How to Replace Cement Window Sills

Cement was commonly used for windowsills in old homes because of its long lasting nature, even when exposed to years of extreme weather. Though the sills were long lasting, they could still develop extensive damage over time like any cement surface. The damage usually consists of wide cracks of scaling across the surface of the sill. While the damage can be repaired with a cement overlay, it’s usually best to replace the sill with a new one to preserve the strength of the sill. Replacement does not necessitate the removal of the window, but is still a messy and laborious process.

Camera
Tape measure
Large angle grinder
Special Direct System, or SDS, drills with chisel bit
Brush
Mortar
Trowel
Replacement windowsill
Carpenter’s level
Pointing trowel

Instructions

- 1
  Take a picture of the existing windowsill with a camera to help in choosing a new sill of the same design. Measure the dimensions of the old sill and write them down for choosing a new sill as well. Take the length, width, height and depth of the sill using a tape measure. Use the dimension measurements and the photo to purchase a new windowsill of the same size.
- 2
  Use the angle grinder to cut through the mortar joint along the top, bottom and sides of the sill. Cut through the mortar completely until you reach the subsurface beneath the sill.
- 3
  Break away the mortar using the cut line to work your way through the material with an SDS drill containing a chisel bit. Use the drill to chip away the edges of the sill against the wall to create a gap between the sill and the wall around the perimeter of the sill. Pull the sill out of its mounting hole once you’ve cleared enough material to loosen its placement.
- 4
  Clean up the opening by cutting away the mortar from the sides with the angle grinder. Brush away any residue from the cutting process.
- 5
  Place a 1/2-inch layer of mortar along the bottom and sides of the mounting hole in the wall. Place the new sill into the hole and then tap it into place with a rubber mallet. Place the carpenter’s level across the top of the sill and tap the sill with the mallet to adjust it so that it’s level in the hole.
- 6
  Fill the gap along the top of the sill with mortar, and then go over the mortar joints with a pointing trowel to level them out and create a smooth mortared surface. Allow the mortar to dry for 48 hours before touching the replacement sill.

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