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The Daily Fix

Can Moldy Framing Lumber Cause Rot?

FHB_WEB, member

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As a tree ages, sapwood is converted to heartwood and often gains decay resistance.

As a tree ages, sapwood is converted to heartwood and often gains decay resistance.

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by Stephen L. Quarles

Mold can grow on framing lumber as soon as it leaves the sawmill. You’ve probably seen the telltale black stains on bundles of 2x material at the lumberyard. But mold growth on studs is mostly a surface phenomenon; if it bothers you, you can wash off the spore-producing fruiting bodies of mold and sapstain fungi with soap, water, and an abrasive pad.

	Like mold, this blue-stain fungus can’t penetrate heartwood. As a tree ages, sapwood is converted to heartwood and often gains decay resistance.

Decay, or rot, is caused by different fungi, ones that can attack the cell walls of wood and eventually cause structural failure. Because of the extractives in heartwood, both mold and decay fungi prefer sapwood. Framing and sheathing are made from tree species with lots of sapwood (southern yellow pine, Douglas fir, aspen, spruce); as a result, today’s houses have more potential mold and decay food than earlier houses built with lumber such as chestnut, oak, hickory, and other hardwoods as well as lumber cut predominantly from the heartwood of softwood species.

Wet framing lumber can support the growth of mold and decay fungi, but by itself, moldy lumber can’t cause rot. It takes a decay fungus to do that. Because the growth rate of decay fungi is relatively slow compared with mold, you are more likely to see mold growth. As long as wetted lumber isn’t wet too long and the dried lumber stays dry, you won’t see decay fungi.

—Stephen L. Quarles, Ph.D., is a wood-durability adviser at the University of California Cooperative Extension and is based in Richmond, California.

Read the complete article... The Mold Explosion: Why Now? Today's houses make it easier for mold to thrive. The cure is quick cleanup and smarter choices in materials. by Joseph Lstiburek

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