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Wood Rot
Carl Brahe, Certified Home Inspector

Sometimes called house cancer, wood rot, is something most home owners will have to deal with at some time. Wood rot is caused by several different fungi that are usually present, but dormant, in most lumber. When paint fails, or wood is in contact with soil or concrete, it can begin to absorb moisture. When the moisture level is sufficient, the wood rot fungi start digesting the proteins and sugars contained in the wood fiber.

There are two kinds of wood rot: wet rot and dry rot. Both require wood, moisture, oxygen and warmth to grow. They do not grow well in water logged wood because of a lack of oxygen. When the moisture level reaches 19% in lumber and temperature is between about 40º F and 105º F the fungi begin to colonize.

Wet rot uses the moisture readily available in wood to digest it. Dry rot may extend tentacles up to 25’ through masonry to tap into moisture sources to bring enough water to the wood to digest it. These tentacles can grow as big around as fingers.

Both types of rot eat wood. It is their function in nature. Wood rot can grow quickly, destroying the strength of lumber. As these fungi digest the nutrients in the wood all strength is lost. The wood may powder, splinter or come apart in sheets. It will spread until it runs out of moisture or wood, or the temperature becomes too hot or too cold.

Before repairing damage caused by wood rot the conditions that caused it must be corrected or it will return. This means drying up the source of water. The required moisture can come from inside or outside. The most common sources of water that feeds wood rot come from plumbing leaks, roof/siding/trim leaks, poor drainage or condensation.

It’s may be obvious when you have a plumbing, or roof, leak but many people over look drainage, and condensation, as sources of potentially damaging moisture. When the earth is sloped toward your house water is brought to your foundation. It can penetrate foundation walls, possibly causing a variety of damage and health hazards.

This seems basic, but the true drainage around a house may be hidden by poor landscaping, or beneath a deck. When landscaping, correct the drainage before you begin. A hard, clay type soil is best for draining water away from your foundation. Ideally, the ground should slope away from the house a minimum of 1/2'”/foot and continue 10 feet from the foundation.

One of the most common, damaging defects I have found in homes in the Denver area is water intrusion through the foundation from ground sloped toward the house and hidden beneath stones or bark. If the water drains toward the house, covering it with a porous medium like stone, will not fix the problem. It only makes it harder to identify the problem.

Homes are being built more energy efficient and air tight. This increases the amount of moisture in the air that is unable to escape to the outside. Bathing, cooking and indoor plants all increase indoor humidity. You may notice small black specks on the ceiling above your bath or shower, or condensation on the inside of your windows on a cold day. These are indication that you need to evaluate condensation and air ventilation.

A solution that can help control moisture, and other indoor air quality issues, is a fresh air heat exchanger. Air from inside the house is forced through a radiator like structure, venting to the outside. Outside air is simultaneously drawn through the heat exchanger collecting heat from the exiting air. These units reclaim 85% of the heat and keep the indoor air cleaner and healthier.

When the source of moisture, allowing wood rot to grow. has been eliminated all affected wood must be removed and replaced. Painting will do no good. The wood will be still be spongy and hold moisture infecting the wood around it.

The beginning of wood rot may be signaled by a water stain. Under the right conditions, wood rot can spread quickly and cause a great deal of damage. Prevention and early repair often require very little time or money. Repairing damage from moisture left to do it’s worst can be very expensive.