Backer rod is typically a foam-like material used to properly configure expansion/contraction joints and to provide a surface that either the sealant will not adhere to or will be so flexible in its own right that, even though the sealant adheres to it, it does not hinder the needed movement capability of the sealant itself. Backer rod usually comes in long lengths of a circular profile and is pressed into the joint just before the sealant is applied. (See " Two-point adhesion")

a circular foam rod

In glazing, a polyethylene or polyurethane foam material installed under compression and used to control sealant joint depth, provide a surface for sealant tooling, serve as a bond breaker to prevent three-sided adhesion, and provide an hour-glass contour of the finished bead.