Should rotted wood be repaired or replaced?

First question: does it hold anything up?

This is the line that matters. A joist, beam, post, ledger, stair stringer, header, or railing post is a structural member. Its job is strength, and decayed wood has permanently lost strength no matter how dry and stable it looks afterward. Levi's training as an architect makes him conservative here: structural members with rot get cut back to bright, sound wood and replaced or sistered with new lumber, full stop.

Trim, fascia, casing, sills, column wraps, pergola rafter tails, and decorative elements are different. They keep water out and look right, but they carry little or nothing. For those, a repair that restores the surface and seals out water can be perfectly sound.

Second question: how far does it go?

Probe around the soft spot in every direction, including along the grain, where decay travels fastest. If firm wood is within an inch or two and the piece is non-structural, consolidation and filling is reasonable. If the soft area runs the length of a sill or through the thickness of a board, replacing the piece is faster, cleaner, and lasts longer than trying to rebuild it in filler.

A quick field test: if more than roughly a third of a trim piece's cross section is gone, or the rot has reached a joint, replace it. Partial repairs of badly decayed trim tend to open up at the seam between filler and wood within a few seasons.

Third question: can the water be stopped?

Any repair, patch or replacement, fails if the wood goes back to being wet. So the decision includes what is causing the problem and whether it can be corrected. A sill rotting because a sprinkler hits it twice a day is a sprinkler problem with a carpentry symptom. A joist rotting because a balcony has no flashing needs flashing before it needs lumber.

When the water cannot be fully controlled, such as a post base that will always see rain, we change the detail: lift the post onto a standoff or a pedestal, use pressure-treated or naturally durable stock, and seal end grain so the new piece can survive the exposure the old one did not.

A balcony is a good example of both

On a second-story balcony Levi rebuilt, the joist ends and the ledger area had rotted where water had been sitting for years. Those were structural, so every joist was cut back to sound wood and sistered or replaced, and the connection to the wall was flashed properly for the first time. The railing, by contrast, was rebuilt new rather than patched because the pickets were failing everywhere, not in one spot. The decking came up and went back as new material because the old boards were the reason water sat on the framing.

• Structural members: cut back and replace or sister
• Trim with shallow rot and sound wood close by: consolidate and fill
• Trim with deep or widespread rot: replace the piece
• Any of the above: fix the water first

What this looks like in a scope

We write it out piece by piece. Which members are replaced, which are sistered, which trim is repaired in place, and which is new. We note the material and finish for each so the repair matches what stays. That level of detail is what keeps a rot repair from growing in the middle, and it is what lets you compare a thorough repair to a quick one honestly.

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