Frequent Rains: Another Decay Hazard for Wood

In Build Green: Wood Can Last for Centuries, Carol Clausen and Sam Glass remind readers that moisture causes wood to decay, and another source of moisture that the homeowner must guard against is frequent rains.

The natural hazard for decay of wood used above ground is greatest in regions with high rainfall and mild climate. In the contiguous United States, this occurs in the Southeast and Northwest.


The map shows three zones of decay hazard in the United States.

What does that mean for the homeowner? Remember–the decay hazard for homes is greatest on the side that is exposed to prevailing rains. This is most noticeable in the region of highest decay hazard, where greater precautions against decay may be needed on the more exposed side of the house than on the remaining sides.

Interestingly, this photo shows that a high-decay hazard can even be artificially created when wood siding is repeatedly wetted by a lawn sprinkler, faucet, or vegetation that is planted too close.


With proper attention, treated wood, and building techniques, the homeowner can guard against the threat of decay from frequent rains even in the most rainy locations.





Proper Soil Grading Helps Keep Decay at Bay

According to Build Green: Wood Can Last for Centuries, a common homeowner and contractor mistake following construction projects is to set the finish grade for soil or mulch above the level of wood framing. Soil contact is one of the primary culprits in wood decay.


Soil graded high against the exterior brick veneer will contribute to decay problems in untreated wood members below the grade line. Similar problems will occur with soil graded high against stucco and siding.

Where untreated wood is used in a structure, it should be at least eight inches above the finish grade for framing members and six inches above finish grade for siding. Composite products should never be used in contact with soil.

Preservative treatments are designated for above-ground use or in-ground contact (buried in soil or touching soil). When planning your building, it is important that you specify the right treated wood for your specific need and that you insist that the treatment be of certified quality and be labeled accordingly.


Here’s what not to do. In this photo, the finish grade on the yard is above the level of wood framing inside the wall. To make matters worse, the lawn sprinkling system is providing constant wetting of the stucco siding on this home. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)

A vast array of treated wood is available for the homeowner. Choosing a preservative approved for ground contact, properly grading soil, and avoiding constant wetting will go a long way in protecting your outside wood structure from decay caused by the moisture in soil.