Walls and attics get insulated; the narrow band where the floor meets the foundation gets skipped more than anywhere else in the house.
Home › Floor structure
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.
Floor structure guides
It's the lowest piece of framing in the house, bolted to the foundation, and usually the first piece of wood an older house loses.
Sistering, releveling and full replacement all get called the same thing online, and they fix different problems.
Fastening new lumber alongside a damaged joist works for one notch or crack. It stops working once the damage spreads further than that.
A sag near a wall, one centered at mid-span, and a floor that's soft everywhere each point toward a different cause.
Batts between the joists keep the floor warmer, but over a damp crawl space they can hold water against the wood.
Missing insulation, air leaks and moisture all feel like a cold floor, and each needs a different fix.
Piers, beams and joists hold the floor above the soil. Moisture and settling are what to watch for.
A slope tilts the whole room one way. That points at the support under one side, and at water, more often than at the framing.
Bounce comes from joists that flex too much. Long spans, notches and damp wood are the usual reasons, and all are visible from below.
Cupped boards mean the underside is wetter than the top. Fix the moisture first and leave the sanding for last.
Soft spots and dark stains point at water under the floor. Find the source before you repair anything.
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.