A rim joist is the full-depth member that runs around the edge of a floor frame, nailed across the ends of the joists. It closes the joist bays, keeps the joists upright, carries wall loads down to the sill or wall below, and gives the exterior sheathing a nailing surface.
You will see it on every floor framing plan as the line that outlines the joist field. Engineered versions are sold as rim board, and on decks the same member caps the outer joist ends.
What a rim joist does
A rim joist has four jobs in a typical platform-framed floor.
- Lateral restraint at joist ends. 2021 IRC Section R502.7 requires joists to be supported laterally at their ends by full-depth solid blocking at least 2 inches nominal thick, or by attachment to a full-depth header, band, or rim joist, or to an adjoining stud, or otherwise restrained against rotation. Trusses, structural composite lumber, glulam, and I-joists follow the manufacturer's recommendations instead.
- Load path for walls. The wall above sits on the subfloor over the rim, and the rim bears on the sill plate or the wall below, so vertical load passes straight down the perimeter.
- Braced wall connection. R602.10.8 says that where joists run perpendicular to a braced wall panel above or below, a rim joist, band joist, or blocking must run along the full length of the panel. Where joists run parallel, a rim joist, end joist, or other parallel member goes directly above and below the panel.
- Closure and nailing. It closes each joist bay at the perimeter and backs the wall sheathing.
Rim joist fastening in the IRC
2021 IRC Table R602.3(1), fastening schedule, items 23 and 27.
| Connection | Fasteners | Placement |
|---|---|---|
| Rim joist, band joist, or blocking to sill or top plate | 8d box (2-1/2" x 0.113") | 4" o.c. toe nail |
| Rim joist, band joist, or blocking to sill or top plate | 8d common (2-1/2" x 0.131"), or 10d box (3" x 0.128"), or 3" x 0.131" nails | 6" o.c. toe nail |
| Band or rim joist to joist | 3-16d common (3-1/2" x 0.162"), or 4-10d box (3" x 0.128"), or 4-3" x 0.131" nails, or 4-3" x 14 ga. staples, 7/16" crown | End nail |
Rim joist vs band joist
In most US framing the two terms mean the same thing. The 2021 IRC itself uses them side by side: R502.7 lists "header, band or rim joist" and Table R602.3(1) lists "rim joist, band joist or blocking." Regional habits vary. Some crews call the member across the joist ends the band joist or rim joist and the outermost joist running parallel to the rest the end joist. Some call both the rim. On a drawing, pick one term, use it consistently, and put the size in the label so nothing depends on the word.
A header joist is different. It is the member across a floor opening that carries the cut joists, framed with trimmers under R502.10. The stair opening guide covers that framing.
Rim board: the engineered version
A rim board is an engineered wood product made to act as a rim joist, usually in I-joist floors where a sawn 2x would not match the joist depth or would shrink differently. 2021 IRC R502.1.7 requires engineered wood rim boards to conform to ANSI/APA PRR 410 or be evaluated under ASTM D7672, with structural capacities set the same way. Boards conforming to PRR 410 must be marked. Thickness, depth, and load capacity come from the manufacturer, not from an IRC table.
Rim board can also act as a header. R602.7.2 allows rim board headers over openings in the wall below, sized from Table R602.7(1) and supported at each end by full-height studs. Rim board headers that carry concentrated loads need engineering.
Rim joists at floor cantilevers
When floor joists cantilever past a bearing wall, the footnotes to 2021 IRC Tables R502.3.3(1) and R502.3.3(2) require a full-depth rim joist at the unsupported end of the cantilever joists and solid blocking at the supported end. For cantilevers of 24 inches or less in Seismic Design Category A, B, or C, Table R502.3.3(1) drops the blocking at the support.
Deck rim joist
On a deck, the rim joist caps the outboard ends of the joists and is often the lateral restraint at the beam line. 2021 IRC R507.6.2 requires joist ends and bearing locations to be restrained against rotation. Where a rim joist does that job, it must be fastened to the end of each joist with at least three 10d (3-inch by 0.128-inch) nails or three No. 10 by 3-inch wood screws. Blocking or hangers used instead must be at least 60 percent of the joist depth.
Guards often mount to the rim, and R507.10.1.1 requires a joist or beam carrying a guard on its side to be connected to the adjacent joists so it cannot rotate. Connections that rely only on nails or screws in end-grain withdrawal are not permitted. That usually means blocking behind each guard post.
The house rim joist matters too. A ledger bolts to it, so R507.9.1.2 requires the band joist behind a ledger to be at least 2-inch nominal solid-sawn spruce-pine-fir or better, or a 1-inch nominal engineered rim board under R502.1.7, bearing fully on the structure below. The deck framing plan guide shows how the ledger and rim joists appear on a permit drawing, and the deck joist span and spacing chart gives the joist sizes they cap.
Rim joist insulation
The rim joist area leaks air more than most of the building envelope because the sill plate, rim, subfloor, and wall plates all meet there, each with a joint. Typical treatments are closed-cell spray foam applied to the inside face of each bay, or rigid foam board cut to fit each bay with the edges sealed with foam or caulk. Fiberglass batts alone do not stop air movement.
The required R-value, any vapor control, and whether exposed foam needs a thermal or ignition barrier depend on the energy code and residential code your jurisdiction has adopted, so check those before choosing a product. A framing plan normally does not show insulation, but a general note can reference the wall section.
What a rim joist looks like on a framing plan
On a plan view the rim joist is drawn as a continuous line, or a narrow double line at scale, along the perimeter of each joist field where the joist ends land. Label it once with size or product, for example 2x10 RIM JOIST or RIM BOARD PER MFR, and add a general note if every floor uses the same rim. Tag anything different: a doubled rim, a rim board header over a wide opening below, or a deck rim joist with its fastening.
Interior blocking lines look similar, so keep the line weights and tags distinct. Joist blocking and bridging covers how to draw and label blocking, and the framing plan symbols guide lists the common line types. For sizing the joists the rim closes off, see the floor joist span table.
Keep it in context
The IRC provisions above are prescriptive requirements for the conditions they describe. The adopted code edition and local amendments govern, and I-joists, LVL, and engineered rim board follow the manufacturer's installation details and load tables.
Framing Plan is a preliminary drawing aid. It draws joist layouts, beams, and openings, and exports PNG, PDF, and DXF for review; it does not check member sizes against the code. Open the framing studio to draw a joist field and its perimeter.