Why Window Sills Are Angled (It’s More Than Decorative!)

Window sills have been around since windows first came into use. While we may not notice them in our daily lives, this minor architectural feature protects your home while looking stylish.

Window sills are often angled slightly to allow rainwater to run off. If the window sill is flat, naturally, rainwater could ingress into the building, causing lasting structural damage. Window sills are also vital to ensure your home is built with longevity and quality in mind. 

A window without a sill wouldn’t be a window, and it’s been this way for many years. Let’s take a look at everything you could want to know about window sills. 

What Is The Purpose Of A Window Sill?

The window sill is an underrated architectural feature. But what purpose does it serve?

  • It’s part of the window structure — The sill shapes the base of the window, providing both structure and support. Without the window sill, the frame would be far more susceptible to shifting in the wall, causing damage to the window and the exterior walls. 
  • A barrier between the outside and inside — The downward slope of the window sill allows snow and water to fall off as quickly as it lands. Water may seep into the walls without the window sill, causing lasting structural damage.

Even the simplest of features like a sill can save you from having to fix compromised structural integrity.

Types Of Window Sills 

When trying to classify window sills, you’ll find that there are generally two categories of window sills: interior and exterior.

As the name states, interior window sills are found inside the home, and exterior ones are located outside the home. 

Interior window sills will have a trim surrounding the window, and most people will use this sill to store or display items:

Coffee mug on top of a window sill
Coffee mug on top of a window sill

There is not need for them to be angled because they don’t have to face the harsh weather conditions exterior sills do.

Exterior window sills are geared towards protection against water damage. These sills need to be angled to direct water away from your home:

A basic UPVC plastic window sill
A basic UPVC plastic window sill

Why Should Window Sills Be Angled?

Alright, so the purpose of a window sill is to direct water away from interior spaces, but why should it be angled?

  • Structural support — As mentioned above, the window sill acts as a support structure for the rest of the window. It also holds the window sash in place when the window is closed. 
  • Energy-saving feature — Window sills protect interior rooms from drafts by creating an airtight seal. Tightly sealed windows mean less electricity for heating and cooling, saving you money on your electricity bill.
  • Protection against the elements — Besides keeping water and wind at bay, window sills also prevent mold from forming on your windows and in your walls. 
  • Design feature — A window sill can act as a permanent shelf. The downward sloping sill will help any water drain away from the house if you plan to install flower boxes. 

Angled window sills simply make sense. If you were to have a flat window sill, any water or snow that finds its way to your window wouldn’t have a means of escaping, other than through evaporation. 

If water is left on your sill and allowed to heat up through sun rays, it could cause discoloration or bubbling of paint on sills, as well as increased deterioration of wooden window sills. 

In the most extreme cases, pooling water on your window sill can penetrate your interior walls, causing damp and flaking interior paint. Other than flaking paint, you can also run the risk of mold growth in your walls and around your windows. 

Any mold growth in your interior walls could pose a threat to your home’s structure and your health, as some molds can be toxic

What Is The Correct Angle For A Window Sill?

To minimize and avoid water infiltration, you want to ensure that your window sill is at an optimal angle. 

If the angle isn’t accurate, you won’t have a tight seal between the window sill and the window frame. A wrong window sill angle will compromise the water-wicking ability of the sill.

Generally, you’ll find that sills range between 7 to 15 degrees in slope angle:

A diagram showing a 7 15 and 45 degree angle for window sill purposes
A diagram showing a 7 15 and 45 degree angle for window sill purposes

If you want to calculate the slope of your window, this post gives you three ways to do so, with diagrams, charts, and various tools to ensure you get the most accurate sill angle measurement. 

Can Window Sills Be Flat?

Technically speaking, your window sill can be flat. Would it be a good idea to make your window sill flat? Probably not. 

Besides the reasons mentioned earlier as to why you want an angled window sill, window sills are necessary for any window structure. 

The sill provides a natural stop to the lower rail of the window and holds the window sash in place when the window is closed. 

Without an angled exterior window sill, your walls and floors risk becoming waterlogged due to water running down the walls and pooling in the floors

What Are Window Sills Made Of?

A block and brick built new construction home with some windows not installed
A block and brick built new construction home with some windows not installed

Window sills are a simple architectural feature that serves a great purpose and is easy to install — especially compared to other home features. 

Because a sill is a semi-permanent fixture, meaning you’ll only remove them if you remove the window, choosing the suitable material is crucial. 

You ideally want to choose a sill material that is durable and complements the exterior of your house.

Previously, window sills mainly consisted of wooden boards prone to absorbing water. These boards tend to rot without proper maintenance due to water damage and mold

In our modern-day, materials are designed to serve a specific purpose. In the case of window sills, the primary function is to be weather resistant. 

While the materials have improved, there still isn’t a large variety of materials available to serve the function of a window sill. 

If you’re looking for window sill materials, consider the following options:

Wood

Modern wood sills have finishes designed to be weatherproof; this makes them tolerant of most weather conditions. These sills also best suit traditional homes. 

With proper maintenance, these sills can last for decades. 

Stone 

Brick and stone window sills best suit homes built with the same material. What makes these sills unique is that they are the most durable and natural option and don’t need any particular maintenance strategies

Marble, granite, and stone are best suited to exterior window sills.

Plastic

A basic UPVC plastic window sill
A basic UPVC plastic window sill

PVC sills are a great option because of the versatility it provides

Manufacturers can produce PVC sills to look like stone, marble, and even wood in the production process. 

Because of the material, these sills also tend to be resistant to mold and stand the best chance against most weather conditions. 

