Controlling wood deterioration due to moisture and fungus

Reply Tue 7 Sep, 2010 08:13 am
By BumbleBeeBoogie

Before my retirement as the architectural administrator for a 3,000 homes master homeowners association in California for over 14 years, I saw a lot of unnecessary damage caused by decay fungus. It is sad to see how much money is spent each year on these preventable home repairs, mostly to trim and siding .

More than 5% of all construction lumber manufactured in the U. S. is used to replace wood that has decayed in existing structures. Damage to wood frame buildings by mildew, mold, staining, and decay is easily preventable. It is usually caused by design flaws, poor workmanship, and or neglected maintenance.

Microorganisms cause mildew, mold, staining, and decay; they belong to a huge group of primitive plants called fungi. Fungi have four growth requirements similar to our basic human needs: food, water, air, and a satisfactory temperature. Decay will be prevented if any one of the four requirements is removed. The most effective way to prevent fungal deterioration is to keep the area dry.


* Mildew

Exterior mildews often appear on unheated projecting parts of buildings such as eaves that cool quickly after sunset causing dew to form. North facing walls and walls shaded by trees and other obstructions that restrict sunlight and airflow are also candidates. While mildew won't grow where siding crosses studs and other thermal bridges, mildew may thrive over the cooler, insulated bays between studs, where dew persists to provide the needed moisture. These mildews are easily removed with a chlorine and detergent solution.

* Mold

Molds need a surface moisture content of about 20% to get started. Molds tend to be a discoloration on the surface of wood. Discoloration aside, molds generally have little effect on wood's integrity. Prevention lies wholly in controlling air moisture levels and condensation through proper site drainage and ventilation.

* Staining Fungi

Staining fungi is found almost exclusively in freshly sawn lumber. If not treated, or if the moisture content is not reduced to an intolerable level, this fungi can destroy certain wood cells leaving the wood susceptible to decay.

* Decay Fungi (Dry Rot)

While discoloration by mildew, mold, and staining fungi is only an appearance problem, decay fungus threatens the structural integrity of wood. Suitably termed the "slow fire", these fungi eat the very cellulose of which wood cells are made. Decay fungi are what we commonly call dry rot, even though it is caused by water.

Moisture content is the critical factor that makes wood susceptible to decay. It must exceed 28%, and liquid water must be present in cell cavities before decay fungi can gain a toehold. Once established, some fungi can carry on their destruction at a moisture content as low as 20%. When moisture content falls below this level, all fungal activity stops. That's one reason why framing lumber is dried to 19% moisture content or less.

It doesn't take a trained eye to recognize decay in its advanced stages. Wood is visibly discolored, spongy, and musty. Surfaces may be stringy, shrunken, or split across the grain. Decay extends deep into the wood and strength loss is significant.

Before repainting the exterior or interior of your house, look for evidence of mildew or mold on the wall surfaces. Remove mildew or mold with an application of a bleach and water mixture. When you purchase your house paint, have a mildicide added to the the paint or purchase the mildicide at your local paint store and add it to the paint yourself.


Like mold, mildew, and staining, drying up the moisture can stop existing decay. But remember that to make the remedy permanent, you've got to cure the disease (water infiltration) not just treat the symptoms (mildew, mold, and decay).

The first and most important step when you find decay is to learn the source of the water penetration. Once the source has been shut off, remove and/or replace as much of the decayed wood as is practical and economical. Decayed wood absorbs and holds water more readily than sound wood, inviting further decay and insect attack. Let any areas you don't restore dry out before making repairs or this moisture will be the fuel for a continuation of the "slow burn".


The best defense against decay is preventing water intrusion through design, good workmanship, and proper maintenance. Properly placed metal flashings are a must with building methods and materials of today. Proper application of caulking also is very important. Proper means not trapping water with a caulking dam.

People appreciate the details of old Victorian homes where every exterior piece of wood harmonized with each other to shed water down and away from the structure. Not many of us could afford to build a house that way today, but we need to keep the principles the same. When building or repairing, try to think like water; how can I get in this house? How can I keep that intruder out?

Annual inspections and qualified repair personnel are key to a healthy exterior. A home owner or property Manager should be on the constant lookout for problem areas. If you don't know what to look for, ask a professional. An experienced contractor can survey your properties and submit a report of the specific conditions, location of each problem, and a recommended repair scenario with materials specifications. This survey can be presented to repair contractors for estimates.

Through proper design, workmanship, and maintenance, many dollars can be saved on wood building repairs each year.
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Reply Tue 26 Apr, 2011 07:20 am
please help i am looking for information based on the statement that says what is the effect of moisture content on there strength of wood and why does wood grain always run straight the stem?plz my assignment is due next week wesnesday.email [email protected]
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Gutter cleaning
Reply Tue 14 Mar, 2017 12:30 pm
A good way to control this is to have your gutters cleaned on a routine basis. Clogged gutters can cause severe moisture problems around your home.
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