Does Painting Protect from Wood Decay?

Carol Clausen and Samuel Glass tell us about another easy and inexpensive trick for preserving your building projects in Build Green: Your Wood Can Last for Centuries. Coatings that form a moisture-impervious film are effective in preventing decay if the seal where two pieces of wood are joined is kept intact. A well-maintained film of paint over the wood and good paint seals in the joints shed water, thus protecting wood by keeping it dry.

Cracked paint seals permit entry of water and contribute to decay. Water seeping through the broken seal where wood is joined together will be rapidly absorbed at the ends of the wood pieces; its loss by evaporation from the sides of the wood member is retarded by the impervious, intact paint film over the rest of the wood. This keeps the wood moist and favors decay, at least near the joints. The effectiveness of a moisture-impervious paint in protecting wood from decay depends upon the quality of the seal in the joints.


Keeping the paint seal at wood junctures intact is critical. These are the points where moisture can seep into wood.

Keeping Wood Dry Isn’t Always Enough: Beware of Dry Rot

Build Green: Wood Can Last for Centuries by Carol Clausen and Samuel Glass tells us that most wood-decay problems only occur when decay fungi grow in wet wood. But one kind of fungus is uniquely capable of transporting its own water from a source of moisture (usually soil) into wood that is typically too dry to decay. While decay by such water-conducting fungi is uncommon, when it occurs, it is devastating.


Dry rot starts when an infected piece of wood forms a bridge between soil and other wood in a house (Photo by Carol Clausen, FPL).

Large areas of flooring and walls can be destroyed each year unless the fungus is stopped. Ironically, it may be the easiest fungus to prevent or control. Unlike typical decay fungi that start growing from airborne spores, water-conducting fungi usually start growing from previously infected lumber that forms a bridge between the soil and other wood in the house. This can happen if old, discarded beams that have been lying on the ground are used in home repairs or additions; new wood has been improperly stored in contact with soil; or infected wood waste is used as fill under a porch or addition. This type of decay can be stopped by simply breaking the contact between susceptible wood and the source of moisture. Once the water supply is broken and the infected wood dries, the fungus will die.

Good news for the homeowner: a potentially destructive source of rot is easy to control with attention and care.

Homeowners: Beware of Plumbing Leaks

According to Build Green: Wood Can Last for Centuries, an obvious source of wood decay is wood that is wetted continuously by plumbing leaks. The leak itself is not always as obvious. Spillage behind a washing machine, leaks in caulk at the top of a bath tub, or plumbing leaks in a shower stall often go unnoticed for a long time. If so, they can lead to a serious decay problem in the floor and lower parts of interior walls. Annual inspections and minor home maintenance can prevent this type of problem.


Leaks in plumbing can be difficult to spot since pipes are typically hidden behind the fixture or the wall. Watch for damage at corners of the floor trim and water stains on the ceiling beneath the bathroom, kitchen, or laundry room. Concealed leaks can cause serious damage over time if they are not located and fixed. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)

Decay Hazard: Water Collecting against Wood

So many things the homeowner must consider when trying to protect wood from decay! According to Build Green: Wood Can Last for Centuries, water pooling on the hard surface of a porch, sidewalk, or patio as a result of draining from the roof presents another decay hazard.

Decay problems arise in the base of porch posts, entryway trim, and door jambs that rest directly on sidewalks, driveways, or porch and patio slabs. Wood resting in pooled water results in water wicking into the end grain of the wood through capillary action. Bases of wooden posts need protection, particularly in regions with high hazard for decay.


Wood resting directly on a sill makes it prone to decay.

Preservative-treated wood is recommended in these applications. Otherwise, be certain that roof runoff is directed away from these structures and that posts, trim, and door jambs do not rest directly on the hard surface.

Beware of Splashing Rain

Build Green: Wood Can Last for Centuries cites splashing rain as another source of moisture that can make wood decay. As an example of this, water flowing off the roof and splashing against the house may also wet wood siding enough to permit decay.

This problem frequently occurs where water flows from the roof onto a hard-surfaced patio, sidewalk, or entryway parallel to the home. Sometimes this problem is inadvertently increased through the use of diverters over the doorway such as an entry canopy. If the stream of water draining from the diverter flows or splashes against exterior woodwork, wood that otherwise could provide satisfactory service may decay.

Usually, this hazard can be prevented through careful design of house and entryway and through appropriate use of preservative-treated products at the time of construction. In existing houses, rain gutters should be installed with downspouts that direct water away from the house.


A wide roof overhang directs water runoff from the roof away from the exterior walls. The splash zone is also moved farther away from the building. (Photo by Carol Clausen, FPL.)