Simply put, the answer is they work hand-in-hand. Mold does the damage, but needs a damp, organic material to flourish in. But let’s back up…as far as nature is concerned, rotting wood is normal. Dead trees decomposing into soil in the forest and the shiitake mushroom log your neighbor is cultivating under his basement workbench are examples of normal, useful fungi completing the process of growth and decay.
Part of building homes with wood, means fighting fungi. Not only to extend the life of the building, but to protect the infrastructure and ultimately, your safety. A home that avoids rot can last for hundreds of years. Conversely, wood rot can cut the useful life of your home to years or even months.
Spotting the Rot
Many fungi species, commonly called "mold" will grow in colonies on the surface of wood if it gets wet even once. It's only a problem if the fungi are among the smaller number that can invade the interior of damp wood, and destroy its integrity.
A quick field test for detecting structural rot is to stab the suspected joist or trim in several places with a dull blade, such as a screwdriver or table knife. If the blade sinks in a fraction of an inch, it's OK. If it sinks to the hilt, you have rot.
It's Rotten. Now What?
You might be asking, “How much should I rip out if I find rot?”
The professional answer is, all of it. Remove enough drywall, subfloor, sheathing and flashing until you can see a clean margin of good wood all around the problem area. It's often easier and less expensive to remove it all down to the joists than attempt to remove the smallest possible amount.
Because rot follows water, and water flows downhill, anything under the damaged area should be exposed and replaced or reinforced. Everything above the damaged area should be examined until you find the source of the water and stop the water infiltration.
Don't Have a Rotten Home Life
Prevention is cheaper than repairs. The key to any pest control is to control the environmental factors that allow it to live: temperature, food and water.
The wood-rot fungi thrive at the temperatures that are comfortable for humans. So that control method is unavailable for most buildings.
Removing the "food" means removing the wood from the structure, which isn't practical for most buildings. Making the wood "inedible" with anti-fungal preservatives is partially effective. But even "treated" lumber will succumb to wood rot.
That leaves controlling the moisture available for the fungi as the best method. If you keep it dry, wood lasts a very long time. Archaeologists have recovered wooden boats in the Egyptian desert that are over a thousand years old.
If you are serious about preventing wood rot and the needless expense of repairs, you must be vigilant about preventing water infiltration. Leaks are the enemy. Regularly inspect flashing, around openings, and around plumbing runs and fixtures. Fix leaks as soon as you detect them and if it’s too late...get restoration professionals involved ASAP.