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
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
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.