Sashco Pre-Caulking Filler Rope Backer Rod Roll, 100' Length x 3/8" Width, Grey Makes professional joints that last Helps fill in and insulate large caulk joints and allows the caulking to absorb more movement Closed cell design repels water to help protect the joint from damage Complies with ASTM C 1330, Type C Measures 3/8-inches diameter by 100-feet length Easy to fit - soft and pliable so it fits into the joint easily Water resistant - unlike open cell backer rod, it repels water to help protect the joint from damage Fills the joint so you use less caulk Fully compatible with all substrates and sealants