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Deck rot repair

Rot on a deck is either a board you replace in an afternoon or a joist that has to be sistered, and the difference is decided by probing, not by looking. Soft wood under a sound paint film is the normal case.

Where rot starts

Post bases, at the concrete line, where moisture sits and air does not reach.
Ledger-to-wall joint, if the flashing was omitted or has failed.
Board ends, because end grain absorbs water 10–20 times faster than the face.
Around fasteners, where a hole lets water into the core.
Stair stringers, at the bottom where they meet grade.
Under planters, mats and furniture, where water is trapped.

Probing, not looking

Use an awl or a screwdriver and push. Sound wood resists; rotted wood yields with light pressure and the tool sinks in. Test in several places, because rot is often local — a board can be sound at one end and gone at the other.

What to look for on the surface:

Discolouration that is darker than the surrounding wood, especially with a white or grey fringe.
Paint or stain that has blistered or lifted in a patch, which usually means moisture behind it.
Soft spots that give slightly underfoot.
Mushrooms or fruiting bodies, which mean the rot is advanced and the wood is being consumed.

Cosmetic rot: replace the board

A rotted deck board is a 20-minute job. Cut it out, or remove the fasteners and lift it. If the board is face-screwed, back the screws out; if it is hidden-fastened, you may have to cut the board along the joist and pry it out in pieces.

Before installing the new board, treat the joist top with a preservative if there is any sign of surface rot, and seal the ends of the new board. A board replaced without sealing its ends will rot at the same rate.

Structural rot: sister or replace

A rotted joist cannot be repaired with filler. Two options:

Sistering. A new joist of the same size bolted alongside the rotted one, spanning from the ledger to the beam, with the load transferred at both ends. This works if the rot is localized and the ends are sound.
Replacement. Required if the rot reaches the bearing points, because a sister cannot transfer load through a rotted end.

A rotted ledger is the serious case. It means the band joist behind it may also be rotted, and that is a wall repair, not a deck repair. A deck whose ledger has lost section should not be used until it is evaluated.

Filler is not a repair

Do not fill structural rot with epoxy putty. It is fine for a cosmetic defect in a non-structural member; it does not restore the strength of a joist.
Do not paint over rot. It traps moisture and accelerates the decay, and it hides the problem from the next inspection.
Do not replace a rotted board without finding the water source. The new board will rot too.

Stopping it coming back

Fix the water: flashing at the ledger, drainage away from post bases, ventilation under a skirted deck.
Seal cut ends during installation, every time.
Keep the deck clear of soil and mulch contact — 6″ of clearance at the frame is the usual minimum.
Use hot-dip galvanised or stainless fasteners, because corroding fasteners are a path for water.

What this page will not assess

It does not assess structural adequacy. If the ledger or the band joist is involved, get it looked at.
It does not cover termites or carpenter ants, which look similar and need a different response.
It does not price the repair — see deck repair cost.
It does not cover composite, which does not rot but can fail at the fasteners.

Questions people actually ask

How do I know if deck rot is serious?

Probe it with a screwdriver. If the wood yields easily, the section is gone. Rot in a deck board is cosmetic; rot at a joist bearing point or in the ledger is structural.

Can I repair a rotted deck joist?

A localized rot away from the bearing points can be sistered with a new joist bolted alongside. If the rot reaches the ends where the joist bears, it has to be replaced.

Will wood filler fix deck rot?

Only cosmetically, and only in non-structural members. It does not restore strength and it does not stop the decay if the water source is still there.

Why does my deck keep rotting in the same place?

Because the water source has not been fixed. Common causes are a missing ledger flashing, a post base sitting in wet soil, or a planter trapping moisture against the wood.

How to flash a ledgerThe most common source of rot. Deck inspection checklistWhere to look before buying a house with a deck. Deck repair costWhat each repair runs.

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