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What Is a Rim Joist?

A concise, illustrated definition of a rim joist, an integral part of a floor’s skeletal structure

Raised Floor Framing Diagram

A rim joist is also called a header joist.

In today’s platform-frame house construction, beneath the subflooring, a series of floor joists, usually 2-by-8 or larger lumber set on edge and spaced every 16 inches, provides support for the floor.

Along the outer foundation walls, the ends of these joists rest on a sill plate and are capped by a rim joist (also known as a band joist or header joist) that is nailed to their ends. It keeps the joists true and provides a flat surface for backing the edge of subflooring and the base of exterior siding. In effect, it completes the “box” of a floor’s structure.

Just as the gist of an argument or question is its base or foundation, so a joist is the base for a house. The word “joist” is a latter form of the Middle English and Old French word giste, meaning “place of rest.” As early as the 13th century, it indicated a timber on which floor boards or ceiling laths were nailed.

About Don Vandervort
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Don Vandervort developed his expertise more than 30 years ago as Building Editor for both Sunset Books and Home Magazine. He has written more than 30 home improvement books and countless magazine articles. He appeared regularly on HGTV’s “The Fix,” and served as MSN’s home expert. Don founded HomeTips in 1996.

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