What is a Rim Joist?

C. Daw

When it comes to framing a home or building, joists are an integral aspect of carpentry because they distribute the weight of the building evenly across the entire expansion. The bigger the structure, the more the joists are relied upon. Rim joists are the final joists that end the series of framing joists, so they are even more important than basic joists. The rim joist is the main support that holds the majority of the weight from the walls and headers that are built above them.

Rim joists complete the box or rectangle shape that makes the floor or ceiling framing in a building.
Rim joists complete the box or rectangle shape that makes the floor or ceiling framing in a building.

The rim joist is basically the outer edge of the joists and effectively completes the box or rectangle that is the floor or ceiling framing. This joist decides the measured area of the home or building, as well as the overall design of the home or building. No matter how many joists are placed between the outer walls, they usually are not required to hold the weight that the rim joists are expected to support.

After the rim joists are installed, the walls or roof trusses are attached directly onto them, so they are responsible for withstanding various amounts of weight while allowing a level, straight foundation for the sections above. It is imperative that the rim joists for a house or building are made from solid wood that is free from bows and twists, and that they have as few knots as possible. The more solid the rim joist is, the more strength and stability the entire building or home will have.

It has become common for some builders to use iron for rim joists in place of the traditional wood ones. The reasoning behind this is to allow even more weight-carrying capacity and a massive increase in the stability of the building. Either way, the rim joist has to be the most stable of all the joists that are being installed.

Rim joists are a necessary and required joist when it comes to building, and it is one aspect of framing that cannot be overlooked. The stability of the home or building depends on various building materials and stages, but when it comes to the stability of the walls and roofing trusses, the rim joist is the most important. A strong rim joist makes a strong framing structure, but a weak rim joist will cause framing problems.

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