information Sheet for creosote, pentachlorophenol, and inorganic arsenical pressure-treated products. Though compliance isn't perfect, the CIS is supposed to be available from treated wood retailers. It's always obtainable from AWPA.

Recommended uses vary with preservative type, but some caveats apply to all treated products:

• use treated wood only where such protection is needed

• wear a dust mask and goggles when machining treated wood

• wash hands before eating after handling treated wood

• wash work clothes separately, and before reuse

• do not burn treated wood; dispose of by burial or ordinary trash collection

• do not use treated wood for cuttingboards, countertops, silage or fodder bins, or where it could become a component of animal feed

If CCA-treated wood is so safe, then why the precautions? Where peoples' health is concerned it's best to err on the conservative side. The two routes by which CCA may enter the body are by inhalation or ingestion, especially of airborne machining dust. Also, there is the case of a U.S. Forest Service employee who exhibited symptoms consistent with arsenic over-exposure after building picnic tables of CCA-treated wood. On the other hand, the State of Wisconsin Department of Health and Social Services reported "No detectable arsenic concentrations were found in the breathing zone of the radial-saw operator or the workers assembling treated wood-foundations" in its study of factory employees building foundation components from treated lumber and plywood. Neither masks nor gloves were worn in this case.

Sensitivity to any substance varies drastically among individuals. An adage among toxicologists says: "The dose makes the poison." The bottom line appears to be that even without mask or gloves, exposure to arsenic from handling or machining CCA-treated wood is well below the average 80 micrograms Americans get daily from their food and water.

Borates
Another water-borne preservative that shows increasing promise is borax, or more specifically, disodium octaborate tetrahydrate. An old preservative that's been rediscovered, borates protect wood from most fungi and wood-eating insects. Borate-treated wood is unchanged in color, noncorrosive to fasteners, and can be readily glued and finished. Non-toxic to people and animals, borates also increase wood's fire resistance.

Borates are applied by dipping-diffusion. Green wood is immersed in a hot, aqueous borate bath, then removed and solid-piled. Over a few weeks' time, the preservative naturally dilutes itself by diffusing into the water in the wood. Sapwood is completely penetrated, as is the heartwood of some woods. Dry wood is treated in a full-cell pressure process. Spray application is used for treating wood in place. Today, borates' biggest use is in treating timbers for log structures and post and beam construction.

The stumbling block with borates is that they remain water-soluble, and readily leach out of treated wood that gets wet. Until a way to render them insoluble after treatment is developed, borate products shouldn't be used where they're exposed to weather. Leaching of borates from the exterior walls of log homes is all but eliminated by applying a water repellant every couple of years.

The future
With creosote use static and penta declining, the volume of wood treated with water-borne arsenicals and borates will continue to rise. Tomorrow's preservatives will combine even lower toxicity to people and animals with greater effectiveness at low retentions. One oil-borne candidate is chlorothalonil, an EPA-registered agricultural fungicide. With performance on par with penta, only its high cost has to be overcome. Already commercialized in New Zealand, water-borne alkylammonium or AAC compounds are also being looked at. With good building practice and preservative treatment, wood can last a lifetime.

Contact Information: Dave Damery, Director Building Materials and Wood Technology 120 Holdsworth Natural Resources Center University of Massachusetts, Amherst, MA 01003 Tel: +1 (413) 545-1770