A Stronger Way to Butt-Join Rim Joists
The Problem
When you're running rim joist (band board) and two pieces of 2x10 or 2x12 need to meet, the standard move is to butt them right at a floor joist. But that gives you a narrow 1.5" edge to nail into — not much meat for a joint that needs to hold the edge of your floor system together.
The Technique
- Run your first rim joist board at least 4 inches past the floor joist where the joint would normally land
- Cut a short piece of the same stock (a ~16" length of 2x10, for example) to use as a backer block
- Set the block on edge between the floor joists, right behind where the joint will fall — its face should sit flush against the back of the rim joist
- Nail the block to the overhanging first board through the full face of the 2x10 — you're sistering the block to the rim joist, so nail generously across the wide face
- Butt the second rim joist board up to the first, tight against the end
- Nail the second board into the backer block the same way — through the wide face into solid wood
Now both board ends are fastened across the full width of the lumber, not just into a joist edge.
Pro tip: The whole point is nailing surface. A standard butt joint gives you 1.5 inches of joist edge. This method gives you 16 inches of backer block face on each side of the joint — an enormous difference in holding power and rigidity.
Pro tip: Adding a bead of construction adhesive between the backer block and the rim joist boards before nailing makes the connection even more resistant to movement as lumber dries and seasons.
Safety note: Rim joist work often means nailing at the edge of a floor platform. Make sure your footing is solid and the floor joists you're standing on are secured before leaning out to work on the perimeter.
Why This Isn't Done More Often
It takes an extra couple of minutes per joint and uses a short offcut of lumber. On a large framing job those minutes add up, so many crews stick with the standard joist-edge butt. But for the effort involved, the strength difference is hard to argue with.