Short answer
Signs of exterior siding rot include soft or spongy spots, flaking or swollen boards, dark staining, crumbly edges, musty smells, and paint that won’t hold. To stop leaks, locate and fix the water entry first (failed caulk, flashing, gutters, or grade issues), then repair or replace damaged siding and trim. Small areas can be consolidated with epoxy; larger sections usually need board replacement and proper flashing/WRB repair.
What rot looks like (and where it hides)
Look for:
- Soft, punky wood that gives under a screwdriver or awl
- Blistering/peeling paint, swollen or wavy boards, cupping
- Dark streaks, green/black growth at butt joints and end grain
- Rusting nail heads, water stains on interior walls near the area
- Gaps opening at trim joints and around windows/doors
- Repeated wetting zones: near downspouts, splashback from decks/patios, bottom course close to soil, roof-to-wall intersections
Tip: A moisture meter reading above ~15–18% in wood siding indicates elevated moisture; above 20% risks decay fungi.
Find and stop the water source first
Most siding rot is a symptom of water management failures. Check these in order:
- Roof-to-wall intersections: missing or short kick-out flashing at the end of step flashing
- Window/door perimeters: cracked caulk, failed head flashing/drip cap
- Penetrations: hose bibbs, light fixtures, vents—often unflashed or poorly sealed
- Gutters/downspouts: clogs, overflows, downspouts dumping near walls
- Ground clearance: wood/fiber-cement too close to grade or hardscape causing splashback
- Sprinklers hitting siding; snow or mulch piled against walls
Perform a controlled hose test: start at the lowest suspect area and work upward, one zone at a time, for 2–3 minutes each, while a helper watches inside for moisture.
Tools and materials
- Inspection: awl/screwdriver, moisture meter, flashlight
- Removal: oscillating multi-tool with bi-metal blade, pry bar, siding gauge, zip tool (vinyl), circular saw set to siding thickness, nail puller/cat’s paw
- Flashing/WRB: butyl flashing tape, self-adhered flashing membrane, metal drip cap, kick-out flashing, house wrap (WRB), cap staples/tape
- Fasteners: stainless or hot-dip galvanized siding nails/screws per manufacturer
- Sealants/finishes: high-quality polyurethane or silyl-modified polymer exterior sealant, exterior primer (oil/alkyd or bonding), 100% acrylic paint
- Wood repair: borate wood preservative, epoxy consolidant and filler (2-part), exterior wood putty for minor dings
- Safety: cut-resistant gloves, safety glasses, dust mask/respirator, hearing protection, stable ladder
Step-by-step repairs
A) Minor rot in wood trim or small spots
- Dry the area: allow 1–3 dry days if possible (use a fan). Target wood moisture under ~15%.
- Remove loose material: scrape paint and decayed wood until sound.
- Treat: brush in borate preservative on cleaned wood; let dry.
- Consolidate: soak with epoxy consolidant; allow it to penetrate and cure.
- Fill/shape: apply epoxy filler, sculpt to profile, sand after cure.
- Prime and paint: spot-prime repaired areas, then two coats of exterior paint.
Time: half day active work plus curing. Materials: $50–$120.
B) Replace rotted lap/board siding (wood or fiber cement)
- Protect and prep: cover landscaping; set saw depth to siding thickness only. Ladder safely.
- Remove damaged boards: cut paint line at seams; gently pry. Use an oscillating tool to cut nails at overlaps to avoid damaging sheathing. On fiber cement, follow manufacturer’s removal method and use respirator when cutting.
- Inspect substrate: check WRB (house wrap), sheathing, and framing. Replace rotten sheathing; treat stained but sound wood with borate.
- Fix water management:
- Patch WRB shingle-style with 6" minimum laps; tape seams as rated by the WRB manufacturer.
- Install or repair metal head flashing/drip caps over windows/trim and add kick-out flashing at roof-to-wall terminations.
- Prepare new boards: back-prime all sides/ends of wood siding; seal end cuts on fiber cement with manufacturer-approved sealer.
- Install: blind-nail per pattern into studs with stainless/HDG fasteners; maintain factory lap and reveal. Leave 1/8" gap at vertical joints; use slip flashing or joint flashing behind butt joints.
- Seal and finish: caulk vertical joints and trim interfaces (not horizontal laps). Prime and paint.
Typical cost: $200–$400 for a small section (DIY). Time: 4–8 hours.
C) Vinyl siding notes
- Rot usually isn’t the vinyl—it’s the sheathing behind. Use a zip tool to unlock panels, fix WRB/flashing, then reinstall. Don’t caulk bottom weep holes or lock seams.
Safety considerations
- Pre-1978 paint may contain lead. Follow EPA RRP guidelines (containment, HEPA cleanup, PPE).
- Cutting fiber cement produces silica dust—use a P100 respirator and score-snap or shears when possible.
- Ladder placement on firm, level ground; maintain 3 points of contact.
- If you see mold inside walls or significant structural decay, stop and reassess.
Key specs for success
Clearances:
- Wood/fiber-cement to grade: 6–8 in
- Bottom siding to roofing: 1–2 in
- Deck/flat surfaces to siding: 1 in minimum
Fasteners:
- Stainless or hot-dip galvanized; length to penetrate studs 1–1/4 in
Sealant joint:
- Ideal depth/width ~1/4 in with backer rod; don’t caulk horizontal laps or weeps
Paint/finish:
- Apply when wood moisture <15%
Flashing laps:
- Minimum 4–6 in shingle-style overlap; add kick-out at roof-to-wall
Tips for best results
- Back-prime wood and seal all end cuts before install.
- Keep gutters clean; extend downspouts 4–6 ft from foundation.
- Re-aim sprinklers away from siding.
- Stagger butt joints and use joint flashing behind them.
- Annual inspection after heavy storms catches issues early.
Common mistakes
- Relying on caulk alone without fixing flashing/WRB.
- Trapping moisture by caulking weep paths and horizontal laps.
- Using electro-galvanized nails that rust and stain.
- Painting wet wood or skipping primer.
- Setting siding too close to grade or roofing.
When to call a pro
- Widespread rot, soft sheathing over large areas, or structural decay.
- Reflashing windows/doors or adding kick-out flashing tied into existing roofing.
- Multi-story work, steep ladders, or limited access.
- Asbestos-cement shingles/siding (requires special handling).
Fix the water path first, then repair the materials. Do that in the right order and your repair will last.