ROANOKE TIMES
Copyright (c) 1996, Roanoke Times
DATE: Sunday, November 24, 1996 TAG: 9611260056
SECTION: HOMES PAGE: D-2 EDITION: METRO
COLUMN: Hints for Homeowners
SOURCE: STEVE ELDER
Some woods are naturally resistant to rot because of their inherent chemical and molecular properties. Cypress and black locust have traditionally been used for fence posts. Redwood was the original outdoor deck material, although its increasing rarity and cost have put it beyond most budgets nowadays. White oak and redwood are the preferred woods for outdoor lawn furniture. Cedar is commonly used for lap siding and was the original roofing material in early America.
All of these woods, however, usually rot within 10 to 20 years if they are in direct ground contact, whereas pressure-treated lumber can last over 60 years.
The wood commonly used for pressure-treated lumber is Southern yellow pine. Its cellular structure makes it accept the preservative particularly well, and it is one of the strongest woods available.
In order of decreasing quality, the grades are: Select Structural, Dense Select Structural, No. 1, No. 1 Dense, No. 2, No. 2 Dense and No. 3. The better grades have smaller knots and fewer defects. Not all grades are manufactured by all mills; some are available only by special order.
The designation ``dense'' applies if there are more than six growth rings per inch in a board section; this indicates slower growth resulting in a harder, stronger wood. The grade most commonly used in deck-building is No. 2, although sometimes a dense board can sneak through the grading process. This is the reason you occasionally find a board that seems twice as heavy as normal.
The first widely used wood preservative was creosote. It is a derivative of coal tar, which is itself a by-product of the coking of bituminous coal. Creosote was and still is used to treat wooden railroad ties, pilings and utility poles. (Note: the modern formulation of creosote is a much cleaner version.)
There are several additional preservatives in use today, including copper napthenate and pentachlorophenol. The one most commonly used to treat wood for use in home construction is chromated copper arsenate, commonly referred to as CCA.
Chrome, copper and arsenic are elements occurring naturally in the Earth's crust. CCA is delivered to a treatment plant in a concentrated water solution, which is then diluted to obtain the formulation actually used in the treatment process. In this form, CCA is a toxin, but when it is driven deep into the cellular structure of the wood fibers under pressure, it bonds tightly with cellulose through a series of chemical reactions called fixation. Once CCA is fixated, the chemical bond can be broken only by strong acids or combustion.
Accordingly, pressure-treated lumber scraps should not be burned, because the smoke can be harmful. The chemical bond of fixated CCA is not broken by water, therefore the risk of pollutants being released from pressure-treated lumber through the effects of water or weather is generally considered negligible or nonexistant.
Responding to consumer concerns about the use of CCA-treated lumber in playground equipment, both the Consumer Product Safety Commission and the Environmental Protection Agency have examined the question of environmental harm. Laboratory and field tests conducted over a span of decades have shown the chemical migration and leachate of CCA to be negligible.
Recently, I had the opportunity to visit a wood-preserving facility in Moneta. J. Eldridge Wimmer, owner and operator of Blue Ridge Wood Preserving Inc., was kind enough to give me a personal tour of his treatment plant. Cured (kiln-dried) lumber is shipped to the plant, where it is sorted by length, banded and loaded onto trolleys. The trolleys are then pulled by cable into a long steel cylinder. A large hatch with a ring seal is then closed and dogged tight.
The computer-controlled treatment process begins by removing any excess moisture in the wood by creating a slight vacuum in the tank; then the preservative is pumped in and the pressure raised to 125 pounds per square inch. The pressure drives the preservative into the wood fibers, where it then fixates to create a permanent bond.
The process at Blue Ridge presently takes about 50 minutes from start to finish, but the company is planning to install a more powerful pump to reduce the time it takes to pressurize the tank. After treatment, the lumber is then stacked on a concrete floor to drip dry. The floor is sloped towards a sump drain so as to recapture the excess preservative for re-use.
The amount of preservative varies according to the end use of the lumber. Wood intended for use above ground is normally treated to a level of .25, which means that there is a quarter of a pound of preservative per cubic foot of wood. Ground contact requires a treatment level of .4 pounds per cubic foot, and wood used for foundations will be treated to a .6 level. The treatment level is noted on plastic tags affixed to each board after treatment.
Pressure-treated wood does not give up its preservative when rained on or touched, but it can be harmful if ingested. This is the reason that termites do not attack treated wood.The main job site precaution is to sweep up the sawdust and shavings from pressure-treated lumber for two good reasons: you don't want to get them mixed in with your Fritos, and you don't want to be caught doing something even a termite knows not to do.
Steve Elder is a Roanoke home inspector. Questions and comments may be sent to him in care of The Roanoke Times, P.O. Box 2491, Roanoke, Va. 24010-2491.
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