What Is a Backer Rod?

A backer rod is used to fill gaps between building materials, such as gaps between tiles and another surfaces.
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  • Written By: Joshua Nuttall
  • Edited By: C. Wilborn
  • Last Modified Date: 27 March 2014
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Backer rod is a round open- or closed-cell foam rod used to fill joints between building materials. It is used to fill most of the void in a joint so caulking or chinking material, a mortar-like sealant, can be applied to finish filling the void and create an air-tight and water-tight seal. Caulking or chinking material can be used to fill the entire gap, but the cure time for large amounts of these products would be a number of days or weeks, depending on the size of the gap.

There are significant differences in the characteristics between open-cell and closed-cell foam backer rod. The closed-cell version repels moisture, provides a firm finish to the gap, and can be set into place with a spray adhesive. It is commonly used for smaller joints in the 1/4 to 3/8 inch (6.35 to 9.52 mm) range, and it possesses a higher insulative value than open-cell forms. Caulking or other chinking materials will not stick to it, which allows free and independent movement in the building materials and the chinking material.

Open-cell backer rod products are very easy to install because they are softer and more pliable than closed-cell versions. Large diameter pieces can be compressed to fit into gaps that vary in thickness in some areas. This type is designed to be used only indoors, and does not possess insulative characteristics that are as good as its closed-cell counterpart.


Chinking materials are flexible sealants that will remain flexible throughout their usable life span. They will stick to the building materials, but not to the backer rod. For this reason, the foam is also called a bond breaker. It allows the building materials to move, bend, and flex based on the environmental factors they are exposed to, such as humidity, heat, cold, wind, and ultraviolet light exposure.

Other sealing materials can be used to fill gaps, including elastomeric and other flexible caulking products. Elastomeric caulks are able to bend, flex, expand, contract, and stretch as needed while keeping a tight bond and an air-tight seal intact. These caulks and sealants are able to stretch to 200% or more of their original shape and size. They paintable with acrylic latex paints, but should be allowed to fully cure before being painted.

Both open-cell and closed-cell backer rod is available in a wide range of diameters, from 1/4 inch (6.35 mm) to 4 inches (101.6 mm) or larger. Some retailers sell it by the foot. Other retailers sell it in small packages. For industrial uses, it can be purchased in 1,650 foot (502.92 meter) rolls.


Discuss this Article

Post 6

We installed backer rod between adjacent precast panels on the second floor of a parking deck. We then placed caulking to completely seal the void. Now sections of the backed rod are falling out of the joints from below. I think this is OK because the backer rod served it purpose- it supported the caulk as it cured, but now it is working itself out of the joint with expansion and contraction caused by temperature changes. The caulk itself is holding up great.

Post 4

Would an "unplanned" 2-inch gap between concrete tilt up panels be better served using a backer rod and sealant, or full sealant joint?

Post 3

You do not want a good bond between the b-r and the sealant! Read the article!

The technical reason is that if adheres to the bottom of the joint ( which is the backer rod) you have 3-sided adhesion which reduces the ability of the sealant to stretch and drastically increases stresses at the bond line. The only case where it might be acceptable is with open cell b-r because that is so stretchy. Of course oc b-r is for indoor use so who cares? (movement is limited)

Post 2

@abiane - You always want a good bond between the backer rod and sealant, so be sure that you have the proper combination. If you're not sure how to go about it, you can always research online or at the home improvement store. You can always call a General Contractor, but sometimes they get pricey with the quotes.

Post 1

Some stores will sell continuous lengths of backer rods so that you can get whatever size you need. Aside from that, backer rod sizes vary and can vary quite a bit so it's best to do some measuring before making a trip to the store.

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