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Q.When inspecting the porch floor joists of an 1880s farmhouse in Illinois, I was surprised to see that the joists and the underside of the tongue-and-groove fir decking appeared velvety, and looked like they were covered with matted dog hair. The surfaces felt not wet, not very dry, but very soft. Is this type of decay caused by a fungus or an insect?

A.Stephen Quarles, cooperative extension advisor on wood durability at the University of California Forest Products Laboratory in Richmond, Calif., responds: Based on your description, the degradation affecting your porch was caused by a decay fungus. The velvety appearance you observed on the surface of the wood was mycelium, a term used to describe a mass of hyphae (thread-like cells of the fungus that break down the chemical constituents that make up wood into food that the fungus can absorb).

The color of the mycelium can vary, depending on the particular fungal species and how dry it is. On the back of building paper installed on a wall, I have seen white mycelium so thick that other people present mistook it for housewrap.

The presence of mycelium indicates that the growth conditions for the fungus were pretty good. With that much mycelium on the wood, you should be able to easily gouge out some wood with a screwdriver or an ice pick. It’s a good idea to check how deep the decay has penetrated.

Even though the surface of the mycelium didn’t feel wet, the underlying wood may or may not be wet. The wood would have to be wet for the fungus to still be active. If decaying wood dries out, and the moisture content drops below about 28%, the decay fungi will become dormant. The fungi can become active again if the moisture returns.

To address the problem with your porch, you need to assess whether the joists and boards are still structurally adequate. In any case, try to remove the source of the water. This may or may not be possible, depending on whether the porch is exposed to rain and snow. Improved air circulation under the deck can help.

If it’s time to rebuild the porch, the replacement joists should be pressure-treated. Instead of tongue-and-groove fir, consider installing deck boards of pressure-treated lumber or wood-plastic composite, allowing space for drainage between the boards.