I'm setting floor joists and some of them have a visible bow along the edge.

Yes, it matters. Install joists with the crown (bow) facing up.

Why:

• A bowed joist installed crown-up will straighten under load, creating a flatter, more stable floor.
• Installing crown-down can exaggerate the dip, leading to squeaks, creaks, and an uneven surface.

How to identify the crown:

• Look down the length of the joist from one end. The crown is the side that curves upward.
• If unsure, use a straightedge or level to check for gaps along the edge.

Code reference:

• Per the International Residential Code (IRC) R502.3, floor joists must be graded and installed according to manufacturer or grade stamp requirements. While the IRC doesn’t explicitly state “crown up,” it’s standard industry practice and required by most engineered joist manufacturers.

Pro tip:

• If the bow is severe, consider sistering the joist or consulting a structural pro—excessive deflection may indicate undersized or damaged lumber.

Bottom line: Crown up, every time. It’s a small detail that makes a big difference in floor performance.

Crown up. Always.

The bow (crown) goes up. Gravity and live loads will push the joist toward straight over time. If you install it crown down, the load increases the deflection, creating a permanent sag and a bouncy floor.

Code basis: IRC R502.6 / IBC 2308.4.2 require joists to be installed with the "crown up" (or "bow up").

Practical check:

1. Sight down the top edge. The center should be higher than the ends.
2. Mark the high point with lumber crayon.
3. If the crown exceeds ½" in 10 ft (or L/240), the joist is technically defective per grading rules—reject it or sister it.
4. For I-joists or LVLs: no crown; install per manufacturer stamp (usually "TOP" marked).

Pro tip: If you have a mix, group similar crowns together in a bay so the subfloor planes out. Don’t pair a ¼" crown next to a ¾" crown.

Bottom line: Crown up. It’s not optional—it’s structural.