Wood Rot at Siding, Trim, Sills, or Fascia: What It Means on Your Home Inspection Report

Your inspector found rotted (decayed) wood outside: siding, window or door trim, a sill, or the fascia, the board behind the gutter. Rot usually shows as peeling or blistered paint, dark or bleached patches, cracks across the grain, or wood that's soft when pressed. Under the InterNACHI standards of practice, one of the standards home inspectors work under, inspectors check wall coverings, trim, eaves, soffits, and fascia, but an inspection won't identify concealed defects, such as damage behind the siding.

Rot is caused by decay fungi, which need water. The USDA Forest Products Laboratory's Wood Handbook (2021 edition) says serious decay happens only when wood is wetted past its fiber saturation point, an average 30 percent moisture content, by rain, condensation, or wet ground; humid air alone won't do it. Wood kept air dry won't decay, and decay already present won't progress. Rotted wood is a water problem as much as a wood problem.

Why It Matters

Strength. The handbook says wood with visible decay may be assumed to have greatly reduced strength in every respect. A 2002 Forest Products Laboratory fact sheet called decay the most destructive kind of microbiological damage because it causes structural failure, sometimes very rapidly.

What's behind it. The Building America Solution Center, the Department of Energy's library of building practices, says rot or peeling paint on window trim can mean a poorly draining window or missing flashing, and that cracked, warped, peeling, discolored, or rotting siding near windows and doors can signal water damage underneath. Where flashing is missing, water can get into the wall and cause mold and rotting of the sheathing, framing, and insulation. At the roof edge, metal drip edge helps keep rain out of the gap between roofing and fascia, where it could cause damage you don't see.

Obvious causes in the Solution Center's list: missing gutters and rain splashing back on the house, sprinklers aimed at it, siding that touches the ground, yards that slope toward the house, and planter boxes against it. A 1973 Forest Products Laboratory research paper said joints of end grain, such as where siding meets trim, are the most absorptive, and that paint seals on joints rarely stay tight.

Urgent (beyond repair-soon): rot in structural wood, such as deck or porch posts, joists, and beams, rafter tails, rim joists, or sill plates; sagging; or a probe that sinks deep into framing. Get a contractor promptly, and a structural engineer for structural members, and keep people off a deck or porch with rotted structure; see our page on deck ledger problems.

What to Do Before Closing

• Ask the inspector where the rot is, how much, whether they probed it, and whether they saw stains inside, insect activity, or other signs of hidden water damage nearby.
• Have a contractor evaluate it and find the water source. Rot at trim or siding can mean damaged sheathing behind it, which may mean removing some siding to see.
• Talk to your real estate agent about asking the seller to repair it or offer a credit within your inspection contingency.

What to Do After You Move In

Fix the water first. The Wood Handbook's rule for repairs: correct the moisture condition that led to the damage. If the moisture source is entirely eliminated, only the weakened wood needs replacing with dry lumber; if it can't be corrected, replace the infected parts with preservative-treated wood or the heartwood of a naturally decay-resistant species.

A carpenter or contractor replaces the rotted wood and checks behind it; a roofer or gutter installer adds missing drip edge, flashing, or gutters. Then keep water off the wood: clear the gutters (see cleaning gutters from the ground), aim sprinklers away from the house, and keep paint and caulk in good shape. The handbook recommends at least 6 inches between soil and siding.

A house built before 1978 may have lead-based paint, and scraping or cutting it can create dangerous lead dust. The EPA recommends hiring a lead-safe certified contractor, and its rule requires contractors doing this work in pre-1978 homes to be certified.

Can You Fix It Yourself?

You can find it. The Solution Center's homeowner check: push a screwdriver or your thumbnail into siding or trim anywhere you see peeling or discoloration. The wood should be firm; any squishiness or sponginess means it's getting water damaged, and you should work with an experienced contractor to find and fix the cause.

Replacing a short piece of nonstructural trim you can reach from the ground can be a homeowner job once the water source is fixed. In a pre-1978 house, the EPA recommends hiring a lead-safe certified contractor; if you do it yourself, it says to use its lead-safe work practices, such as misting before you scrape and containing the dust. Anything structural, anything behind the siding, and anything needing a tall ladder belongs to a pro; our DIY vs. hire-a-pro guide explains why structural work is always a pro's job.

Frequently Asked Questions

What is dry rot?

Mostly a misnomer. The Wood Handbook says brown, crumbly rot is sometimes called dry rot, but the term is incorrect because wood must have moisture to decay, although it may dry out later. Some fungi can carry water into wood from the soil, but they still need water.

Can I just paint over rotted wood?

No. Paint doesn't restore strength, and the handbook says visibly decayed wood has greatly reduced strength. Fix the moisture source and replace the weakened wood.

Is wood rot on trim a deal breaker?

Usually not by itself. What matters is whether it reaches the sheathing, framing, or a deck or porch structure, so get a contractor's evaluation before your contingency ends.

The Bottom Line

Rotted trim, sills, siding, or fascia means water is reaching the wood, and visible decay means lost strength. Before closing, have a contractor find the water source and check behind the rot. Then fix the water first, replace the weakened wood, and probe suspicious spots yourself. See more findings in our inspection report decoder.

Sources