Backer Rod for Asphalt Cracks: When and How to Use It

Backer rod is a round foam cord you press into a deep crack before sealing it. It fills most of the depth so the sealant only has to fill the top part. That saves material, keeps the sealant from sinking out of sight, and helps the seal stretch properly as the pavement moves. Use it on driveway cracks that are wider than about 1/2 inch or deeper than about an inch. Pick a rod slightly wider than the crack, push it down evenly, and leave room on top for the sealant, typically around 1/4 to 1/2 inch.

Asphalt crack filler guide → Crack sealing vs patching →

What Backer Rod Does

When you pour crack filler into a deep crack without backer rod, three things happen:

• It disappears. The filler runs down into the gravel, and you keep pouring and pouring.
• It sinks later. As the filler settles into the void, a dip appears where the crack was.
• It sticks to the bottom. Sealant stuck on three sides, both walls plus the bottom, can't stretch when the crack opens in cold weather, so it tears.

Backer rod solves all three. It's a floor for the sealant to rest on, and sealant doesn't bond to it, so the seal only grips the two sides and can flex like a rubber band.

When to Use Backer Rod for Asphalt Cracks

Crack size	What to do
Hairline to about 1/4 inch	Pourable or squeeze-tube filler alone
About 1/4 to 1/2 inch	Filler alone, or a thin rod if it's deep
About 1/2 inch to 1 inch or more, and deep	Backer rod, then sealant
Wider than about 1.5 inches or crumbling	Treat it as a patch job, not a crack

The deeper the crack, the more backer rod helps. Some cracks go all the way through the asphalt into the base. For help choosing a sealant to go on top, see our rundown of asphalt crack filler types, and for cracks that may need more than sealing, read crack sealing vs patching.

Choosing the Right Backer Rod

Type. Closed-cell foam is the usual choice for pavement because it doesn't absorb water. Open-cell rod soaks up moisture like a sponge, which you don't want under a driveway seal.

Heat rating. If you're using hot-pour rubberized crack sealant, buy heat-resistant backer rod. Standard foam can melt or shrink under hot sealant. For cold-pour or tube sealants, regular closed-cell rod is fine.

Diameter. Pick a rod about 25% wider than the crack, or roughly 1/8 inch bigger for narrower cracks. It needs to squeeze in snugly so it stays put. A few examples:

• 3/8-inch crack: about 1/2-inch rod
• 1/2-inch crack: about 5/8-inch rod
• 3/4-inch crack: about 7/8- or 1-inch rod

Cracks usually vary in width along their length. Buy a size that fits the typical width, and keep a smaller roll handy for narrow stretches.

How to Install Backer Rod

1. Clean the crack. Pull weeds, dig out dirt and loose asphalt with a screwdriver or crack hook, then blow or vacuum it out. The crack should be dry.
2. Unroll and push it in. Start at one end and press the rod into the crack with your fingers, then push it down to depth with a blunt tool. A paint stirrer, a putty knife, or a small roller made for backer rod all work.
3. Set the depth. Leave the right space for sealant. For most driveway cracks, that's about 1/4 to 1/2 inch below the surface. Follow your sealant's label.
4. Don't puncture it. A sharp tool can pierce closed-cell foam and release gas, which can cause bubbles in the sealant. Use a blunt edge.
5. Keep it continuous. Butt the ends tight where one piece ends and the next starts. Don't stretch the rod as you install it, because it shrinks back.
6. Seal. Apply the crack sealant over the rod, filling to the surface or slightly below. Tool it smooth if the product calls for that.

How Deep Should the Sealant Be?

A common guideline for sealant is that it should be about half as deep as it is wide on wider cracks, with a minimum of around 1/4 inch. Some hot-pour products call for equal width and depth. The key is that the sealant layer isn't too thick, which makes it stiff, or too thin, which makes it weak. Check the product's instructions, since recommendations differ between hot-pour and cold-pour sealants.

Rules of Thumb

• One roll goes a long way. Measure your total crack length first, adding 10% extra.
• Seal cracks in dry, mild weather when the asphalt is neither frozen nor very hot. Late spring and fall are ideal because cracks sit at a mid-range width.
• If you plan to sealcoat, fill cracks first and let the sealant cure according to the label before sealcoating.

Planning the whole job? Our driveway sealer calculator estimates how much sealcoat to buy after crack repair.

Bottom Line on Backer Rod for Asphalt Cracks

Backer rod belongs in any driveway crack that's wide and deep. Choose closed-cell rod about a quarter wider than the crack, use a heat-resistant type under hot-pour sealant, push it to a consistent depth with a blunt tool, and leave the right space on top for sealant. You'll use less sealant and get a seal that stretches instead of tearing.