Replacement Window FAQ: Pocket Sill vs Sloped Sill Window

One of the biggest decisions when replacing your windows is whether you opt for a pocket sill window or sloped sill window. Of course, there are plenty of factors you need to balance to make the right choice for your lifestyle and budget, but deciding between sloped and pocket sill windows is a good place to start. 

Window designs have dramatically changed over the years, but one thing that has remained consistent is the two types of sills offered by window manufacturers. In this article, we will look into the advantages and disadvantages of each type of sill so you can make an informed choice for your next replacement window project. 

Firstly – What is a Sill?

Let us start at the top: what is a sill? The sill is the bottom section of the window.  This is where all the bugs, dust, dirt and debris accumulate over time. We like to think of it as the area you need to vacuum first when you open your windows for the first time when the weather heats up!

So What is the Difference Between a Pocket and Sloped Sill?

Pocket sills: Pocket sill windows are designed to surround the bottom edge of the sash when the window is closed. The best part about this design is that in most instances you will receive better air infiltration and more glass to your window. A big disadvantage, however, is that the sill looks and acts like a gutter for your window. As such, over time it can fill with dirt and debris and create major structural issues (not to mention an unsightly mess)!

Sloped sills: Sloped sill windows feature a sill that flushes the dirt and debris out each time it rains, using gravity to flush water out of the sill of the window. A huge benefit to this design is that there is little to no maintenance required, and less chance of structural erosion of your window frame over time.

Why Drainage Matters

Perhaps the most important consideration when deciding between a sloped sill or pocket sill is the type of drainage pathway you have away from the window. Moisture drainage is important because if the accumulated moisture cannot find a successful route away from your home, it is going to infiltrate into your home and can cause some significant structural damage inside your walls. 

Pocket Sill Drainage

The gutter type of design in a pocket sill window relies on a series of weep pathways to remove accumulated moisture out of the window frame. However, these weep pathways are known for their high failure rates because they often become blocked by dirt, debris and bugs. When this happens, the water has no place to drain out, so the water is forced into the home. 

Most pocket sill window makers continue to manufacture their double hung windows in this way because they do not possess the updated welding machinery that is needed to weld a sloped sill. Also, a pocket sill double hung is easier to construct and weld.

However, we should stress that leaks in a pocket sill design should be the exception and not the rule. There are plenty of pocket sill designs out there that work well for the life of the product. 

Sloped Sill Drainage

When it comes to sloped sills and drainage, slope sills tilt down and outwards and offer unobstructed drainage, so there is no risk of water leaking into your home. When replacing windows, Windows on Washington uses the Starmark Evo double-hung windows because it offers a one-piece solid slope, which improves drainage and eliminates the need for weep holes (which tend to clog and freeze).

Is a Pocket Sill Window or Sloped Sill Window Best?

Unfortunately, there is no one answer (although, we certainly have a preference) as it really depends on your home and individual needs. There are plenty of people who favor pocket sill windows for the following reasons: 

• Minimizes any problems with viewable glass (in some cases)
• Pocket sill windows are stronger because they shut into a fully supported pocket
• It provides a more airtight seal when the window shuts into a pocket that has additional weather-stripping layers

While the above points are indeed true, they are only most accurate when comparing pocket sill windows to badly designed sloped sill windows. There certainly is a good argument for choosing pocket sill windows for air-tightness and visible glass, but that is only if the sloped sill market lacked performance in those categories – which it does not. 

It is our expert opinion that newer model sloped sill windows will almost always outperform the majority of pocket sill windows in:

Design pressure (strength)

• Air infiltration (airtightness)
• Visible glass

That is why in most cases, we recommend choosing a sloped sill window for your replacement window project. If you need more information about sloped sill windows, please get in touch with the WoW team today.