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.

Step 7: The End

Like always, I hope you enjoyed this article and learned something from it. If you have any questions or comments please do leave them down below.

As always, have a wonderful day!

Andu

PS: If you enjoyed the article and video please consider subscribing! Thanks!

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1 Person Made This Project!

Flixm made it!

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15 Comments

djohnson

5 weeks ago

Looks great! I’m tackling this same project for my 2 basement windows. The first one is just about done. The 2nd basement window is way out of square. Any tips for how to tackle this? I used 1/2”red oak, 8” wide plank on the first window. The 2nd well is deeper so my 8” board would not have the reveal needed. should I just buy a larger board to work around the square problem?

Kekshexe

2 years ago

It really looks nice and is quite exactly what I want for our 250 yr old studwork house. I fugure the good old fashioned granite sill is way too solid so this wooden sill it shall be!

NoahEyler

Question 2 years ago

If you have preexisting wooden window frames, is this something that can be done to modify them, I.e taking out the old, dark and outdated wood and putting in this new more modern type sill? Do you have any links to that type of woodworking projects?