subject: Select Good Lumber For Deck Building [print this page] Dry rot and damage sneaks up on you after the laborious process of deck building. Boring insects and wood rots operate just under the surface of a board, weakening the wood slowly and invisibly. But termites and ants give themselves away by leaving little piles of sawdust or mud tunnels. Occasionally, they bore through the surface of a board, exposing a portion of tunnel. But these social insects don't live in your house; they maintain queens and brood in nests in the soil. Their entry can be barred by keeping wood out of direct contact with the ground, inspecting frequently for paths or tunnels, or by hiring an environmentally responsible exterminator.
But dry rot fungus doesn't need a pathway to your outdoor deck. It sprouts from tiny airborne spores that are literally everywhere. Once a spore is wind-blown onto moist wood, it sprouts microscopically and begins to grow down into the wood--silent, invisible, and insidious.
Only when the dense network of dead-white filaments that make-up the body of the fungus plant have permeated and fed off the wood will the plant make its presence evident by sending out fruiting bodies. These range from foot-wide gray-white half-rounds of shelf fungus to the small pink, black, or gray fleshy blobs or mini-mushroom-tipped branches of less grandiose species.
Dry rot, by the way, isn't "dry" at all. It consists of a group of terrestrial fungi (related to common mushrooms) that, like all land plants, require plenty of moisture, oxygen, and moderate temperatures. Colorless and lacking the chlorophyll needed to create organic carbohydrates from sunlight and components of inorganic air and water via photosynthesis, they live in the dark and feed on dead plant material, indeed they are the prime medium of nature's recycling process.
New or Old Ideas for Deck Building?
Especially when adding service life to wooden decks made from cedar and other untreated woods, carpenters are using an old material tar paper, or more properly, heavy building paper or builders felt. This is a loosely woven, absorbent paper-cloth infused with asphalt that is commonly tacked over wooden roof sheathing to provide a waterproof underlayment for roof shingles.
Cut into strips that are wider by an inch than deck framing, it is stapled to the top surfaces of joists so it overhangs each side by a half-inch. Decking is fastened over the paper. Especially if planks are fastened tight with deck screws, sun-heated asphalt will soften and form a watertight gasket between bottom plank and top of joists, and seal around fasteners. Rain water flowing through cracks between planks can't soak into bottom of plank or top of beam, but drips off the edges of the paper.
You can cut tar paper strips from standard rolls, or buy precut strips at most lumberyards. The black paper will make a sharp color-contrast with decking at first, but after stain, sealers, and preservatives are applied, wood and asphalt will gradually fade to a uniform shade.
Lumber and Hardware for Deck Building
Pressure treated lumber, or PT, used in most deck building is made from standard "dimension lumber", from a good-building wood such as one of the southeastern yellow pines or western Douglas fir. It is heated and dried to less than 10 percent moisture content in a big kiln, then saturated under pressure for an hour or so in a bath of hot preservative.
By standards promulgated by regional wood-treatment associations, the lumber must be impregnated to at least 85 percent of its thickness and to a preservative content of between .25 to .60 pounds per cubic foot of wood. Low-rated timbers are cheaper than the higher rated, but using them in your deck is false economy. Use .25 outdoors, but for sheltered applications (a roofed deck), and save .60 for especially hostile environments such as along the ultra-humid Gulf Coast, inland in subtropical southern Florida, or as posts for a freshwater dock or saltwater pier.
The standard wood preservative is CCA, or Chrome-Copper-Arsenate. It is a long-acting but relatively benign heavy-metal poison--but a poison nonetheless--that kills mold spores and bacteria and deters bugs and gnawing animals. Older formulations give the wood a corroded-copper-green tint that fades to a pleasant silvery grey in 6 months or less. Newer compounds lack the green color.
