Fact Checked

What is a Backer Rod?

Joshua Nuttall
Joshua Nuttall

A backer rod is a small foam rod or cord that is used to fill joints between tiles or other pieces of building material. Builders and contractors often use this tool initially in grooves or cracks and then they apply mortar, sealant, or chinking around it. The rod adds stability, precision, and also typically save time. There are typically two types of backer rod, open-cell and closed-cell; they can often be used interchangeably, though open versions tend to be better for indoor environments where there isn’t a lot of moisture. Decks, patios, and walkways that sit outdoors more commonly use closed-cell rods as a way to add insulation and waterproofing. In either case these tools aren’t usually essential for any application, but they can improve the appearance and durability of many different projects.

Primary Uses

A backer rod is used to fill gaps between building materials, such as gaps between tiles and another surfaces.
A backer rod is used to fill gaps between building materials, such as gaps between tiles and another surfaces.

Builders use these sorts of foam rods for a number of different construction projects, but almost always in the context of filling gaps where materials join together. They’re most commonly used in tiling and flooring, but they can also be found in decorative wall paneling, window sealing, stonework, and counter tops. The main idea is to use the foam to fill most of the void in a joint so that caulking or chinking material, a mortar-like sealant, can be applied to finish filling it and create an airtight and watertight seal. The rods also add accuracy and firmness that can give a more finished look to the end result.

Rods are usually 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 them to order, or it may be available in small, pre-cut packages. Large distributors or busy contractors often buy their rods in bulk, often in 1,650 foot (502.92 meter) rolls. It’s usually the case that different specifications are best for different projects, so the type of rod needed for tile flooring in a bathroom may be different than that needed for kitchen counters or stone walkways.

Open-Cell Rods

Rods are typically also available in two main types, namely open-cell and closed-cell. The distinction usually has to do with how dense the foam is. In an open-cell rod, the material is usually airy and breathable, while it is much denser in a “closed” version.

Open-cell products tend to be the easiest to install because they are soft and very pliable. Large diameter pieces can be compressed to fit into gaps that vary in thickness in some areas, which means that this style is more universal, too; it can be compressed to fit a range of different sizes and projects. Open-cell products are usually only designed to be used indoors, though. This type of material is not a good insulator and it won’t typically repel water.

Closed-Cell Versions

Rods made with closed-cell foam are denser and firmer, which makes them harder to bend and maneuver — but also better suited for outdoor and heavier-duty settings. 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.

Biggest Benefits

Rods of both varieties allow 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. In most cases 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.

There are only a few scenarios in which backer rods are essential, but they’re almost always really helpful. Builders can often use caulking or chinking material to fill the entire gap, but the “cure” or dry time for large amounts of these products would be a number of days or weeks, depending on the size of the gap. Things are a lot faster with the rod, and the results are often more dependable, too.

Alternative Products

Other sealing materials besides rods 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 airtight seal intact. These caulks and sealants are able to stretch to 200% or more of their original shape and size, and they’re often paintable with acrylic latex paints, too.

You might also Like

Discussion Comments


Does backer rod absorb or wick water?


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.


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


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)


@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.


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.

Post your comments
Forgot password?
    • A backer rod is used to fill gaps between building materials, such as gaps between tiles and another surfaces.
      By: MAK
      A backer rod is used to fill gaps between building materials, such as gaps between tiles and another surfaces.