A protective layer of high-density material, such as gypsum board, wood fiber, or perlite, placed on top of softer foam roof insulation. It shields the foam from the extreme heat of hot-applied bitumen, prevents thermal blistering, and provides impact resistance against hail and foot traffic.
Heard on the job
Lay the cover board down with staggered joints over the polyiso before you start the hot mop.
Insider tip
Skipping the cover board over foam insulation usually leads to crushed insulation under foot traffic and melted foam during hot-mop applications.