A Weather-Proof Deck

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Dreaded Dry Rot

Deck damage sneaks up on you. 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 wand 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 deck. It sprouts from tiny airborne spores that are literally everywhere. Once a spore is windblown onto moist wood, it sprouts microscopically and begins to grow down into the wood— silent, invisible, and insidious. I've seen many (and replaced more than a few) support timbers under decks and porches and in the cellars of old buildings that looked perfectly sound at a casual glance. But you could sink your penknife blade in 'em up to the hilt; the beams were little but shells with the inside turned to frass by dry rot fungus.

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 boatwood that supports "dry" rot. Sea water (though it does harbor wood threats of its own) does not support fungi. Neither will standing fresh water; 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°F 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.

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