How to Repair Rotted Wood on a Fence

Rotted fence wood starts invisibly—fungal decay working from the inside out. By the time you see soft, discolored boards or spongey sections, the damage is real. The good news is that not every rotted fence board means replacing the whole fence. Catch it early, and you can patch sections. Let it go, and you're pulling posts. This guide covers both: surgical repairs for surface rot and the harder choice of full board replacement when the decay runs deep.

1. Know What You're Dealing With. Press a screwdriver or awl into the damaged wood. If it sinks easily past the surface, the rot is deep. If it stops in a quarter-inch, it's surface rot. Check at least three spots on the affected area—rot spreads unevenly. Mark the boundaries with a pencil so you know exactly what needs to come out.
2. Excavate Decay Completely. Use a chisel and mallet, or a reciprocating saw with a wood-cutting blade, to excavate the soft wood. Work aggressively—hesitation leaves decay behind that will spread back under any patch. Chisel perpendicular to the grain when possible, creating clean edges that will accept filler or a new board. Cut away all discolored, soft, or spongy material until you reach hard, sound wood.
3. Clean to Bare Wood. Once you've removed the soft wood, scrape the cavity walls with a paint scraper or wire brush to remove all debris, fungal growth, and loose fibers. Wipe the area with a dry cloth. The cavity should be clean enough that you wouldn't cringe touching the inside of it. Moisture and mold spores remaining in the cavity will seed new rot beneath your repair.
4. Stop Decay Dead. Brush a wood fungicide into and around the cleaned cavity. If the rot reached the interior of a post or extends behind facing boards, inject fungicide into the wood with a pump applicator. Fungicide stops dormant spores and prevents decay from advancing into wood you're about to cover. Allow the wood to dry completely before patching—typically 24 hours in dry weather, longer if it's humid or the cavity is deep.
5. Fill the Void Solid. For surface rot or small cavities (under 1 inch deep), use a two-part epoxy wood filler or exterior-grade polyurethane filler. Mix per instructions and press it firmly into the cavity with a putty knife, overfilling slightly. Smooth it flush with the board surface. These fillers cure hard and can be sanded, stained, or painted. This method works on boards you're keeping; it's not a solution for heavily damaged posts.
6. Start Fresh, Start Right. For rot deeper than an inch, or any decay in fence posts, remove and replace the entire board. Unscrew or unbolt the damaged board from the rails and posts. Measure its length and width carefully. Install a new pressure-treated board of the same dimensions, securing it with corrosion-resistant fasteners (stainless or coated deck screws). Treat the cut ends of the new board with wood preservative before installation.
7. Seal Out Water Forever. Once filler has cured (check product instructions—usually 24 hours), sand the repair smooth with 80-grit sandpaper, then 120-grit. Prime and paint, or stain and seal, to match the rest of the fence. Sealing is not optional: unfinished repairs will re-rot faster than the original wood did. Use an exterior wood stain or paint that sheds water rather than a film-forming finish that can trap moisture.
8. Stop Rot Before It Starts. Walk the fence line where you made repairs. Look for water pooling at the base, ground contact above the fence trim, or vegetation pressed against boards—all accelerate rot. Clear debris from fence corners and along the bottom. Consider a landscape fabric or gravel border to move dirt away from the fence base. Annual sealing of the entire fence extends its life by years.