First of all, Happy 2021!

In this article, you will learn how to build and install a wooden window sill!

It is really nothing complicated. A few power tools are needed but other than that it is a pretty simple project. I hope it helps you if you are wondering how to build one.

Of course, there are ready-made window sills on the market. Sadly they all tend to be made out of unnatural materials which is a big no-go for me. There is already enough plastic on this Earth so why not put in some effort and make window sills from wood?! It will certainly feel a hundred times better to a touch and make the room feel much cosier.

Let´s get started!

⇩⇩Be sure to watch the video if you prefer more of a visual presentation!⇩⇩

Step 1: Tools and Materials

Tools needed:

Circular saw - preferably on a track
Jig saw
Random orbital sander (or a similar sander)
Measuring and marking tools
Wood plane or coarse sandpaper
Caulking gun
Hammer

Materials needed:

Wood - I used a glued up board of ash. This is readily available material in my area. It costs around 100€ per slab, is 31 mm thick, has no defects and is already nicely sanded. I used around 1/4 of the board so it cost maybe 15€ per sill. And it has probably one of the nicest colour and grain that I know. It is perfect for something like this! Of course, a solid piece of lumber could also be used but that probably requires a jointer and a planer. Plywood is also an option.
Caulking
Wood finish - Osmo transparent Topoil in my case.

Step 2: Measure and Cut

I measured the opening where the sill will be. I had managed to build the walls somewhat straight so everything was pretty close to square (nothing a bit of caulking could not hide). If the walls would have been crooked I would have had to make a template from thin plywood or cardboard.

To make the straight cuts I used my track saw (my favourite tool!). The little squares on the ends were cut using a jig saw. Since the material was quite thick (31mm) and dense the blade started to wander and made a crooked cut. To avoid this I cut most of the material with a circular saw and then just finished off the cut with a jig saw. This gave satisfactory results.

I did cut the sills pretty precise. I am not worrying about the wood movement too much since ash is a pretty dense wood and it does not expand too much in longitudinal direction anyway. It can still easily expand in the tangential direction.

Step 3: Adding the Chamfer

I do not really know if this step is a must.

If you have ever done some welding you probably know that it is a good idea to chamfer both edges before welding them. This results in a much stronger and nicer weld. Well, I thought why not give it a try on a window sill. The chamfer was made using a wood plane. If you do not have one then I think coarse sandpaper on a solid sanding block works as well. The end result came out pretty nice so I think it worked. You can see more in the Caulking step.

Step 4: Round Over the Edges

This is really a personal preference. I decided to round over the two sharp corners. I did not make the roundover a specific size. I just used the first round object near me - the inner circle of electrical tape. Most of the material was cut using a jig saw. The rest was removed using an orbital sander and 100 grit sandpaper.

Step 5: Sand and Finish

Next up, I sanded all the visible surfaces first with 100 and then with 180 grit. That left a pretty smooth surface. There were some burn marks from a circular saw. I removed those with a hand plane but rougher sandpaper would have done the same.

For the finish, I used Osmo transparent Topoil. This is a good quality finish and it really popped the colours and the grain of the wood.

Step 6: Install

Before adding the adhesive it is a good idea to try it out one last time to avoid any surprises. I used some leftover heat-proof (of course it does not have to be) adhesive from my Wood Stove Podium project. I applied quite a few thick beads to ensure good adhesion. The sill was installed and tapped with a hammer and a waste block (to avoid dents).

All the edges were caulked. I really do not have the patience to tape everything so I just used a soapy finger to push in the caulk and hope for the best. I am quite satisfied with the results I get but it does require some practice.