Metal

A modern option for window sills is aluminum. This material is durable, cheap, and can last longer than wood with proper maintenance. The only downside to choosing a metal window sill is that most companies don’t manufacture the windows to isolate the sound of rain.

Tile

Tile is a less common option but a great way to add personality to your house exterior. Many older homes boast beautiful mosaic designs in their sills. 

This type of window sill requires the most maintenance, as you want to keep the tile clean to show off the design. 

If you choose to tile your window sills, be sure to use an exterior grade grout to outlast any harsh weather conditions. 

It’s also wise to seal the grout to prevent mold and water damage. 

An Overview Of Window Sill Materials

Of these materials, you’ll find that timber and aluminum sills are the most common window sill material options, with PVC sills slowly rising in popularity

What makes these the best materials for window sills is that they offer the best protection against rainwater while simultaneously suiting most home exteriors. 

Choosing a metal window sill will be cheaper while installing stone window sills will incur the most costs. Unfortunately, this is a situation where the highest investment upfront will ensure minor maintenance and the most prolonged duration.

How To Clean And Maintain A Window Sill

Various microfiber clothes
Various microfiber clothes

All windows are susceptible to dust, debris, rain, and the occasional bug. Because of this, windows, as much as their sills, become dirty. 

Fortunately, maintaining a clean window sill doesn’t require heaps of effort. With some simple cleaning supplies and elbow grease, a dirty window sill will be something of the past

To clean a window sill, you want to ensure that you have suitable materials for the job. To clean your window sills, you will need the following supplies:

  • Warm water with some cleaning detergent, like dish soap, mixed in.
  • A clean cloth or rag.
  • A brush or a vacuum with a brush attachment.
  • A spray bottle with soapy water.

How To Clean A Window Sill

Use a brush or vacuum to remove as much dirt, dust, and bugs from the interior and exterior of the window sill as you can. 

If you dust off everything you can initially, you minimize the time you will have to spend cleaning away extra grime and dust later when you use water and a cloth. 

After cleaning away the dust with a brush, you then want to soak your cloth in warm soapy water and give the sills a good wipe down. 

Once you’ve wiped down the sills with your soapy cloth, it’s time to tackle the window corners and the window tracks. 

To do this, you want to fill a spray bottle with some of the soapy water and spray the hard-to-reach areas. This way, the dirt will become loose and freely flow off the sill

After leaving the solution for a few seconds, take the corner of your cloth to wipe away any excess dirt that dripped down. 

If the areas are still dirty after the first pass, repeat spraying down the areas with soapy water. If this still doesn’t get out the dirt, use a cotton swab to clean out the rest of the dust. 

It’s important to minimize the amount of water you use on a wooden window sill to reduce potential damage

Also, remember to keep the bucket of water handy to regularly rinse your cloth and avoid wiping dirt over your window sill. 

Be mindful not to be too forceful when removing dirt to avoid chipping away paint or scratching the sill. 

How To Maintain A Window Sill

Once you’ve cleaned the window sills, maintenance is simple. 

To avoid deep cleaning the sills again in the future, regularly dust your window sills. If you reduce the dust present, there won’t be much grime to build on

If bugs or leaves find their way onto your sill, brush or vacuum them away.

Keeping an eye on your window sills and ensuring they’re well maintained will leave them lasting for years to come. 

Frequently Asked Questions

If you have any questions about window sills that weren’t answered above, here are some common questions people tend to ask:

Are Window Sills A Must?

Window sills are essential to any window structure. 
Without them, the possibility of water damage is very likely. The problem with water damage is that it’s very gradual and generally only noticed when it’s too late.

Why Are Bricks Angled On The Window Sill?

The brick window sill is simply a window sill style. 
They are angled to allow the water to drain away from your house, keep moisture out, and act as aesthetic exterior decoration. 

What’s The Difference Between A Window Sill, A Window Ledge, And A Window Stool?

Generally, you’ll find that the terms window sill and window ledge are used interchangeably. 
Both acts as structures to drain water out and away from the house and are located exteriorly. 
The window stool faces the interior of the house. You can also adjust the stool depth to become a window bay. 

What Is A Sloped Sill Adapter?

A sloped sill adapter is designed to bridge the gap between the bottom of a window frame and the window sill. They are easy to install with exterior-grade adhesive tape and pre-scored tracks to adjust to your window and sill difference. 

What Is Sill Pan Flashing?

A pan flashing is installed below every window and door to direct any water that may flow into the house back outside. It’s also used as an exterior design feature in modern homes. 

Do I Need To Add Damp-Proof Course Around My Window?

There is a cavity around your windows and doors that prevents insulation. To keep your house well insulated, you want to add a Damp-Proof Course, or DPC, around your window and door fittings. DPC is a membrane applied to prevent moisture transmission between exterior and interior spaces. 

Do I Need To Add Damp-Proof Course Under My Window Sill?

You only need the DPC to separate the interior wall and sill from the exterior sill. 
You don’t need to install DPC underneath an exterior sill.

Conclusion

Window sills are the unsung heroes of windows. With them, your house won’t be susceptible to water damage. 

At the right angle, a window sill will decrease condensation between window panes, reduce water infiltration, and leave you with a home that will withstand any wet weather conditions.

About Charlie D Paige

Charlie is a massive DIY fan, with dozens of DIY projects under his belt - ranging from tiling to electrics, and concrete pads to walls. Charlie loves tinkering, seeing how things works, the outdoors and playing with power tools... so is it any wonder that he's completed so many DIY jobs over the years?

Charlie loves spreading his hard-won DIY experience with the world via this blog.