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Millions of wood decks are built every year. Unfortunately, many of them will deteriorate long before they should. As a deck builder in the Pacific Northwest, I have a lot of experience demolishing and replacing rotted decks. I want what I build to last, so when I tear down a rotting deck, I try to figure out what went wrong.
Know Your Enemy
Rot is a fungus, an organism that feeds on and destroys natural materials like lumber. The spores it uses to reproduce are nearly everywhere and will grow wherever conditions are right. There are many different types of rot, but they all require food and moisture to survive. If you understand how these organisms grow, you can build decks that are less hospitable to them.
Food source. Most decks--particularly the structural framing--are made from woods that are susceptible to rot. Because the companies that treat lumber use local materials, the wood you use depends on where you work. From Denver east, builders use pressure-treated southern pine, a kind of wood easily penetrated by chemical preservatives. On the West Coast, we use pressure-treated hem-fir, a species group that does not accept chemicals very well. Frequently, only the outer surface contains preservatives, so the interior of the lumber is unprotected and susceptible to rot (see Figure 1).
[FIGURE 1 OMITTED]
Moisture. Wood won't deteriorate unless moisture is present. Green lumber often contains enough moisture to rot, but even materials that start out dry can become wet when they're exposed to the weather, washed, or used as a surface for potted plants.
Some kinds of rot fungi, the wet rots, require a wood moisture content of 30 percent or more to survive. Many types of rot will do just fine as long as the wood has a moisture content of at least 20 percent, a threshold easily reached in damp climates like ours.
People often speak of "dry rot," but if lumber is dry, it will not decay. So-called dry rot fungi are a specific type of brown rot that sends out hyphae, strands of tissue that can transport moisture from surrounding wood. But this can't happen unless the surrounding wood is wet. If you can keep a deck below 20 …