Tip for Caulking Wide Cracks

Using foam backer rod, followed by caulking, to fill a wide crack in a sidewalk.
Using foam backer rod, followed by caulking, to fill a wide crack in a sidewalk.

To full gaps wider than 1/4″ use foam backer rod before caulking to prevent the caulk from cracking over time.

To caulk wide gaps:

  1. Clean out the crack to remove any debris.
  2. Push foam backer rod into the crack.
  3. Caulk over the backer rod for a long lasting repair.

Foam backer rod is available at home centers in several different thicknesses. Watch this video to find out more.

Further Information

Joe Truini: When using caulk to seal cracks and holes around the house or outside, it’s important to make sure the crack is no wider than a quarter of an inch, because if you just put in caulk, the caulk will fail, because the crack is too wide.

So, here’s the trick. Go to a hardware store and pick up foam backer rod. It’s just this rope of soft foam rubber. And you can force that in through the crack first, and that’ll serve as a foundation for the caulk itself.

So, start by sweeping it out. Sweeping out the crack so there’s no dust or debris in there. You want it as clean as possible. In this case, since we’re working with concrete, we’re going to be using a concrete repair caulk.

But before putting in the caulk, because this crack is way too wide, we’re going to stuff it with this backer. Then you can fill it up. The entire crack will be filled, but not entirely with caulk.

The backer rod provides the support, and you’ll have a perfect repair that will last years.


  1. We had a insulated sunroom added to the back of our house using a contractor. The exterior wall was completely removed and this room was added. We had problems with the contractor showing up and completing the job. Our house has been very cold since. The walls were insulated with blown in foam and I feel this was done very well (the kind that is sprayed in and then hardens). About a year later, we had a patio added outside this new sunroom and while busting up remnants of an old patio with drills, dust kept coming in at the base of the walls. I think no insulation or caulking were done before a 6 inch baseboard and floor molding were added. Air can be felt all along the baseboard area. This is a 20×20 room and it would be too expensive to remove/replace the baseboard and floor molding. The house is built on a slab as is the new addition. There is concrete siding on exterior walls.
    My question: Can we use the spray foam under neath the siding at the base of the sunroom to insulate it from the outside? We plan on adding/gluing decorative rock around the very bottom to cover where the concrete meets the ground as the former owner had done around the rest of the house. We thought tnis might help insulate that area before adding the decorative rock. Would this work (the spray foam) or is there something else we could do to help insulate that area? No hopes of contractor returning, he took his money and ran!


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