Backer rod is a flexible, closed-cell foam rope pressed into a concrete joint or wide crack before sealant is applied, controlling the sealant depth and helping it bond correctly.
In depth
Sealant works best in an hourglass shape, bonded to the two joint walls but not the bottom. Backer rod creates that shape. Without it, sealant bonds on three sides and tears itself apart as the slabs move seasonally, which is why joints sealed without rod often fail within a year or two.
Backer rod also stops sealant from sinking endlessly into a deep joint, saving material and keeping the finished surface flush and clean-looking.
How Acme applies it. After we raise and level slabs, we recommend sealing the joints so water stops feeding the erosion under the slab. For wider joints, we use closed-cell backer rod with a quality self-leveling sealant so the seal flexes with the slab instead of failing.
Related terms: Joint Sealing, Closed-Cell Foam, Expansion Joint, Soil Washout
Frequently asked questions
Do I really need backer rod, or can I just fill the joint with caulk?
Deep-filled caulk fails early. Backer rod gives the sealant the right depth and shape so it stretches with slab movement instead of tearing.
What size backer rod should be used?
Roughly 25 percent wider than the joint, so it compresses snugly and stays put while the sealant cures.