How to Repair Uneven Concrete Under a Sill Plate


During foundation construction, the contractor forms and pours the walls, leveling the wet concrete with the tops of the forms. While this usually provides a relatively level concrete surface on which to install the sill plate, some types of wall forms have support boards that span the tops of the forms, making it difficult to get the wet concrete perfectly level. The risk of an uneven wall increases if the concrete begins to harden before the tops of the walls are even. Leveling the uneven spots provides a better base for the sill plate.

Things You'll Need

  • 10d nails
  • Hammer
  • String
  • Transit
  • Wheelbarrow
  • Hoe
  • High-strength mortar mix
  • Mason’s trowel
  • Hammer two 10d nails into the opposite ends of one foundation wall. Leave about 1 1/2 inches of the nail sticking up.

  • Tie a string taut between the two nails, and use a transit to level the string as close to the top of the concrete wall as possible without touching it in any spot. A tiny gap of about 1/16 inch is sufficient.

  • Mix water and high-strength mortar mix in a wheelbarrow. Follow the mixing directions on the mortar mix, and stir the wet mortar with a garden hoe or a small shovel.

  • Scoop up the wet mortar with a mason’s trowel and deposit it on top of the concrete wall at one side.

  • Spread and tamp the wet mortar flat against the concrete until it’s level with the string.

  • Continue using the string as the guide and work your way down the wall, adding mortar and leveling it with the mason’s trowel as you go.

  • Repeat with the other concrete walls, but leave the original string in place. Just drive a new nail at the end of the next wall and tie a new string between the two nails, pulling it taut and leveling it with the transit each time.

  • Let the mortar dry before removing the nails and setting the sill plate.

Tips & Warnings

  • You can roll a wheelbarrow along with you, but if you mix the mortar in a large bucket, carrying it can be cumbersome.
  • The top of the wall need not be smooth like the concrete of a sidewalk, because it's going to be covered. It should be level, however.
  • If the contractor set the sill plate on an uneven foundation wall and the house is already built, your options are limited. You can caulk the joint between the sill plate and the concrete from the interior or install a strip of flashing under the siding that covers the sill plate to prevent water and air from entering through gaps beneath the plate.
  • Photo Credit Jupiterimages/ Images
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