Standard patching cement from a hardware store will fail within a year. You need . These products contain acrylic or vinyl resins that bond to old concrete and allow slight flexibility.
This is the most critical step. If you apply new concrete over loose debris, the repair will fail.
Exterior concrete window sills are designed to be tough—they face scorching sun, freezing rain, and settling snow. However, over time, even the most durable masonry succumbs to the elements. If you’ve noticed chipping edges, cracking surfaces, or spalling on your sills, addressing the damage early can prevent costly water intrusion and maintain your home's structural integrity.
Once fully cured (check your mortar packaging for timeframes), apply a masonry sealer to the entire sill. This protects the repair and the original concrete from future water absorption.
Signals trapped moisture or mold growth. Step-by-Step Repair Process 1. Surface Preparation
Profile the sill to shed water. The top should slope away from the window (minimum 15 degrees). Run your trowel over the surface to a smooth, but not polished, finish.
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