The Highly Technical Half-Circle Trim Installation


Alright, to get you ready for this one lets start with a joke:
Q: What kind of tree do math teachers climb?

A: A geometry

Corny, but hey – without geometry, life is pointless.

So Drew and I recently added trim around the half circle window on the front of the house. It draws the eye and is a bit of a showpiece for our house, so we wanted to make sure it looks especially good. Seeing as how we weren’t sure how to go about turning rectangular pieces of wood into a smooth radius, this proved to be a challenge.

The entire process took a couple days. We had kept our cardboard template from when we had routed the hole for the window, so we decided to use that when calculating the trim. We’d bought large pieces of 2×8 cedar and planned to lay them under the template, draw the trim, and then cut and install it. But this posed a couple problems. For one, we had to fit our trim smack dab in the middle of the triangular roof and the circular window, which only provided a clearance of a few inches from either edge. About half way up both sides of the window is where the half circle window comes the closest to the roof. The roof has a 42 degree pitch, so it becomes its most narrow in those sections. To make the shakes easier to install, we wanted to butt the trim right up against the roof in those sections. The problem was the window wasn’t quite in the center of the wall – the maximum clearance on the left side was 4 3/8″ from the roof, and 4 9/16″ on the right. So whatever trim we installed would need to be wider on one side than the other. Not only that, but the narrowest points on either side were not symmetrical! (I don’t remember the exact measurements, but it was roughly 18.5″ on one and 19″ on the other) so this made our original plan difficult.

Our original cardboard templates – the larger one is the half circle window that goes over the front porch. The smaller one is the dormer window on the end wall.

Then we came up with another idea: we decided to tape a string to the bottom center of the half circle template and lay the whole thing on another large sheet of cardboard. We outlined the existing half circle on the new cardboard, and then pulled the string taught out past the edge of our template and drew marks of a consistent width on the new cardboard. We were essentially creating a shadow, or an up-scaled model of our original template that would be the exact size and shape of our window.

Here we are trying the string idea. The front piece of cardboard is our template. We used the square (the metal ‘L’ shaped thing) to measure an accurate distance. We’d then mark the cardboard all the way around until we had a half circle trim template.

This allowed us to make a geometrically accurate model of our old template on a larger scale, and it also allowed us to make small width discrepancies where needed to make the trim narrow or thicken as we saw fit. At the end, we had an inverted “U” shape that we cut out of the cardboard. This was to be the exact size and shape of the trim we needed to cut.

Here is the half circle window template with the finished trim template above it.
We completed the same process for the dormer window.
On the half-circle template we placed marks where the trim would be hitting the roof. (This is the left long side of the trim.) We also measured where the center of the trim would be and matched it up to the center of the window template (not pictured).

Next, we marked the center point of our new template and placed a cedar board underneath. We centered it and copied the shape of our trim onto the board. We now had our first piece. We drew its placement on the template also (where it began and ended) so we knew how to place the next piece.

The board we used underneath the trim template was actually a bit thicker than this one, but this gives you an idea of the process. We placed the  trim template on the board, marked around it, cut it with a band saw, and then continued on and made the next piece.

Drew then went ahead and cut it out with the band saw as I figured out the next piece. We did this for all the pieces until we had five trim pieces.

Sorry it’s not the best picture quality-wise, but it gives you an idea of how the trim looked around the original half circle template. (You can see the green tape where we had attached a string to draw the trim template.) We later finished the trim by sanding and oiling it.
We completed the same process for the dormer window. We left the side legs long until we were able to place it around the window on site and more precisely fit it with the window’s bottom trim.
Same thing, just different angle. We’re waiting to install this piece until we being working on the back wall.

Next, we glued and screwed all the pieces together and let them dry. After that we tested it out. Unfortunately I don’t have any pictures of this, but I’ll sum it up for you and say it didn’t fit quite yet, but it was a good way for us to see what we’d especially need to sand down. So we set to sanding both the interior and exterior of the trim. This process took a long time, but we finally had finessed it enough that it looked great and fit perfectly! So we took it down, oiled it, then installed it, and voila.


Next I’ll be installing shakes around that window… Wish me luck.



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