Select Good Lumber for Deck Building

Hiring a good professional contactor for deck building can ensure a worry free outdoor deck for many years to come.  However, one should become familiar with the dreaded dry rot that can creep into a deck and what precautions you can take yourself.

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. Sometimes in the quiet of the night you can even hear them munching! 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 and inspecting frequently for the paths or tunnels the critters make into the house, 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 that range from foot-wide gray-white half-rounds of shelf fungus to the small pink, black, or grey fleshy blobs or mini-mushroom-tipped branches of less grandiose species.

Dry rot, by the way, isn’t “dry” at all, but consists of a group of terrestrial fungi (related to common mushrooms) that, like all land plants, need plenty of moisture, oxygen, and moderate temperatures. Colorless and lacking chlorophyll to work nature’s magic and 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.

“Dry” rot got its name because it attacks timbers of wooden ocean-going boats that have been hauled out, “high and dry,” so that planking shrinks, seams open up, and the hull needs to be “soaked up” before launching or pumped out continually for hours after . . . or the boat will be on the bottom next morning. It is rainwater trickling in to moisten boat wood that supports “dry” rot. Sea water (though it does harbor wood threats of its own) does not support fungi. A deck post that’s sunk into consistently wet ground will remain sound below the soil and above it. It rots right at the surface line where it stays just moist all the time. Temperatures must be moderate for fungus to grow–above freezing but less than 100 Degrees Fahrenheit or so–so if you build your deck somewhere above the arctic circle or in the Mohave Desert, it will last with little care, though its utility will be limited.

New or Old Ideas for Deck Building?
Especially when adding service life to wooden decks made from cedar and other un-treated 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.

Water does its worst damage at frame joints–where several rim joists meet at corner posts.

To minimize water intrusion, sandwich sheet tar paper between butting surfaces of posts and beams and fasten tight with lags. Then use a modern clear, flexible silicone sealer to caulk all the top-side and vertical seams.

Cut a gasket to fit over the top of the entire joint–covering top of post and meeting joists and overlapping all edges by at least a half-inch. Spread a bead of caulk around the edges of paper, and apply it before fasting planks.

If some water does get into tar-papered joints, creosotes in the tar will give your wood the same protection that keeps creosote-soaked railroad ties and telephone poles standing for 50 years–but in your deck it can’t wash off.

A modern-caulk-and-old-fashioned-tar-paper-protected deck should last as near to forever as a wood structure can hope to.

Lumber and Hardware for Deck Building
Pressure treated lumber, or PT, used in most deck building is made from standard “dimension lumber”–square timbers and nominal 2″ thick boards cut in even-inch widths from 2″ to 2″ and in even-foot lengths from 8′ to 20′ or more–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. A .40 rating certifies that lumber will survive full-time ground contact for 40 years. Kept off the ground and treated annually with preservative and waterproofed, the wood should last as long as the rest of the house. 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.