windowsill

A windowsill, or window sill, is the horizontal ledge at the bottom of a window opening. It can be an integral part of the window frame or a built-up feature of the surrounding wall. Interior windowsills provide a finished edge, support light objects, and help seal the window against drafts. Exterior sills project outward to shed rainwater and protect the wall beneath the opening.

Materials vary widely. Interior sills are commonly wood, stone, or synthetic materials such as PVC or engineered wood. Exterior sills are typically stone, concrete, metal, or weather-resistant wood. The sill’s depth and profile depend on the window type, architectural style, and local climate.

Design and installation considerations include proper drainage, a slight outward slope to shed water, and weatherproofing at the junction with the wall. Interior sills emphasize finish and insulation, while exterior sills focus on durability and moisture management.

Maintenance involves protecting wood with sealants, paint, or varnish to prevent rot, and cleaning to prevent staining or mold. Stone or metal sills require periodic cleaning and, when needed, resealing or repainting.

In homes, windowsills are commonly used to display plants, photographs, or decorative items. They may also support light gardening, such as herbs, in sunny locations. The term may also describe functional exterior ledges that are integral to window performance.