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Building Science

Strategies for Preventing Wood Rot

Correctly using and detailing lumber and other wood-based products adds to their longevity, which means we should be doing it on every job

Wood can last a long time on the exterior of a house, but not if you don't take steps to protect it and give it a chance to dry out after getting wet.

To be green, a house has to be durable for the long haul. That means the wood used to build it needs to be installed in such a way that it won’t rot.

Decay organisms like fungi eat wood. They need four conditions to live. There must be food (wood), water, oxygen, and hospitable temperatures. Remove any one of these variables, and wood doesn’t rot. As a practical matter, food and water are the only two within a builder’s control.

That said, rot does slow during cold temperatures, and practically stops when the mercury drops below 32℉. That means wood rots more quickly in warmer climates, making rot-control measures all the more important in the South, particularly in the wet, eastern states.

Using wood right

Wood makes great houses—plus, it sequesters carbon. But it needs to be kept as dry as possible. If you can’t keep it dry all the time, you need to create conditions that allow it to dry if it does get wet.

As an example, a house I worked on two years ago had exterior porch columns made from treated 6x6s wrapped in pine. Whoever trimmed those columns wrapped 1×10 PVC as a base detail over the main pine cladding. He probably figured that the most vulnerable part of an outdoor column is the base, and PVC won’t rot. But the pine behind the PVC rotted out entirely, while all the wood above was fine. Water that had gotten to the pine behind the PVC couldn’t dry out.

To make that base detail work, I stopped the new pine about an inch below where the PVC would top out. I sealed the pine on all sides with a high-quality primer (I like Ben Moore’s Fresh Start). To support the PVC…

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