A lot of projects require days of setup and then are completed in a couple of hours. The bedroom ceiling is an example. It took us about a week to get the forms perfected. I’ve done a lot of test versions over the past ten months – now for the real thing.
The ceiling is arched, caps the partition wall on the south to the library, bonds with the bedroom doorway brick arch, locks around the masonry wood stove structure interweaves through the closet and a bookshelf over my aquarium area and then attaches to the north wall and east walls.
You’ll notice the granite shelves and beam in my office around the coral reef aquarium tank wall opening have been covered with plastic film. We did the same on the expensive double hung Marvin window. This was just incase I got my math wrong and we dump three quarters of a ton of wet cement down into the bedroom. Fortunately I was right on spec.
The ceiling is two layers, one of white concrete with stone inlays and then an additional two inches of structural grey concrete for the attic floor. All the concrete has PVA fibers in it and the is a layer of 661010 Welded Wire Mesh to lock it all together. This ceiling’s not coming down.
The form work had to be strong enough to support me (180 lbs) plus 1440 lbs of wet concrete in the first layer of white concrete. It consists of four 2×4 posts in the corners of the room plus one on the east window side, two 2×6 joists plus one 2×4 joist on the posts and then 2×6 planks on 16″ centers. There is also an emergency center post which I used to monitor the loading on the system. I wanted to know if my wooden support structure was being overloaded and bending – it never did which is excellent news.
On top of the planking we put 2″ thick foam, bent to form the curve of the ceiling. This was shimmed after bending and foamed with spray in place foam to lock it exactly to the curve I wanted.
This detail shot shows the complexity of the area over the closet and the bookshelf. The wall there is not flat – it has the built in granite shelving and space for clothing in a small closet behind the air ducts. On top of that I had not gotten the beam quite perpendicular when I poured it before so I did a lot of shimming to get it lined up. Now it is right and will be from here on. I used bricks to hold everything down tight and applied a bead of silicone around the perimeter to lock the foam down tight and to prevent concrete from slipping down the walls through any cracks.
Due to the size of the room, ~80″x80″, we needed two partial sheets of the foam board. That created a seam. I siliconed the seam and then added another final layer made of two partial sheets of 1/2″ foam, also siliconed together. To hide the seam I did a row of small pieces of granite marching across the ceiling. Later we’ll stencil the rest of the ceiling edges to work with that design. The effect will be that the colored pieces of granite are inlayed into the ceiling. I had visions of doing the constellations but that will have to wait for another ceiling, perhaps the kids’ loft next year – Will is researching fluorescing and glow in the dark materials.
Over the arch we laid plastic cling wrap to protect the bricks while making the forms. The forms are sealed with expanding foam. This will leave a little bit of a rough surface, all the better to grip on when we do the final interior buff stucco parge coat.
The final form work ready for the big pour. After all the days of prep it only took 17 buckets of concrete and about two hours to actually pour the ceiling. Holly sifted sand so we would have a fine mix for the ceiling layer. 10 year old mix master Ben made white concrete using two mixers at once. He averaged one bucket every seven minutes and the rest of us were just keeping up with him. Will, age 15, hefted and toted concrete from Ben, into the cottage and up to me in the attic. My math says he lifted over 12,000 lbs that morning in 62 lb increments – that definitely counts as Physical Education for the day! I spread and vibrated the concrete from my catwalk above the forms. Hope, age 4, fed us bananas and crackers.
Looking closely in the photo above you’ll notice a bar clamp in the upper right. I find these invaluable when building forms and scaffolding. They hold the wood tightly together as I screw. I then shim to lock everything tight against the walls of the house. I don’t like wiggles.
While applying the concrete I worked it hard. Once the ceiling had been poured we vibrated it by using a large concrete vibrator to shake the entire structure. I attached the vibrator to the central joist. The goal is to get all the air bubbles to migrate upward away from the bottom of the form, the ceiling. The vibration will also densify and strengthen the concrete. I’ve used this same method on three other test pieces with excellent results.
In the photo above you can see the catwalk I used to reach all the areas of the ceiling without actually having to walk on the foam board. I wanted to keep the top plastic sheet on the foam board as pristine as possible, this is my negative image of the ceiling. Any dents in the foam would be bumps in the ceiling. The catwalk, sitting on the tank beam, air ducts and scaffolding gave me about six inches of work space above the concrete as I poured and worked it.
After seven days of curing Hope is demonstrating the height of the attic space. The actual attic floor will be about 2″ higher than that after we pour the grey concrete layer. The ceiling white concrete layer’s top is purposefully rough to enhance bonding between the layers. In it’s ‘green’ state the concrete looks buff colored but when it cures it will turn white.
It is very tempting to pull the forms and peel it so we can see our work. But I must resist and trust my test tiles. I did not use accelerant in this concrete because I wanted more time to work it and get the bubbles out. It is best to leave the structure there for as long as possible up to the 28 day cure time. All in good time…
Outdoors: 35°F/23°F Mostly Overcast, Spots of Sun, Snow
Farm House: 54°F/51°F Pig sorting, whey driveway work
Tiny Cottage: 49°F/39°F Exterior Sills, Exterior Scaffolding