What is a nailer board?

A nailer board is usually a lumber board (like a 2x4) that is attached to the top of a metal structural element (usually a purlin

• a horizontal truss or beam). Metal structural elements usually

have attachment points or through-holes for bolts at preset spaces. The nailer board is attached using those attachment points, and then anything can be readily nailed to the nailer board at any position, including subpurlins (cross-beams), plywood platforms, you name it.

TLDR - it's the soft, wooden board attached to a metal beam so you can nail stuff to it.