What is the purpose of backer rod in expansion joint installation?

• A. To create a void for air
• B. To control depth and provide a substrate for sealant
• C. To absorb moisture
• D. To color the joint

Answer: (B)

Backer rod is placed into expansion joints to set the depth of the sealant and provide a backing for it. This keeps the sealant from filling the entire joint depth, giving the bead the correct thickness so it can flex with joint movement without cracking or pulling away. The backing also creates a solid surface behind the sealant so it bonds primarily to the joint faces rather than to the bottom, which helps the sealant form a proper, durable seal and maintain its shape over time. Because the rod is closed-cell foam, it resists moisture and won’t wick water into the sealant. In short, backer rod ensures the sealant has the right depth and a stable substrate to stay adhered to, enabling effective sealing and movement accommodation.