Rim Board Calculator

About the Rim Board Calculator

The rim board (also called a band joist or rim joist) closes off the floor framing at the building perimeter. It sits on top of the sill plate or wall plate and provides a nailing surface for the exterior sheathing, a fire stop between floors, and lateral support for the ends of the floor joists.

Modern construction uses engineered rim board—1.125″ thick LVL or 1″ OSB rim board—instead of solid sawn lumber. Engineered rim board eliminates the differential shrinkage between I-joists and solid lumber band joists that causes drywall cracks, squeaky floors, and nail pops.

This calculator computes the total lineal footage of rim board needed based on the building perimeter, the number of pieces (typically sold in 12', 16', or 20' lengths), and estimated cost. It accounts for waste and any interior bearing walls that also need rim-board closure.

When This Page Helps

Rim board is a must-have for I-joist floors and is increasingly used with solid lumber joists too. This calculator ensures you order the right quantity for your floor perimeter.

How to Use the Inputs

1. Enter the building perimeter in lineal feet.
2. Add any interior bearing wall lengths that need rim board.
3. Select the rim board stock length.
4. Enter the price per lineal foot.
5. Read the total footage, pieces, and cost.

Example Calculation

Tips & Best Practices

• Use rim board depth that matches your joist depth exactly—standard depths match I-joist sizes (9.5″, 11.875″, 14″, 16″).
• Butt joints in rim board should fall on a framing member (joist or stud) to maintain a solid nailing surface.
• Nail rim board to each joist end with 3–16d nails (8d if pneumatic).
• Apply sill seal under the rim board on exterior walls to reduce air infiltration.
• OSB rim board is less expensive than LVL rim but doesn't have the same bearing capacity for point loads.
• Consider ordering rim board in lengths that minimize waste—match the stock length to your wall lengths.

Rim Board vs. Solid Lumber Comparison

Engineered rim board advantages: no shrinkage (dimensionally stable), matches I-joist depth exactly, lightweight and easy to handle, and available in long lengths (up to 20'). Solid lumber rim (2×12): heavier, shrinks 1/8″–3/16″ as it dries, and may not match I-joist depths precisely.

Air Sealing at the Rim

The rim board area is one of the biggest sources of air leakage in a house. Best practice: seal the bottom with sill seal, the top with subfloor adhesive, and the cavity behind with spray foam insulation (R-19 minimum in most climate zones).

Ordering Tips

Rim board is sold by the piece (each piece is a stock length). Order based on your wall lengths to minimize cuts and waste. Many suppliers stock 12' and 16' lengths; 20' may be special order. Check availability before designing your cut list.

Frequently Asked Questions

• Can I use a 2×12 as a rim joist with I-joists?

  It's not recommended. Solid lumber shrinks as it dries, while I-joists don't. This differential movement causes floor-to-wall separation, drywall cracks, and squeaks. Use engineered rim board for best results.

• What is the difference between rim board and rim joist?

  They're the same thing. "Rim joist" is the traditional term for the solid lumber piece. "Rim board" typically refers to the thinner (1″–1.125″) engineered product designed specifically for this purpose.

• Do I need rim board on interior bearing walls?

  Yes, if the floor joists overlap or butt at an interior bearing wall, you need closure material. This can be rim board, solid blocking between joists, or a combination.

• How thick is engineered rim board?

  LVL rim board is typically 1.125″ thick (standard lumber 1.125″ = 1-1/8″). OSB rim board is 1″ thick. Both are thinner than solid 2× lumber (1.5″), which can be a consideration for siding attachment.

• Is rim board structural?

  Yes. Rim board transfers vertical loads above the floor to the wall below and provides lateral bracing. LVL rim board has published bearing capacities for point loads. Squash blocks may still be required under heavy concentrated loads.

• How many pieces of rim board do I need?

  Divide the total perimeter (plus interior bearing walls) by the stock length, add 5–10% waste, and round up. Typical stock lengths are 12', 16', and 20'.