How to Fill the Gap Between the Basement Foundation & the Sidewalk

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Things You'll Need

  • Long wire brush

  • Chisel

  • Hammer

  • Measuring tape

  • Foam backer rod

  • Utility knife

  • Putty knife

  • Silicone latex caulk

  • Scissors

A walkway adds an inviting touch as long as it doesn't threaten a basement foundation.
Image Credit: Jupiterimages/Brand X Pictures/Getty Images

Protecting a basement foundation from water damage and cracking is essential. A gap between a basement foundation and a sidewalk leaves both structures vulnerable to severe water damage. Filling the space with semi-flexible, rot-resistant material blocks moisture while allowing the structures to shift without cracking. Do-it-yourself people with little or no experience can complete this project in an afternoon plus a full day for the caulk to set.


Step 1

Clean out the gap between the foundation and sidewalk with a long wire brush. Chip off any hard debris or caulk stuck on the sides with a hammer and a chisel.

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Step 2

Measure the width of the gap with measuring tape. If it's wider than 1/2 inch, you'll need to fill in the gap with an expansion joint like foam backer rod. If it's less than a 1/2 inch wide, you can fill in the whole gap with caulk.


Step 3

Place a strip of foam backer rod on top of the sidewalk parallel to the gap. Use a utility knife to cut the rod to fit the length of the gap. If the rod is not long enough, cut another piece to fit the remaining space.

Step 4

Lay the foam backer rod on top of the gap and push it down with your hands. If you're using two lengths of rod, push these down so the ends are flush. Use the blunt end of a putty knife to hit the rod down to the bottom of the gap.


Step 5

Load silicone latex caulk into a caulk gun and snip off the end with scissors. Fill the remaining gap with the caulk until its level with the sidewalk. Hold a putty knife level with one side over the pavement and the other over the caulk. Run the knife slowly down the length of the gap to remove excess and to ensure that the caulk is level.


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