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Wonderboard ceiling
Monday, April 13 2009
Even at this late stage of its construction, the flat part of the greenhouse ceiling was still unfinished. It consisted of a single sheet of plastic keeping the bats of fiberglass insulation from falling out from between the ceiling joists. I'd already bought the half inch Wonderboard (fiberglass-mesh-reinforced concrete) which I planned to use as a ceiling material, and today I finally got started installing it. It was a straightforward job; I only needed to cut a single hole for the ceiling-mounted electrical outlet and a slot for a bit of electrical wire. Beyond that, I could just screw the sheets in place. Since I intended to "weld" the edges of the sheets to each other with more fiberglass mesh and Portland cement, those edges didn't need to coincide with joists, which was good, since the joists are two feet apart while the Wonderboard sheets measure three feet by five feet. It turns out that the ceiling, which measures about five feet eight inches by fifteen feet two inches, could be entirely tiled by five uncut sheets and one small rectangular piece. The result would leave a five inch strip sticking out past the flat ceiling into the higher part of the greenhouse airspace beneath the sloped south windows, but I can easily come along later and snap that part off.
The main problem when using Wonderboard as a ceiling material is how wickedly heavy it is (despite visible voids, it's twice as heavy per unit of volume as drywall). A single three by five foot sheet of the half inch stuff weighs fifty or sixty pounds (depending on the humidity). Raising that kind of weight all the way to a seven foot ceiling isn't just difficult, it's also dangerous. If such a sheet were to fall on me, it could crush whatever part of me happened to be in the way. The key to getting a sheet in place was to insert one edge onto a ledge near the ceiling and then lever up the rest in a single movement, locking it in place with carefully-sized T-shaped pole (I'd made my pole adjustable by using large C-clamps to attach two pieces of lumber to each other). Essential to this technique is the presence of a ledge near the ceiling, which the greenhouse has all along the top of its concrete walls. For sheets that didn't abut this wall, I had to build a portable artificial ledge from scrap lumber. This device (which had its own adjustable support pole) allowed me to position relatively solid ledges three feet out from the wall. Using these techniques, I was able to install three sheets of Wonderboard ceiling over the course of a couple hours this afternoon without the assistance of anyone.
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