Wood Rot Repair: Finding, Fixing and Preventing Rot

Why wood rots

Rot is caused by decay fungi that feed on wood. According to the USDA Forest Products Laboratory's Wood Handbook, decay fungi need wood with a high moisture content, generally above the fiber saturation point of roughly 30%, to get started, and wood kept below about 20% moisture content is generally safe from decay. On the Gulf Coast, warm temperatures and long humid spells make the conditions easy to reach wherever water collects.

The usual suspects are the parts of a house that stay wet: window sills and the bottoms of window and door trim, the lower courses of siding near soil or mulch, fascia boards behind overflowing gutters, porch posts and railings, and anywhere caulk or paint has failed and lets water into end grain.

Finding it

• Press with a screwdriver or awl. Sound wood resists; rotted wood is soft, spongy or crumbles.
• Look for paint that blisters or peels repeatedly in the same spot, dark stains and cracked, cube-like checking in the wood.
• Check the bottoms of door jambs, the corners of sills and anywhere two boards meet.
• In the Gulf region, distinguish rot from termite damage: mud tubes, hollow-sounding wood with galleries following the grain, or discarded wings suggest termites, which need a pest professional.

Repair or replace?

Repair with epoxy when the damage is small and localized, the piece is decorative or hard to replace (an old sill, a molded trim profile), and the surrounding wood is sound:

1. Dig out all soft wood back to firm material.
2. Let the area dry thoroughly.
3. Apply a penetrating epoxy consolidant to harden the remaining fibers.
4. Fill with a two-part epoxy wood filler, slightly proud of the surface.
5. Sand or shape when cured, then prime and paint.

Replace when the rot is extensive, when the piece is structural (posts, joists, sill plates, rim boards), or when a standard board is cheaper to swap than to rebuild. Use primed or treated material, back-prime all sides and seal end grain before installing, and leave a gap between siding and roofing, decks or soil as the manufacturer specifies. Structural rot, or rot that extends into framing behind siding, needs a qualified contractor and may need a permit.

Stop the water

• Clean and repair gutters and extend downspouts away from the house.
• Keep soil, mulch and plantings several inches below siding.
• Replace failed caulk at butt joints, windows and trim, and make sure flashing directs water out over the siding, not behind it.
• Fix leaking hose bibs, sprinklers that spray the walls, and AC condensate lines that drip on wood.
• Indoors, rot under windows or around tubs points to condensation or leaks; the EPA's mold guidance stresses fixing the moisture source first.

Paint it properly

A sound paint film is the wood's protection. Prime bare wood and epoxy repairs promptly, use a quality exterior acrylic, and keep up with touch-ups. On pre-1978 houses, scraping and sanding around rotted trim can release lead dust; see how to test for lead paint and the RRP rule. For the paint job itself, see exterior painting and how long exterior paint lasts.

Tools and materials for small repairs

• A stiff putty knife, chisel or carbide scraper to remove soft wood.
• A moisture meter, to confirm the wood has dried before treating it.
• Two-part epoxy consolidant and epoxy filler made for exterior wood repair.
• Exterior-grade primer and paint, and a good-quality exterior caulk for joints.
• Gloves and eye protection; epoxies can irritate skin.

Work in dry weather and follow the product's temperature limits. Epoxy cures slowly in cool conditions and fast in heat.

When to call a professional

Bring in a carpenter or contractor when rot reaches structural framing, when it keeps returning in the same place despite repairs, or when siding must come off to find the extent of the damage. Ask them to identify and fix the water source as part of the job.