What is wood rot?

MMH Expert_Matt T EXPERT April 24, 2008

Rot is actually a fungus, and the fungus spores are always in the environment ready to attack. When the moisture content of the wood is above 20 percent, and external temperatures are between 40F and 115F, rot should be expected. The source of the water can be roof or gutter leaks, ice dams, open joints allowing rain penetration, condensation as a result of warm, moist air escaping from inside the building and cooling, or wood too close to grade, absorbing moisture from the soil.

Rot is caused by

• water penetration
• poor drying

Water penetration is common where

• wind-driven rain is common
• roof overhangs are small
• details at penetrations (e.g., windows and doors) are done poorly

Wood loses its structural integrity when rot sets in. Rotted siding is usually replaced unless the rot is very localized. In special cases, such as historical buildings, rotted wood can be injected with epoxy to restore it.

Inspect by probing and pressing as well as looking at wood. In some cases, rotted wood which has been painted over looks to be in good condition. Probing the wood will show whether rot is present. Pay particular attention to areas where rot is most likely to get a foothold. These include areas close to the ground, at joints and end grains, or where siding is within an inch of a roof surface, on dormers or porches, for example.

Wood is more susceptible to rot at the end grains than along the length of the board. Since the ends of boards are susceptible, pay attention to these areas. Butt joints should always occur over studs or other rigid supports. Wood siding may have the bottoms of the boards cut back on a 45-degree angle (bevel), which helps water drip off the face of the board rather than be wicked into the end grain. While this detail is not required, it indicates good work.

If the outside corners are mitered, pay particular attention to these. Any imperfection in the joint exposes end grains of wood to moisture. Rot can get a foothold here. Corner boards or caps are a better way to finish outside corners.

Butt joints on vertical siding boards should be cut on a bevel so the upper board overlaps the lower board and drainage will be by gravity to the outside of the joint. Cut ends should be sealed with paint or stain to prevent moisture penetration. Inspect butt joints in any board siding carefully.

When siding is installed on the diagonal, the joints should be cut so that they are both mitered and the joint is vertical. Joints at 90 degrees to the long dimension of the board may trap water in the joint.

Watch for joints in alternate courses of horizontal or diagonal siding or shingles lining up vertically. It is much better if these joints are staggered.