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The wood under the floor: joists, sill plates and rot
Damp under a house eventually stops being a moisture question and becomes a carpentry one, and the first parts to go are the ones closest to the ground.
Where persistent damp shows up as damage
Everything above a crawl space rests on wood, and wood in contact with persistent moisture eventually loses strength. This section is about what that looks like before it becomes dramatic.
The sill plate — the horizontal timber sitting on top of the foundation wall, carrying the framing above — is the most exposed piece in the building. It is at the bottom, it is in contact with masonry, and masonry moves moisture. Rot there is serious precisely because of where it sits in the load path. The rim joist, the band closing off the ends of the floor joists, is the other classic trouble spot: it is a cold surface at the edge of a conditioned floor, which makes it a place condensation collects, and it is often the last part of a house to get any insulation attention.
Joists themselves fail more gradually. A floor that has developed a noticeable bounce, a slope that shows up when something rolls across a room, doors that have started to bind — these are worth investigating from below rather than explaining away. Sistering, the practice of fastening a new member alongside a damaged one, is the standard repair, and the reason we describe it here rather than instruct it is that whether a joist can be sistered, or whether the load needs to be carried differently, is a judgement about the structure.
Two adjacent things are deliberately out of scope. Termites and wood-boring insects are their own subject with their own inspection regime, and although damp wood and insect damage often turn up in the same crawl space, telling them apart is specialist work. And anything involving temporarily supporting a floor while a member is replaced is not a homeowner activity in any form — the load has to go somewhere while the wood is out, and getting that wrong has consequences that are not reversible. As everywhere on this site: the assessment belongs to a licensed structural engineer.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.