When repairing larger cracks and joints in concrete or asphalt, properly installing a backer rod before applying crack filler or sealant can make a big difference in the quality and longevity of the repair.
What Is Backer Rod?
Backer rod is a flexible foam material that is pressed into a crack or joint before sealant is applied.
What Are the Benefits of Backer Rod?
Backer rod serves two important purposes: it helps control how much filler or sealant is needed, and it helps prevent 3-way adhesion, which can cause a repair to fail prematurely.
What Is 3-Way Adhesion?
Sealants are designed to bond to the two opposing sides of a crack or joint while remaining free from bonding to the bottom.
This allows the sealant to stretch and compress as concrete or asphalt expands, contracts, and moves with temperature changes.
Without backer rod, the sealant can bond to:
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One side of the crack
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The opposite side
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The bottom of the crack
This is called 3-way adhesion.
When sealant is bonded to three surfaces, its movement becomes restricted. Instead of stretching freely between the two sides of the joint, it is pulled in multiple directions at once. This places additional stress on the sealant and can cause it to tear, crack, or pull away from the sides of the joint prematurely.
Backer rod acts as a bond breaker at the bottom of the joint, helping the sealant bond only to the two opposing sides where it should. It also helps control sealant depth and prevents this three-sided adhesion.
Use Less Filler and Sealant
Deep cracks can require a surprising amount of material if they are filled completely with sealant. Backer rod fills the unnecessary empty space beneath the repair and creates a consistent base for the sealant.
This not only reduces the amount of expensive filler or sealant you need, but also helps create the proper sealant thickness for a stronger, more flexible repair.
Choosing the Right Backer Rod Size
Backer rod should be slightly larger than the width of the crack or joint so it compresses and stays securely in place.
As a general rule, choose a backer rod approximately 20–30% larger than the width of the joint, with about 25% larger being a good rule of thumb. For example, a 1/2" wide joint would typically use approximately a 5/8" backer rod. The slight compression creates a secure friction fit without needing adhesive.
For larger cracks and joints, our 3/4" Backer Rod provides 20 ft. of flexible, closed-cell foam that helps control sealant depth, reduce wasted sealant, and prevent 3-way adhesion.
How Deep Should Backer Rod Be Installed?
The depth of the backer rod is just as important as its diameter because it determines how thick the layer of sealant will be.
For many common concrete joint sealants, the finished sealant should generally have a 2:1 width-to-depth ratio. In other words, the sealant should typically be about half as deep as the joint is wide.
For example:
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A 1/2" wide joint should generally have about 1/4" of sealant above the backer rod.
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A 3/4" wide joint would typically have about 3/8" of sealant.
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A 1" wide joint would generally have about 1/2" of sealant.
A common guideline is to maintain at least 1/4" of sealant depth and generally no more than 1/2", although the exact requirements can vary depending on the sealant being used. Always follow the sealant manufacturer’s instructions.
The backer rod should be pushed into the crack far enough that the proper amount of space remains above it for the sealant.
A Small Step That Makes a Better Repair
Taking a few extra minutes to install properly sized backer rod can make a significant difference in a concrete or asphalt repair.
It helps:
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Reduce the amount of expensive filler or sealant required
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Maintain the proper sealant depth
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Prevent 3-way adhesion
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Allow the sealant to expand and contract properly
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Create a longer-lasting, more professional repair
For the best results, choose a backer rod slightly larger than the crack, install it to the proper depth, and always follow the recommendations for the specific crack filler or sealant being used.
Get the Right Start for Your Next Crack Repair
Before sealing deep cracks or expansion joints, make sure the repair is properly prepared. Shop our 3/4" Backer Rod to help control sealant depth, reduce wasted material, and create a better, longer-lasting repair.
