Where does wood rot usually start on a house or deck?

The pattern behind every rot spot

Wood rots where it gets wet and cannot dry. That sounds obvious, but it predicts almost every repair we do. Look for three conditions: end grain exposed to water, horizontal surfaces or joints that hold water, and wood touching something that stays damp, like concrete, soil, or stucco. Anywhere two of those overlap is where you should put the screwdriver first.

Post bases at grade

A post is a bundle of straws standing on end. Set it on a patio slab or in a concrete footing and the bottom end grain drinks up every rain and every sprinkler cycle, then dries slowly because the concrete holds moisture against it. Older North County homes often have porch and patio posts set directly on pavers or concrete, and the bottom few inches are the first thing to go.

The fix is to get the wood up off the surface on a standoff base or a masonry pedestal so the end grain can dry. We rebuilt one porch column this way, shortening the post and setting it on a brick base with a cap so it never sits in water again.

Ledgers and joist ends

The ledger is the board bolted to the house that carries one side of a deck or balcony. Without proper flashing, water running down the wall slips behind the ledger and soaks it, along with the rim joist and sheathing of the house behind it. Joist ends are end grain too, and on an open balcony they sit under a drip line from the deck above.

These are structural members, so damage here is a safety problem rather than a cosmetic one. Our balcony rebuild answers go into more detail, but the short version is that if the joist ends are soft, the decking comes up and the structure gets scoped before anything else.

Fascia, barge boards, and gutters

Fascia carries the gutter, and a gutter that overflows or lacks a drip edge pours water down the back of it. Barge boards on gable ends take the brunt of afternoon sun on their faces and weather at every miter joint. Both are usually painted, so the first sign is paint that bubbles or peels along the bottom edge.

Railing posts, stair stringers, and deck boards

A railing post bolted to the rim joist traps water where the deck boards meet it, and the notch many builders cut at the bottom of the post invites rot right at the point of greatest stress. Stair stringers rot where they land on concrete. Deck boards rot at the ends and wherever leaves and dirt sit between them.

Window and door trim

Sills are horizontal and face up. Casing meets the sill at a joint that opens as the wood moves. On stucco houses, trim pressed tight against the stucco wicks moisture from it. When caulk fails at any of these joints, water gets behind paint and the trim rots from the back. Door jambs and thresholds that face the weather, especially the bottom six inches, are another frequent find.

• Post bottoms on concrete, pavers, or soil
• Deck and balcony ledgers and joist ends
• Fascia behind gutters and barge boards on gables
• Railing post bases and stair stringer feet
• Window sills and the bottom of door jambs and casings

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