What Is a Windowsill?
Apr 20,2026
Windowsills are more than a place to store knick-knacks and stage houseplants. Both the inside (called a window stool) and the outside of the sill are critical to the structure and function of the window. Often referred to as a window shelf or window bottom, the windowsill is the flat piece of the window frame that meets the bottom of the windowpane.
Window Stool vs. Sill—What is the Difference?
A windowsill is typically constructed of a single piece of wood or framing material. In a double- or single-hung window, this sill is bisected by the windowpane. The exterior part of the sill is often sloped to promote drainage. The interior portion of the sill—the part on the inside of the house—is level and referred to as a window stool.
Related Topic: Replacing All Windows in a House—or Replacing Gradually
What Is the Purpose of a Windowsill?
While windowsills do serve an aesthetic purpose, there’s more to their construction than meets the eye. Windowsills:
Offer structural integrity: By creating a solid base, a sill is important to a window’s overall strength and stability. It also holds the sash steady when the window is closed.
Improve your home’s energy efficiency: A sill keeps your window airtight. This prevents drafts and the loss of heat or air-conditioned air to the outside.
Protect homes against the weather: The outer sill is angled so that the water or snow drains away from the house.
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