On Monday I described the model of molding a plaster ceiling in place using a technique I’ve worked up for the tiny cottage. It worked beautifully and since the plaster is up on the ceiling it is well protected from water. In the bathroom where water does get splashed we’re going to use white cement instead of plaster to get a more durable finish.
To get good at doing the plaster before doing the real thing we’re going to do a bunch of small projects on the tiny cottage. After the model our first project is a set of shelves in the bathroom that will go under the aquarium. I’m actually using plaster on the bottom of these shelves, modeling them just like the ceilings in the bedroom and front room to come despite the fact that these shelves will be in a wet area. It will give me a place to test how the plaster does perform in a real life wet place. The structural element of the shelves are ferro cement with PVA fiber. I also mixed PVA fiber into the plaster to increase its strength. If the plaster fails it won’t be a big deal because the cement will still be plenty strong enough for this non-critical application.
Like with the model the other day I setup a mold out of 1/2″ rigid pink foam insulations. The pink board is slightly longer than the 43″ width of the shelf space so it forms a curve. I used concrete blocks below to support the ends and middle, shimming it all up to hold the coming weight of the plaster and concrete. Wooden shims support the foam so that I can easily remove the foam when the shelf is hard. The edges of the foam sheet were sealed to the wall with minimally expanding foam which I allowed to harden fully. Holes drilled in the end partition blocks received 16 penny nails which helped lock the shelf into the partition wall. I also scored the partition block sides with the hammer drill to make sure the concrete I poured for the shelf would bond well with the concrete of the wall.
My mix master Ben made up a half pail batch of plaster of paris at a 2:1 plaster to water ratio using one of our mortar mixers. I poured it into the mold starting at each end and working towards the middle to distribute the weight. Then using a 12″ hand float I spread the plaster. It was like cream cheese – very smooth.
The plaster has a 20 to 30 minute set time from when the water first makes contact. Mixing was 4 minutes. Pouring was 1 minute. Spreading was 3 minutes. It was still soft as I embedded a sheet of expanded metal lath while Ben mixed up a half pail batch of very spreadable (high slump) concrete. Within four minutes we had the concrete on the still soft plaster so they would bond together. Now that the time crunch was off Ben and I took our time to work and float the shelf. Total time was 25 minutes.
Doing the ceilings, especially the 13’x5′ expanse of the front room ceiling is going to be a lot more challenging since it is such a larger area. We’ll work our way up to that with progressively larger projects like more shelves, a desk, the bedroom ceiling, etc. We also do have three mortar mixers so we can crank out plaster very fast with a bit of careful prep and organizing.
Here’s the finished shelf, still green but demolded. In 28 days it will be close to maximum strength. It is a very tempting bench but we’ve all successfully not sat on it. After five days the shelf sounds like a taunt drum when rapped with my fingers. Today as we worked on other things I set tools on the shelf. It still looks green but is harder than my finger nail. This has been a very successful second test of the ceiling molding technique. Next I’ll do the shelf for the wood stove, entirely out of cement and thicker, and then another bathroom shelf, the aquarium shelf, a high bathroom shelf, a desk and a small ceiling. Progressively larger projects to test ideas and perfect the technique before we do the highly visible front room ceiling.
Ah, the underside of the shelf. There is just a slight curve to the bottom side – very pleasing. This is what our ceilings will look like – Except we plan to stencil them and the walls will be white plaster as well. Pretty cool! I left a rough groove along the front edge with metal sticking out so I can hand form a rounded lip. The actual ceilings there is no exposed edge like that. It may seem odd, even very odd, to have plaster on the bottom side of a shelf, but you must admit that it was an excellent test of the technique.
Outdoors: 42°F/23°F Sunny, Windy
Farm House: 56°F/50°F no fire
Tiny Cottage: 55°F/46°F