What Is a Rim in Regards to Building a Deck?

Definition

• A deck's rim is the edge created by the outermost floor joists. Like the other floor joists, the rim joists, also called band joists, support a portion of the deck's total weight. Rim joists might extend beyond the support beams if a deck has an overhang that extends it out into your yard. The rim also forms the border for the deck, which means it is made up of the only jousts that are visible from outside the deck area once the deck is completed.

Site Plans

• Besides serving as an essential structural part of a deck, a deck's rim is also important when drawing up a site plan. Site plans are the plans that builders use to construct decks. They are also necessary when applying for building permits, which many jurisdictions require for certain types of decks. A site plan includes an outline of your property, a diagram of your home's position and an outline for the proposed deck. The deck's outline is formed by the rim, which is used to measure its area and total size.

Function

• Besides providing support for the outermost floor boards, a deck rim serves several functions. A rim gives a deck a finished, solid look, making its floor appear thicker than it actually is from the outside. The rim also serves as a structurally sound mounting point for railing posts, stairways and canopy frames. If you want to enclose the space below your deck with paneling or wood lattice, the rim is the most convenient place to mount the material.

Materials and Construction

• A deck's rim is usually made from the same material as the rest of the deck's structural elements, including vertical posts and floor joists. In most cases this means pressure-treated lumber, which can resist exposure to the elements and insect infestation. Because they are visible as part of the deck, rim joists can be painted or stained once the deck is fully assembled.