This backer rod is 5/8 Inch in diameter. It is cut to length based on the amount of linear feet you order. Our Foamlite Backer Rod is a subfiller for expansion joints and crack repair. It is a Polystyrene rope. For use underneath as a base for Thermolastic and Permaseal fillers. Prevents the need to completely fill the gap as it provides a surface for the filler to harden on top of. Significantly reduces the amount of filler required to fill gaps.Since the foam does contract, it is suggested that you purchase a diameter slightly larger than your crack to ensure a tight seal. Sold per lineal foot.