Step-by-step: How To Add A Window Sill

Window sills can make or break the look of a room. Sills and stools are a must-have feature of any window in a modern home. Luckily, they aren’t all that hard to install and do yourself.

1. Strip Window To Basics

Window sills are a very important aesthetic but require a delicate balance. If there is too much going on around them it can easily overwhelm your eye and make the whole area look unpleasant.

To prevent this, you must first make sure that there is nothing around the window except the skim coat and paint. The skim coat is basically a thin coating under the paint that smooths and hides any drywall repairs.

You likely won’t run into a skim coating unless you are repairing drywall or removing paint.

It also helps to hide beads. Beads are the corners of a wall. Typically they are metal or vinyl. They basically hide the corners that drywall pieces naturally form and replace them with either smooth or sharp edges.

Your window will have four of these. One on each edge.

You must strip your window down to these basic parts. It must look very plain before you can start building your window sill. Otherwise, there will be too much going on to look good.

Not to mention that leaving things around your window will make building the frame harder than necessary.

2. Decide The Look You Want

Before you start any project you must have a clear idea of what you want. If you just start measuring and cutting willy nilly then it’s likely that you will end up with a wreck.

A good idea if you need help deciding the right window sill is to hire a designer. A designer will have all sorts of ideas and will know what will look best for your house.

If you’d rather do it yourself, no worries. There are plenty of ideas everywhere if you keep an eye out. There are some great online resources and lots of magazines you can search.

A big perk to hiring a designer is speed. They have already done all the footwork and will expedite the process. Doing it yourself though will save you money and it frankly is not too hard. It just means you have to be patient while you look. Don’t decide unless you absolutely love it.

The designer won’t make your sills though. They will only advise you on the style you should choose. Making sills is up to you or a contractor. It’s not hard though. It will require some basic woodworking skills and tools. If you are uncomfortable with power saws then consider hiring a contractor.

3. Get Your Measurements

You can only make your measurements after you know the look you’re going for. Measuring can get a little confusing, but we’ll walk you through it.

Your first measurement needs to be of how much your sill be overhanging the base of your window. This is generally between 3/4 of an inch and 2 inches. To do this you need to measure the depth of your window.

After you know the depth of the window, transfer that measurement to your board. So measure from the back of your board (the part that will be up against the window) and draw a line horizontally at the end of the measurement.

Then you make another line where you want your sill to dog-ear. This is the bit that will extend past your window that will give it a nice framed look. This is normally between one and four inches.

You will need to repeat this on the other end of the board. This will give you a box on each end of the board that you need to cut. Before you start cutting though there is one more important measurement.

The angle of the wall. Walls tend to lean and bend and not be straight. To get the correct angle, use a sliding t-bevel. Place one end against the window and the other snug up against the wall. This will give you the angle of your wall.

To transfer this to your board you need to keep the angle. To do this, place one end flush against the long side of your board. Then place the side with the angle up against the first mark you made; the mark that has to do with the depth of your window.

You will need to do the whole process for both sides.

After you have measured for your sill, you need to measure for your stool and case. This will go pretty fast. All you need to do is know how big you want your case to be and base your measurements on that. Keep in mind that the case should be about as wide as your dog-ears are.

For your stool (also called the apron) you need to measure the angle you want your corners cut at. Your apron can really be any size. It depends on the style you want.

4. Make Your Cuts

After you have all your measurements it’s time to make the cuts. If you have never used power tools before then be careful. These are serious machines that have the potential to do a lot of harm.

Operating saws is pretty straightforward though. Put on your safety gear (eyes, ear, and hand protection). Turn on the saw. Then slowly move the wood through the blade. This would be easiest with a table saw or a jigsaw.

Cut along your lines to ensure the best fit possible. Don’t go too fast. Especially if you are new to power saws. Going too fast could cause you to lose focus and get an extremity nicked by the saw. Wearing thick gloves will also help prevent harm.

If you did not measure correctly you will not cut correctly which means your board will not fit correctly.

If you are adding a rounded edge or curve to the front of your board now is the time to do it. Use a router if you have it. If you do not you can add a curve with sandpaper. Use rough grit to start and get your basic form. Then move down to fine grit to smooth it out.

5. Place The Sill

Placing the sill is also pretty easy. Push the sill up against the window. Make sure it is flush on all sides and the bottom. Use a level to make sure the sill is level front to back and side to side.

If it is not level and you want it to be then you will need to use shims. Place the shims in the low spots and try again.

Hold it there while you nail it in. Use nails and not screws. Screws will make removing it much more difficult and will cause the sill to move around while you secure it.

After it is secured, hammer a few nails in just to properly secure it. The sill really shouldn’t have more than six nails. That may even be too many for the sill.

6. Create The Stool

Creating the stool, or apron is pretty easy. If you want something fancy you will need to buy a molded piece of wood. The stool should be about the length of the sill. This really depends on the look you’re going for.

Cut the bottom edges at the angle you want. Too sharp an angle will be distracting. Too shallow is boring. Find what works best for you. It’s perfectly fine to have an apron without a cut angle too. It all depends on the look you’re going for.

7. Place Stool And Case

Place the stool flush against the bottom of the sill and hammer in some nails. Then start placing the casing. These should form a simple box around the window. If you are wanting any reveals, make sure you plan accordingly so it looks like you did it on purpose.

If reveals are done half-heartedly then it will look sloppy and ruin your brand new window sill. Reveals aren’t necessary, so if the idea of a reveal is too much then don’t worry about it.

8. Paint

The last step has a few little steps.

First, you need to spackle your nail holes. Spackle is just a hole filler for screws and nails. You glob some into the hole, then with a straight, flat edge, scrape off any extra. Keep scraping until the spackle fills the hole and is flush with the wall.

Second, you need to prepare for painting by placing protection on anything in the area that you do not want to paint on. Protection can be painter’s tape, paper, plastic, or tarps. Just anything to keep paint from getting in unwanted areas.

Lastly is painting. A nice even coat of your favorite color will bring everything together.

I have included a video that goes over just about everything you need to know about adding your own window sill. Keep in mind, even though it’s pretty simple to do yourself if you have not done something like this in the past you might want to consider hiring someone.

Ricky Kesler

With all of the projects I've done over the years, you'd think that I work on my house full-time. But I actually enjoy other things like spending time outdoors and time with my family.

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