Guide To Window Sills

Our Guide To Window Sills will try to explain what a window sill is and what you should be looking for when shopping for new construction and replacement windows. A window sill is an elemental part of a window frame that is located below the lower window sash. The purpose of an exterior window sill is to help direct water and moisture away from the window and house to insure that no water gets behind the walls and causes wood rot and/or mold inside your walls.

There are a 3 main types of window sills that will discuss. What's interesting about the window sill discussion is the amount of disagreement out there in terms of how important sills are. There are some industry experts who feel that the sill discussion is overblown and that all three types work effectively, while another group feels that the type of window sill is a key design component. My take on it is this; in places that get lots of rain and humidity, the type of sill is one of the defining features of a window. In places that are more mild and not subject to lots of rain and humidity, the sill is not nearly as important.

Snap In Window Sill

The snap in window sill (also called a pocket sill or a capture sill) is designed to drain water and moisture through the frame itself. The snap in sill "allows" water to funnel into the frame and out the weep holes at the bottom. The problem with these pocket or capture sills is that over time dirt and debris can clog these weep holes and trap water, moisture and mold inside of your window frames. This is not what the manufacturer(s) inteded, but it is often the long term result.

Why then would a window manufacturer use a snap in sill? It is cheaper to make a snap in sill than it is to cut an extruded, true welded sloped sill (see below). One example of a window that use the snap in sill is the Alside Excalibur window.

Stepped Window Sill

A stepped window sill is a step up from the snap in sill (pun intended) as it does not allow water to drain through the window frame. Like the name indicates, the sill has an exterior sloped design that sort of resembles a step to shed water away from the sill.

The stepped sill is a preferable design to the snap in sill and is, for example, used on Simonton 5500 Reflections windows, which generally get good reviews from contractors and homeowners alike.

Type Of Window
Frame Material
Quality Of Window
Type Of Installation
Number Of Windows
Enter Your Zip Code

True Welded Sloped Sill

A true sloped sill is one that is machine cut on a slight angle so that water and mositure are sloughed off and away from the window, instead of drained through the frame, as with the snap in sill. This is a better system than the snap in and prevents any possibility of water or rot issues. The true welded sloped window sill is more expensive to make as it requires a piece of machinery to make the extusion angle. One example of a window that use the snap in sill is the Okna 800 - check out our Okna 800 windows reviews.