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The Slope Required for Window Sills

Bridgette Austin

Window sills are important for draining water and ensuring that windows are protected from water damage and decay. The slope on a window sill helps achieve this task despite the sill's nearly horizontal appearance. A high-quality window sill also contains a slope that funnels water over the edge and onto the ground. Adjustments can be made if a slope is not already built into the installation using siding or back dam to facilitate drainage.

Capillary Breaks

The slope on a window sill is important for preventing moisture buildup.

Properly installed window sills have a pronounced slope, according to Par Carlson Dunlop, author of “Principles of Home Inspection: Systems & Standards.” Sills that tilt slightly forward away from the window can adequately drain water that collects along the sill’s surface. Capillary breaks — grooves cut into the underside of the sill that run parallel to the wall — help stop water from traveling along the underside of the sill and toward the wall underneath. This portion of the sill is also referred to as the drip edge. A bead of caulking can serve as substitute for a capillary break to properly drain water.

Draining Sill Pan

Materials such as beveled cedar siding or beveled pine siding can be used to create a draining sill pan. Draining sill pans can be flexible membranes, rigid plastic or metal material that cover the surface window sill. Although the membrane adheres to the sill and creates a watertight surface, the installation of a sill pan can increase the opening sill height approximately 3/8 inches, according to Green Building Advisor. Unlike back dams, draining sill pans create a slope leading toward the bottom edge of the window sill.


Because they are constantly exposed to extreme elements such as rain, heat and snow, even high-quality window sills can rot and decay. The horizontal positioning of window sills makes them vulnerable to large volumes of water, which can seep into corners and soak the wall below. As a result, rot sets in, insects invade and structural damage occurs. Mortar joints between the sill and face of the wall can loosen if mortar is constantly exposed to excess moisture. Slopes on window sills also help drain condensation from air conditioning units.


If you discover metal rust, cracking, rotting, loose mortar or other signs of structural damage, your window’s slope may be insufficient for proper drainage. Check the slope of your window sill from the inside and outside of the window opening. However, the slope on window sills on upper floors is best observed from the inside of the building. Attempting to view window sills beyond the first floors will likely provide an inaccurate estimate of the slope.