Repair Cracked Concrete Window Sill

Remove the excess material build up on the putty knife.

Repair cracked concrete window sill.

Fix any small or medium cracks in your concrete window sill with caulk. Any cracks can be sealed with the mortar mix or cement based caulk. I have an exterior concrete window sill that has a crack in it that i want to patch pic attached. If the sill is painted repaint the whole sill as this will give an overall better finish.

Dust from the sill will rub into the filler making it dirty but also blending it in with the rest of the sill. Resist the urge to keep touching and fiddling with the look of the smoothed caulking you will probably make it look worse. Other option measure it all up make up a timber mould and cast a new one fix either a piece of half round dowel or sash cord to create the drip cast the cill upside down this method will be better than trying to cast in situ plus no weak joints. What would be an appropriate material to fill it with.

Brush the sill with a soft brush to remove dust and dirt before applying the first coat of masonry paint after any of the following work has been done if required of course. However you should be able to purchase grey exterior filler which will be a close match to the concrete. If you have the pieces just clean off any loose mortar from the block. Steel reinforcing bars are cast into the center of the sill to strengthen it.

I have on hand a tube of big stretch caulk and a tub of dap bondex ready mixed concrete patch. Finishing the mortar joint. Scrape away any mess with the edge of the knife. Here is the smoothed caulking.

Sand down the filler when it is completely dry. Concrete has high compression strength but low resistance to bending tensile strength so the steel reinforcement is necessary. Fit the narrow tip of a bottle of silicone latex caulk into each crack and overfill it by about inch. Water is sometimes able to penetrate the concrete because of damage to the surface or just deterioration over time.

Spread and flatten the caulk with a small putty knife. If the sill is plain concrete it will not need to be painted.