Building a deck is an exhilarating adventure for some, and a nightmare for others. Understanding the terms, construction practices, and the purpose of different framing members is helpful. A deck rim joist is an important structural member and provides support when attaching railing posts or stairs.
The rim joist is commonly the same dimensional material as the joists. It is attached perpendicular to the open joist ends to close the joist cavity, maintain the spacing, and to prevent twisting. It helps transfer vertical and lateral loads, so the deck frame works together.
In this guide, we’ll explain what a rim joist is, different terms it may be called, and its structural purposes, plus, sizing and different installation options. The guide will provide you with a better understanding of the purpose of the rim joist and how best to install it for your project.
What Is a Rim Joist on a Deck
Rim joists are the perimeter joists of a deck. The two outside parallel joists, plus those that run perpendicular to all the joists, form the rim of the deck or a band that ties the structure together. Hence, they are known collectively as band boards and rim boards.
Some builders only refer to the outside parallel joists as band boards and those that close the open joists ends as rim boards or rim joists. Others use them interchangeably, and some call them faceplates. Regardless of the name, they are a structural component of decks and improve the rigidity and cohesiveness of the structure.
The outside parallel joists are structural supports for deck boards and also railing posts and stairs if required by the design plan. They are part of the deck’s physical size and commonly spaced equally with the other joists. There is often additional blocking between the outermost joists and the next nearest joist to help make the deck edge more rigid and to share railing post loads.
Section R502.7 of the 2018 International Residential Code (IRC) is copied by most local Building Codes. It requires the open ends between deck joists to be blocked or closed with a rim joist. The blocking or rim joist must be the same depth as the joists and at least nominally 2-inches thick.
The blocking or rim board prevents joist rotation, or twisting, maintains spacing between joists, and provides lateral and unifying support. It stiffens the deck by fastening the joist ends together, reduces vibration, bounce, and movement. Plus, it aesthetically finishes the outer edge of the deck. The rim board also provides structural support for railing posts, posts supporting other deck structures, and stairs.
Band Joist vs Rim Joist
Building a deck extends the living space outdoors and is an extension of floor construction. The terms used for both and their purposes are often interchangeable. The outer joists in a building usually support studs while the outer deck joists often support railing posts.
In deck construction, the rim joist commonly refers to the boards that run perpendicular across the open ends of all parallel joists. The end joists, sometimes also called side rim joists, are the two outermost parallel joists that form the deck platform. The rim joists and the end joists are collectively known as the band joists.
Confusion often arises as terms are used interchangeably. Since the rim joist is also a band joist, it may be referred to with either term. Similarly, the end joists are band joists, which then lead some to call them rim joists.
Are Rim Joists Load Bearing?
Decks attached to buildings often have vertical loads carried by support posts, drop beams, and a ledger board. The posts support the drop or under-mount beam and the joists rest on top of the beam and connect to the ledger with joist hangers. The structural profile is thick and can be difficult to fit close to ground level.
Deck rim joists provide lateral support when used to close off the open ends of joists. They prevent the joists from twisting, maintain spacing, and provide support for stairs or railing posts.
The rim joist also helps unify the deck and transfers lateral loads between parallel joists. When deck boards are laid diagonally to joists, the rim joist also supports the vertical load of and on those boards.
Attached or freestanding decks may use a double thick rim joist for a flush or inset beam instead of a drop or under support beam. The joists fasten to the double rim with joist hangers, and the supporting rim joist-beam provides a lower profile deck structure. In this type of construction, each rim joist-beam will carry up to 50% of the vertical load of the deck.
Deck Rim Board Size
The deck joists, according to the 2018 IRC Section R502.7, must be blocked at the open ends using nominally 2-inches thick pieces the same depth as the joists or a rim joist of similar dimensions. The blocks or rim joist prevent the parallel joists from twisting and closes the cavity between their ends, so it needs to be the same depth. For example, the rim joist for 2×8 deck joists must be 2×8 or larger.
Some deck builders use larger dimensional lumber for the rim board. They attach it so that it is raised above the level of the joists and flush with the top of the deck boards. This creates a picture frame finish and hides the end grain of deck boards.
Blocking the open ends of the joists increases the size options for the deck rim board. The blocks fulfill the Code’s structural requirements and allow for some other options. One option often used is to cover the blocking and joist ends, and sometimes deck board ends, with a 1-inch thick faceplate of different or exotic wood to enhance the visual appeal of the deck.
Do Deck Rim Joists Need to Be Doubled?
Double rim joists commonly are only used for inset or flush beams, or to create a picture frame finish. An exception is to strengthen spliced rim boards, to minimize bounce, or for more substantial mass to fasten railing posts to. People tend to stand at the railings near the deck edge which can increase bounce and flex, especially on cantilevered joists.
Doubling the rim boards on runs longer than one board length allows for staggered seams which would be stronger and less flexible. Some builders use a rim joist the same dimensions as the joists, and then double it with a larger depth plank. This creates a picture frame border and hides end grain on deck boards for a neater finish.
