Structural wood rot in a raised New Orleans home

Moisture is the common thread

The National Park Service calls uncontrolled moisture the most prevalent cause of deterioration in older and historic buildings, leading to rot and eventually to structural components. The U.S. Forest Service’s wood handbook explains the wood science: serious decay occurs when wood stays wetter than its fiber saturation point, about 30 percent moisture on average, and wood that is kept air dry, usually at or below about 20 percent, has a reasonable margin of safety against decay. Humid air alone does not wet wood enough to support significant decay, though it can allow mold; liquid water from rain, condensation, leaks, or wet ground does.

Those are general lumber figures, not a test for your house. Moisture content varies board by board and over time, and measuring it is a job for someone with the right instruments.

Wood tends to get wet from rain splash, blocked or misdirected drainage, plumbing leaks, soil or masonry contact, and damp crawl spaces. In the Gulf climate, floors above open crawl spaces can accumulate moisture even when built to code, according to an ICC and Home Innovation Research Labs technical note that cites a study of twelve raised houses in southern Louisiana.

Where it starts in a New Orleans house

The City’s historic-district guidelines note that wood located on or next to a brick or concrete foundation or pier is more likely to absorb moisture and rot, as well as attract termites, and that wood on a pier or within six inches of the ground is a target. They also point to grading and downspouts: water should be directed away from the foundation.

For termites, the same federal handbook names poorly drained crawl spaces without a moisture barrier, and substructure wood near the ground, as the places of greatest hazard. Those are also places where wood stays damp.

Signs to note, not diagnose

Why the water usually comes first

The park-service guidance is direct: avoid remedial treatments without careful diagnosis, and address ground-related moisture and rain runoff thoroughly. In practice that means the moisture source, whether drainage, a leak, or a damp crawl space, needs attention or replaced wood faces the same conditions. The City’s guidelines for historic buildings say to eliminate the moisture source first when you find soft wood, and replace it in kind.

What repair can involve

Depending on the member and how far the damage extends, repair can range from replacing a decayed section to replacing a whole sill, beam or post, with the floor temporarily supported while it is done. We do not publish steps and do not recommend homeowners do this work. See beam and sill repair. Structural changes generally need a City permit; see structural repair permits. For serious problems, the City’s guidelines note that a qualified architect or engineer can do a more detailed inspection, and termites are handled by a separately licensed pest control professional.