5/8" X 1550' Backer rod is a continuous length of flexible, round, closed cell polyethylene. Black in color and available in a wide range of diameters.
 
Uses
Backing for standing seam metal roofing panels to mitigate the effects of oil canning.
Backing for elastomeric and other applied caulking sealants.
Expansion/construction joints in concrete and precast concrete walls, floors, partitions, bridge construction, parking decks, curtain walls, glazing, log home construction, highway construction, and pavement maintenance.