Another common practice is to block the openings between joists as the fasteners go through side grain instead of weaker end grain. The blocks space the joists, improve structural rigidity, decrease bounce, and provide better backing for railing posts.
The rim board is then fastened to the side grain of the blocks instead of the joist end grain. The look and material used are similar to doubling the rim board but the result is stronger.
Deck Rim Joist Attachment Options
Before installing the deck rim joist it is a common practice to run a string line from one end joist or band board to the other at the very tip or end. The string will identify joists that are longer or shorter.
Either move the string to align with the shortest or replace it (or them) with a longer plank. To ensure the rim board is straight, trim the joists to be flush with the string line.
Section R502.7 of the 2018 IRC requires 2” thick dimensional lumber at least the same depth as the joists for end blocking or rim joist. Some builders use blocks or a rim joist to comply with the Code, others use both. The purpose is to prevent rotation of the joists and firm up the deck edge.
There are different ways to install and fasten the rim joist to the deck. Mark the location of the joists of the inside face of the rim joist using a square and place an X where the joist goes. You should extend the line onto the top edge for ease of alignment.
If shorthanded or doing it alone, attach a block to the underside of the end joists so it extends 3” beyond the joist end. This will provide a shelf rest to support the rim plank and is easy to remove when done. Make sure the rim plank is crown up before installing it.
The traditional method is to use 3 – or more depending on joist depth – galvanized 3-1/2” nails hammered through the rim board into the joist ends. The nails provide shear strength but don’t hold well against lateral or perpendicular twist forces, especially when applied against the top of railing posts. Structural screws are more popular and are replacing nails as they have greater holding power.
One method is to begin fastening at the middle joist and work outward to the end joists. Line up the joist and mark on the rim joist, ensure it is flush with the joist or decking, and drive in one nail or structural screw 1” to 1-1/2” below the upper edge of the joist.
Move outward in both directions aligning and fastening with one nail or screw at each joist. Another method is to begin fastening the rim at one end joist with one nail or screw and work toward the other end.
Once the rim is fastened to each joist, go back and drive in the rest of the fasteners. Use a speed square to make sure joists are square with the line; some joists may need a bit of persuasion to align correctly. Structurally, the rim joist is to prevent joists from twisting, so it’s a good practice to start with them all straight.
Joist HangersJoist hangers are not commonly used to fasten the rim joist to the joist ends due to the added expense. However, if securing it through from the outside isn’t possible or convenient, or the look of nail or screw heads unsightly, then joist hangers are a great solution. The hangers make installation easier and also provide a stronger connection than end grain nailing or toenailing.
Make sure the joist ends are the same length using a string line. Trim if too long, or replace or move the string if too short. It’s also a good practice to use the string to check that all joists are within 1/16” of each other in height – shim if necessary.
Mark the location of the joists on the rim joist using a square and place an X on the side the joist goes – measure twice. Depending on the situation, the hangers may be attached in their regular manner or inverted so the saddle will rest on top of the joist and support the rim while fastening it to the joist ends.
Use a cut-off block of joist material when setting, aligning, and fastening the hangers to the rim board with 1-1/4” structural nails or screws. The cut-off block should be level with the top edge of the rim board when using the hangers normally, and the saddle level with the rim edge when inverted. Attach the hanger in the prescribed pattern to the joist and rim using appropriate fasteners.
Deck corners can be finished in a variety of different ways. The rim board can overlap the end joists to create the corner, or miter both ends of the rim joist and the two end joists at 45° for a corner finish that hides the end grain and looks seamless.
Another method is to frame a corner railing post in and butt the rim and end joists into the post base. The end grain is protected and the post connection is often stronger.
Splicing Rim Joists
Splicing rim joists usually occur when the deck is longer than available dimension lumber. The butt joint where two pieces meet at the end grain should occur at the middle of a joist. Some carpenters will scab a block to the side of the connecting joist to provide greater nailing space. Others prefer to install a block perpendicular between two joists for a stronger joint.
Doubling the rim joist is an alternative method to splicing. The layers strengthen the joint between planks by staggering the seams of each layer by 4-feet. Butt joints are commonly used when boards are doubled.
Building Codes require blocking between joist ends or a rim joist to prevent twisting, install blocking. The blocking means the joint between rim boards need not occur at the end of the joist but can be in the blocking between joists. Miter the ends of the rim boards and fasten them tightly together. The mitered joint all but disappears and minimizes the visibility of shrinkage at the seams.
Pro Note: All cuts and holes in pressure-treated lumber should be painted with preservatives to protect the wood. Use nails, screws, brackets, and bituminous tape or tar paper designed for treated wood to prevent corrosion and rot.
The rim joist is often of the same material as the joists and closes the cavities between them. Structurally, it prevents the joists from twisting, helps maintain spacing, reduces bounce, and helps transfer loads between joists. It also provides mass for fastening stairs, railing posts, and other deck structures.
I hope the guide proved helpful and informative, and you have a better understanding of the purpose of the rim joist and how to install one. If you found the guide useful, please share it with others. Your comments and suggestions are always appreciated.