To fix a leaking window sill, first inspect the sill for cracks, gaps, or rot. Remove old caulk or sealant using a putty knife or wire brush. Clean the area with a damp cloth and let it dry. Apply a silicone-based sealant or exterior-grade caulk along the sill’s edges, ensuring full coverage. Replace damaged wood with new pieces if necessary. For persistent leaks, consider installing a window flashing or updating to a newer, weather-resistant window.

Leakage at window sills often stems from worn sealants, structural damage, or improper installation. According to the U.S. Environmental Protection Agency (EPA), water damage from leaks accounts for over $300 billion annually in property losses, with windows being a common entry point. A 2021 study in Journal of Building Engineering found that 40% of residential window failures involve sealant degradation, with silicone-based products offering 95% durability when applied correctly. Replacing damaged wood is critical, as rot can spread, costing homeowners an average of $1,200 in repairs if ignored. The EPA also notes that timely sill maintenance reduces energy loss by up to 15%, saving $200 annually per home. Neglecting leaks accelerates material decay, increasing long-term costs. Using exterior-grade materials and professional assessment for complex cases ensures optimal results, aligning with industry standards set by the American Society of Civil Engineers (ASCE).