What is the difference between a rim joist and a band joist?

They are the same thing — the term varies by region. Both refer to the perimeter board that caps the ends of the floor joists. Some regions also call it a header joist, though this should not be confused with window or door headers.

Should rim joists be insulated?

Yes. Rim joists are a major thermal bridge and can account for up to 15% of a home's total heat loss. The most effective method is cutting rigid foam (XPS or polyiso) to fit snugly in each bay and sealing the edges with spray foam or caulk.

What size rim joist do I need?

The rim joist must match the depth of your floor joists. If you are using 2x10 floor joists, you need 2x10 rim joists. For engineered I-joist floors, use the manufacturer-specified engineered rim board, which is typically 1-1/8 inches thick.

How many rim joist boards do I need?

Divide your total floor perimeter by the board length you are using (8, 12, or 16 feet). For a 30x40 ft house with a 140-foot perimeter using 16-foot boards, you need 9 boards plus 10-15% waste for cuts at corners.

Do rim joists need to be pressure treated?

Only if the rim joist sits directly on a concrete or masonry foundation. If the rim joist sits on top of a pressure-treated sill plate (the normal configuration), standard untreated lumber is fine.

How do I attach rim joists to floor joists?

Nail through the rim joist into the end grain of each floor joist using three 16d nails per connection. Also toenail or use metal connectors to attach the rim joist to the sill plate below. In high-wind or seismic zones, metal framing clips may be required.

Rim Joist Calculator

Rim joist calculator: How many boards for your floor perimeter? Free tool for common timber sizes — includes board count and linear metres.

Ad — 728x90 — Leaderboard

Enter Your Measurements

How to Use This Calculator

1Measure your floor frame perimeter in feet.
2Enter the board length you'll be using.
3Click Calculate for your rim joist count.

Ad — 728x90 — Mid-Page

About This Material

Rim joists (also called band joists or header joists) are the perimeter boards that cap the ends of the floor joists, forming the outer edge of the floor frame. They must match the depth of the floor joists, so a floor built with 2x10 joists uses 2x10 rim joists. Common sizes are 2x8, 2x10, and 2x12 in standard dimensional lumber, though engineered I-joists often require a specific engineered rim board (such as 1-1/8-inch LVL rim board). Rim joists serve three critical functions: they transfer vertical loads to the foundation or wall below, provide lateral bracing for the floor joists, and close off the floor cavity from outside air and pests. Because rim joists are a major source of heat loss, many energy codes now require insulation in the rim joist cavity. Costs range from $6 to $20 per board depending on lumber dimension and length.

Next Steps:

Floor Joist Calculator →OSB Panel Calculator →

Installation Tips

•Install rim joists flush with the top of floor joists.
•Nail through the rim joist into each floor joist end.
•Consider insulating rim joists — they are a significant heat loss area.

Common Mistakes to Avoid

Not blocking or insulating the rim joist cavity in conditioned spaces.
Using the wrong lumber dimension — match your floor joist depth.

Rim Joist Calculator

How to Use This Calculator

About This Material

Installation Tips

Common Mistakes to Avoid

Frequently Asked Questions

Related Calculators

Floor Joist Calculator

Bridging Calculator

Subfloor Adhesive Calculator

OSB Panel Calculator