Which is better – A Pocket Sill or a Sloped Sill Window?

When it is time to replace your windows, there are a quite a lot of things to consider. Balancing factors of your lifestyle and budget, along with understanding which window material or type of glass is best for your home, can be overwhelming.  One of the most common questions our clients ask is “what is the difference between a pocket sill and a sloped sill window - and which is best?” In 2012, we addressed this question in this article.

The purpose of this article is to provide an update on answers to your questions about the pocket sill versus sloped sill window. If anything has remained constant in the design of windows over the years, it is the two types of sills offered by different manufacturers. There are advantages and disadvantages to both, which we will be discussing in this article.

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. It is the area you need to vacuum when you open your windows for the first time when the weather warms up.

What is the Difference Between Sloped Sills and Pocket Sills?

Pocket sill windows are designed to surround the bottom edge of the sash when the window is closed. The advantage of this design is that in many cases, you will receive greater air infiltration and more glass to your window. A major disadvantage, however, is that the sill looks and acts like a gutter for your window. Furthermore, with time it can fill with dirt and debris and create major structural issues. 

Sloped sill windows feature a sill that flushes the dirt and debris out each time it rains. A sloped sill uses gravity to flush water out of the sill of the window. One major advantage of this design is that there is little to no maintenance and less chance of structural erosion of your window frame over time.

Understanding Drainage

When choosing a window sill, it is important you choose a sill that has a good drainage pathway away from the window. It is also important you understand how both the pocket sill and the sloped sill drain moisture, because if the accumulated moisture cannot find a successful route away from your home, it is going to infiltrate into your home.

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. 

Below is an example of what a pocket sill window drainage path looks like. You can see from the image that it is a complex way to get rid of accumulated moisture and there are plenty of places along the path where bugs and airborne debris can become trapped. 

Many pocket sill manufactures continue to manufacture their double hungs in this manner because they do not possess, or have not invested in, the updated welding machinery that is needed to weld a sloped sill. Furthermore, a pocket sill double hung is easier to construct and weld.

With this being said, we should highlight that leaks in a pocket sill design should be the exception and not the rule. There are many pocket sill designs utilized in windows and doors that work well for the life of the product. 

Which is better – Pocket or Sloped? 

As with almost all questions and answers, it really depends on who you ask. Those who favor pocket sill windows will probably tell you it is far superior than sloped sill windows for these 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

Though the above points can be true, they are only accurate when you are comparing pocket sill windows to poorly designed sloped sill windows.

While there might be a rationale for going with a pocket sill window for the reasons of air tightness and visible glass impact if the sloped sill market lacked performance in those categories.  The reality is that newer designs of sloped sill windows almost always outperform the majority of pocket sill windows in:

• Design pressure (e. strength)
• Air infiltration (i.e. airtightness)
• Visible glass

So if you can avoid potential leaks and not sacrifice any performance, why wouldn’t you? A good sloped sill window does not need to rely on any weep holes to drain out accumulated moisture, making it a significantly more fail-proof design compared to pocket window sills.