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How to Build a Ceiling Dome |
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Adding the Foam First a warning... insulating foam is actually pretty messy stuff. If you get it on anything you can pretty much count on never getting it clean again. This goes for clothes, tools, etc... I figured before I messed up the finished frame I'd do a test on a small piece of wire mesh. In my first attempt I sprayed the foam on the mesh and then tried to shape it a bit figuring it would be easier to shape before it cured. That didn't work at all as working with the foam simply collapses it as all the little gas bubles burst. My second attempt was to let the foam cure and then try to sand it. The results are shown below: 
Not only was the foam very rigid and well secured to the wire mesh, it was very easy to sand. This seemed like the way to go... unfortunately a single can of foam only got me about 25% of the way around the dome. This meant back to Home Depot to get a few more cans. I had originally started with Hilti foam but switched to Great Stuff foam as it's about half the price. In the end I should have stuck with Hilti foam. The Hilti foam cures much more rigid than the cheaper foam which makes it easier to sand. In addition, the cheaper foam didn't cure all the way through so when I sanded it down to the correct shape it tended to shrink a bit more over the next hour or two. Of course this didn't really matter since I was going to cover everything with plaster, but if I was to do it again, the Hilti foam was certainly easier to work with. The image below shows the dome mostly covered in foam with about half of it already sanded to the correct shape. 
In order to shape the foam I built a simple tool from a brass door hinge, steel rod, a pair of rollerblade wheels, sandpaper, and a few scraps of wood. As you can see in the picture below I used a scrap to mount the steel pin perpendicular to the center of the dome frame. I then cut a scrap of plywood to the exact shape of the dome and mounted the hinge on to it. This let me spin the shaper around the pin. However, since the dome has to have a perfect finish, the shaper must remain level at all times to ensure that the dome is symmetric. This is where the rollerblade wheels came in. By mounting the shaper to a scrap with the wheels attached, I could apply downward force on the shaper until the wheels hit the drywall frame. 
I then covered the edge of the shaper with short segments of sandpaper which you can see in the photo below: 
Sanding the foam was mostly a process of rasing the shaper above all the bumps and swinging it back and forth quickly while pushing it down slowly. While large chunks tear out at the beginning, it results in a virtually perfect finish when you're done and it goes pretty quickly. If you look closely you can see little ridges formed by the overlapped seams in the sandpaper. This is fine for the foam layer but it was going to be problematic for the finish coat of plaster. NEXT > | ||||
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