The Versatile Nature of Backer Rod Foam

If you’ve ever strolled across a smooth sidewalk, or put your feet up in a log home that stayed toasty as the wind whipped outside, you owe a debt of gratitude to a product you’ve likely never heard of. That product is polyethylene backer rod, and it is a vital building material for masonry and construction that creates durability, flexibility, and longevity in the structures it is a part of, all while remaining out of sight.

Backer rod, or expansion joint filler, is a waterproof foam that is packed into the joints between cement, logs in cabin construction, and cracks in other building materials. It functions as a bond breaker between the surfaces and the waterproof sealant that is typically applied over the fissure, because if caulk is used to completely fill a joint or crack, a three-point bond would be formed. If this happens, during normal material expansion and contraction, the adhesive would pull away from the two materials it is sealing and the substrate, causing damage. The reason joints in cement and similar materials are filled at all is because of moisture’s ability to cause damage of building materials. If left unsealed, joints can fill with water and freeze for example, cracking freshly poured cement.

However, caulk and similar adhesives do not bond to the backer rod pieces that are tightly packed into the cracks. By only allowing a two-point bond, the masonry or building material is able to expand and contract with the elements, while the sealant stretches and compresses like a rubber band. Filling the majority of the crack with foam is also smarter from a cost standpoint, as opposed to filling it completely with more expensive sealant.