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 the video above to find out more.


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.

Further Information

Editorial Contributors
avatar for Joe Truini

Joe Truini

Radio Show Co-Host

Joe Truini is a contractor, author, and the host of “Simple Solutions” on Today’s Homeowner TV and the weekly Today’s Homeowner radio show. He has worked on both large commercial projects and residential remodeling, and has written for national publications such as This Old House and Popular Mechanics. He has also written eight books, including three best-selling shed-building books. Joe lives in Connecticut with his family and enjoys hiking, traveling, and baseball in his spare time.

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