What Are the Rim Joist and Sill Plate?
The sill plate is the horizontal board bolted to the top of the foundation wall. The rim joist (also called the band or rim board) is the framing member that runs around the outside edge of the floor system on top of it, capping the ends of the floor joists.
Explanation
Together these two pieces form the joint where your wood house meets the masonry foundation. From inside the crawl space, look up at the top of the foundation wall. The sill plate is the flat board lying on the wall. The rim joist is the board standing on edge above it, around the perimeter.
That joint matters for three reasons:
- Moisture: The sill sits right on masonry, which can wick ground moisture upward. The rim joist faces the outdoors, often with little insulation behind it. Both are common spots for condensation, rot and mold in humid weather.
- Termites: Subterranean termites build mud tubes up the foundation to reach this wood first. North Carolina's Residential Code requires a clear termite inspection gap between the top of a crawl space wall liner and the bottom of the wood sill: at least 3 inches in the 2012-based text, and 3 to 4 inches in the 2024 edition. Wall insulation needs a similar gap.
- Air leakage: Gaps between the sill, rim and foundation let outdoor air into the crawl space, which works against a closed crawl space. Encapsulation contractors often seal and insulate the rim area, while keeping the inspection gap open.
When either piece has rotted, repair usually means supporting the floor temporarily while the damaged section is cut out and replaced. That is structural work.
When to seek help
- Wood at the sill or rim feels soft, crumbles or reads high on a moisture meter. See wood moisture content.
- You see mud tubes on the foundation wall or damage along the sill.
- Floors near exterior walls sag, slope or bounce.
- A liner or foam has been installed right up against the sill with no inspection gap.
See crawl space repair for structural work, and termite inspections and closed crawl spaces for inspection rules.