What it usually means, the likely causes in order, what to safely try first, what a pro charges in Canada, and when to stop and call one.
This guide covers the common cases. Describe your exact problem and get a personalized diagnosis — free to start, with a DIY vs. call-a-pro verdict and a real cost estimate.
Rotting or soft boards mean water has been getting into the wood long enough to break it down, not just stain it. Rot is a fungal process that feeds on damp timber, so it favours boards that stay wet: low decking with poor airflow, boards under planters and doormats, fence rails where water sits, and anywhere leaves collect. Surface boards are the visible part, but the same conditions attack joists, posts, and footings underneath, often earlier and more seriously. The job here is to find how far the softness has travelled before deciding whether you are replacing a few boards or rebuilding a section.
Rot does not heal or pause; it spreads through damp wood and weakens the structure a little more each season, so this genuinely worsens if delayed. Prioritise any soft board you stand on, any rot in steps, railings, or posts, and any sponginess near a ledger board against the house. A single soft fence picket can wait for a convenient weekend; a soft deck board over a drop, or a post that crumbles at ground level, should come out of service until it is dealt with.
Boards that were never sealed, or whose finish has long since failed, absorb rain and snowmelt and dry slowly. Repeated wetting breaks the fibres down from the surface and the end grain inward. This is the broad, general cause behind most deck and fence rot, especially on older, untreated, or neglected timber.
Decks built low to the ground, boards boxed in by skirting, and fence sections shaded by shrubs stay damp far longer than open, sunny runs. Planters, mats, and stored items sitting on decking trap water against the surface. Rot that is worst in the shaded, enclosed, or covered areas points directly to this cause.
Where a post meets soil or sits in a damp footing is the wettest, least ventilated point in the whole structure, and it is often where decay begins, well before anything shows on the boards above. A fence that wobbles or a deck post soft at its base is a structural problem, not a cosmetic one.
Water enters most readily through cut ends, screw and nail holes, and joints where two boards meet and hold moisture between them. Butt joints on decking and the tops of fence posts are classic entry points. Rot blooming around fasteners or at board ends, while the middle of the board stays sound, fits this pattern.
| Job size | Typical estimate (CAD) |
|---|---|
| Simple fix — the straightforward version of this repair | $150–$350 |
| Major job — complex, hidden, or replacement-level work | $1,000–$8,000 |
Estimates only, calibrated to Greater Toronto Area trade rates and adjusted by region on this site — not a quote. Your exact price depends on the cause, parts, and access. A personalized diagnosis narrows this range using your specific symptom.
Replacing individual surface boards or fence pickets that test soft, where the frame underneath is sound, is manageable DIY for someone comfortable with a saw and careful measurements, and matching the board profile matters for a tidy result. Call a pro when the poke test finds soft joists, beams, posts, or ledger boards, when rot is widespread across a whole section, or when steps and railings are involved. Structural members carry the deck and the people on it, and a pro will also judge whether the footings below a rotten post are still doing their job.
Surface treatments and fillers can tidy a very small, shallow area of decay in a non-structural spot, but they do not restore strength to wood whose fibres have broken down, and they fail quickly underfoot. On a walking surface, a post, or anything that carries load, replacement is the honest repair. If the screwdriver sinks in, the board is done, however good it might look after filler and paint.
Test in layers. Poke-test the surface boards, then, where you can reach safely, the joists and beams directly beneath the worst areas and the posts at ground level. Rot confined to a few surface boards over sound framing is a board job. Softness underneath, at a post base, or at the board against the house means the repair is structural and larger than the visible patch suggests.
No. Finish protects sound wood from taking on water in the future, but sealing over active rot traps the moisture and the decay process inside, and the board keeps weakening under a tidy-looking surface. The sequence that works is to remove the decayed wood, fix the moisture source, let sound surrounding timber dry, and then protect the finished repair with stain or sealant.
Keep water moving off and air moving through: clear leaves and debris from between boards, lift planters onto feet, trim back shrubs shading a fence, keep soil and mulch below the wood line, and direct downspouts away from posts. Renew the finish on sound wood before it fails completely, and poke-test the known damp spots each spring so a small replacement never becomes a structural one.
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