Backer Rod is an easy to compress rope-like celled polypropylene / polyolefin based foam. It is inserted into a clean joint at the required depth before adding caulk / sealant. Backer Rod creates an even depth and supports (back stops) the sealant / caulk allowing it to form the proper shape (about 2x as wide as deep) and also prevents 3 sided adhesion (bond breaker). It is especially important to use it with Self Leveling Expansion Joint Sealants. Very large and Soft Backer Rods are also used for fill.