What is a Window Sill? - Definition, History & Design

Instructor: Sunday Moulton

Sunday recently earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

We may not always notice window sills in our daily lives, but this small architectural feature serves both decorative and functional purposes. In this lesson, we'll see what the sill really does and discuss some design elements.

Sitting on the Window Sill

When you think of a window sill, called a window ledge by some, the image that might come to mind is of a flat, shelf-like surface where people regularly place potted plants or other decorations. In fact, window sills serve an important function of window construction in addition to offering people a useful shelf or even a place to sit. Let's start by looking at what a window sill is in relation to the window, what it does, and how it came about.

Man Sitting on Window Sill
Man sitting on window sill

Water, Water Everywhere

A window sill is part of the window trim, the decorative covering around the window's edge that protects where the window attaches to the wall and prevents drafts and moisture from getting into the home. The window sill is the flat piece at the bottom of the window. The other parts are the head casing at the very top, the side jambs to the left and right of the window, and the apron positioned directly below the window sill. Together, they offer complete protection around the window.

Parts of Window Trim
Window with trim labeled

The sill plays another important part in protecting the home. If you place a level on the window sill, you'll see it's probably angled slightly to the outside. This feature helps water run back outside rather than all over the floor or into the wall through the window tract. While closing a window is usually enough to keep the rain outdoors, any slight opening, either from leaving it ajar before the storm or through a defect in the window, could mean costly repairs without this simple ledge.

The first windows, usually just openings in a wall, heavily relied on sills to protect the walls and floors around them. This innovation goes so far back in time that we really don't know where they first were used. However, archaeologists have uncovered silled windows as far back as ancient Egypt.

Stylish and Practical

While window trim comes in a variety of styles and designs, window sills are fairly plan. However, their material construction can provide an incredible accent to the ornamental trim around the rest of the window. Today, we enjoy many options in building materials for sills, including wood, stone, and tile. Each material offers benefits and weaknesses but ultimately the buyer, designer, or builder must weigh their options to select the best materials for their needs.


Wood, the most common material in constructing window sills, offers homeowners an attractive look with excellent thermal control, the power to prevent heat from leaking out of or into the home. It also helps with ventilation, preventing overly musty or mildewed air in the walls. Unfortunately, the moisture resistance of wood is very low. Without proper sealing and regular maintenance, wood can warp and pull away from the window to expose the home to the same drafts and leaks it was designed to prevent. Wood can also be a costly investment but the temptation to use cheaper alternatives like plywood or particle board will only result in greater moisture damage.

Wooden Window Sill
Wooden window sill

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