Sashco 30501 1/2" x 100' Coil Pre-Caulking Backer Rod (1 Pack)

Backer rod is 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, does not hinder the needed movement capability of the sealant itself. Backer rod comes in long lengths of a circular profile and is pressed into the joint just before the sealant is applied.