Backer Rod: What Size to Use and How to Install It in Concrete Joints

What does backer rod do?

It sets the depth of the sealant and keeps the sealant from sticking to the bottom of the joint. The Federal Highway Administration describes backer rod as a compressible material that controls the sealant’s shape and prevents three-sided adhesion (opens in a new tab).

Three-sided adhesion is the main reason joint sealant tears. Sealant stuck to both sides and the bottom can’t stretch when the slabs pull apart, so it splits. With rod underneath, the sealant grips only the two concrete faces and stretches between them.

Depth matters too. The FHWA explains that the ratio of sealant depth to width helps minimize stresses on the sealant (opens in a new tab). Too deep and it can’t flex; the rod holds it at the right depth.

What size backer rod do you need?

Buy rod about 25 percent wider than the joint. The FHWA says the rod diameter should be roughly 25 percent larger (opens in a new tab) than the joint width, because a snug fit keeps it from moving. Round up to the next size you can buy.

Measure the joint at its widest point after you clean it out. Old joints are rarely even, so check a few spots along the run.

Sizing rule from the FHWA joint sealing tech brief: rod about 25 percent wider than the joint. Rod sizes are common store sizes.

Closed-cell or open-cell backer rod?

Closed-cell. The FHWA recommends closed-cell, non-absorptive backer rods (opens in a new tab) and says open-cell rod should be avoided because it holds moisture. Outdoors, a wet rod under the sealant keeps the joint damp.

The FHWA also says to confirm the rod is compatible with your sealant before you use it. Check the sealant label for the type of rod it calls for.

How deep should backer rod go?

Deep enough to leave the sealant thickness its label calls for. The right depth depends on the sealant, and the maker’s label gives it.

The FHWA’s guide shows how much it varies. On highway joints it sets silicone sealant at half as deep as the joint is wide (opens in a new tab), and hot-poured asphalt sealant as deep as the joint is wide. A home driveway sealant has its own number on the tube, so follow that.

Set the top of the sealant flush with the slab or slightly below it. Sealant that sits proud of the surface gets scuffed by tires and shovels.

How do you install backer rod for concrete joints?

Clean and dry the joint, press the rod in to an even depth without stretching it, and seal over it the same day. The FHWA says to install the rod right after cleaning (opens in a new tab) and seal as soon as possible after that.

1. Clean out the joint

   Pull out old sealant, weeds and dirt. Scrape both faces down to bare concrete, then brush and vacuum.

2. Let it dry

   The joint has to be dry. The FHWA says compressed air is fine for drying a joint, and to use oil-free air so it doesn’t leave a film on the concrete.

3. Measure the width

   Check the widest point and a few spots along the run, then pick the rod size from the table above.

4. Press the rod in

   Lay the rod over the joint and push it down with a blunt tool to the depth your sealant needs. Work along the joint so the depth stays even.

5. Don't stretch it

   Lay the rod in relaxed, along the whole joint. The FHWA says not to stretch it more than about 5 percent of its length.

6. Seal the joint

   Fill over the rod with a concrete joint sealant. Self-leveling sealant flows flat on a level slab; non-sag sealant has to be tooled smooth.

Does a polyurea joint filler need backer rod?

Not according to the polyurea joint kit’s instructions, which don’t mention it. Joint fillers like that one work differently from sealants: the FHWA notes that a filler’s performance doesn’t rely on adhesion to the sides (opens in a new tab), and it’s typically poured into a narrow sawcut.

The kit’s coverage chart lists joints 1/8 to 1/4 inch wide and 3/4 to 2 inches deep. For a joint that fits, fill the groove as the kit’s instructions say. For one that’s wider or deeper, the expansion joint repair guide covers your options.

Not sure whether you have a control joint or an expansion joint? The joints hub explains both.

What mistakes make backer rod fail?

Most come from rod that’s the wrong size or the wrong kind. A loose rod lets sealant flow past it, and an open-cell rod stays wet.

• Rod too small. It slips down, or sealant runs around it and sinks. Go about 25 percent wider than the joint.
• Open-cell rod outdoors. It soaks up water and holds it against the sealant. Use closed-cell.
• Stretching it into place. Stretched foam pulls back toward its length. Lay it in relaxed.
• Puncturing the rod. A sharp tool can tear it. Press it in with something blunt.
• Uneven depth. Too deep leaves thick sealant that can’t flex; too shallow leaves a thin skin that tears. Keep it at the depth on the sealant label.
• Waiting too long to seal. Dust and dew get back in. Seal the same day you set the rod.