The roof is on, at least the entire first layer. The photo above is from this morning after we put on the final lath and started packing on concrete. The roof is a barrel vault with a typical concrete thickness of about 3/8″ of ferrocement using PVA fibers on expanded metal lath. This is actually enough to get through the winter but given the glorious warm weather we are supposed to get from now until the start of true winter (Solstice = December 21st) we’re going to go for the gusto and do the second layer of concrete this week.
The process of add concrete to the ferro cement roof begins with Ben, our mix master, making up a five gallon pail of concrete. Ben uses an accelerant (AcceleGuard 80) since we’re doing cold weather work and dish soap to reduce the amount of water needed to get a stronger mix. Will divides the five gallon mix bucket into two smaller carrying buckets (about 30 lbs each) and brings it up to me on the scaffolding or sometimes all the way to the top on the cat walk. I dump the bucket and float it along as I dump it. This begins the process of spreading the concrete rather than it being all in one glob that could roll down the roof’s slope.
Here you can see older concrete, expanded metal lath, fresh floated concrete and the new carrying bucket’s worth of concrete – about 2.5 gallons or about 30 lbs. In an ideal world all of the concrete would go onto a project at once and there would be no cold joints. We don’t live in an ideal world. Fortunately there is a large margin of error and a number of tricks to get cold joints to adhere including wetting the concrete, leaving the surface rough and having the PVA fibers sticking up out of the old surface as they naturally do. In this particular instance I didn’t wet the surface as the batch was plenty wet but usually I do wet the older concrete.
Next I spread the concrete across the lath using a concrete hand float. The goal is to get an even coating on all of the metal and leave a surface I can bond to with the next layer. I’m not trying to float it perfectly smooth at this point.
The floated roof ready for curing. Note that curing is not drying. Concrete needs water to properly cure. It is a chemical reaction. If the concrete dries it will not reach full strength. In the summer this is a big issue, especially if the concrete is out in the hot sun. The solution is to mist it. Inside the tiny cottage we leave buckets of water about with no lids on to humidify the air so the concrete will cure properly. Watching the color of the concrete gives me an idea of how well it is going. If it gets light we spray it with water or splash water on the floor to raise the humidity in the envelope.
The cat walk is our upper scaffolding that allows me to reach the higher parts of the roof. Will, seen mounting the ladder on the left, climbed 2,240 feet today carrying 30 lb buckets of concrete. Pretty impressive! He was bringing the buckets to me, seen here lying down in the middle of the cat walk so I can reach half way down the curve of the roof. The lower scaffolding allows me to reach half way up. Between the two of them I can just barely reach any spot on the roof.
Near the end of the day I unscrewed the cat walk and slipped more pieces of lath under it to get where the boards had been. This felt much like the cartoon of the guy sitting on the branch while cutting it off… :) I even leaned out a bit to make the boards lift up on the pivot so I could slip the lath under!
Garden sprayers are perfect for wetting the concrete. With warm water and a few pumps I can ready the old cement. At the end of the day as the sun went down the garden water sprayer turned into a snow making machine. It was amazing how quickly the temperatures dropped once the sun was gone. We quickly shut down and began covering up.
Here’s how things looked at the end of today before we covered the roof with insulation and two layers of 6 mil plastic sheeting to keep in the warmth. Interesting, in the past the temperature of the cottage didn’t start rising until a while after we got the plastic on. Today, even before the plastic was on the temperature inside was already above the outdoor high for the day. The concrete roof, while not a terribly good insulator, was better than nothing. With the roof now virtually sealed up, except for three holes for cat walk supports, the solar heating through the windows is now being retained. My goal is a self heating house that will naturally stay reasonably warm by itself even through a Vermont winter (-45°F). Add a little bit of heat from lights, our bodies, appliances and the cook stove for a comfortable environment.
On the top of the roof you can see our new experimental insulation technique. It is three sheets of 1/2″ pink foam insulation bound together as the diagram shows below in pink (foam) and green (hinge):
Hope and Holly are carrying one insulation unit from the assembly site to the front of the house to pass up to me. The two long flaps hang down the curves of the roof and the top shingles above them. The green line represents a binding with three zip ties that allows the 4’x8′ sheets of pink foam to fold. The 1/2
” material conforms to the roof’s curve when the plastic is pulled tight. This appears to have worked very well. Lapped layers is the final goal for the roof insulation. We also plan to put in a radiant barrier of Foil-Bubble-Bubble-Foil. I’m trying to figure out if I want that above or below the insulation.
This makes us closed in and much closer to moving in. Once this roof is cured it would be good enough for this winter although I do hope to add a second layer of concrete before we remove the scaffolding and do final insulation. Hopefully the mild weather will persist!
Outdoors: 39°F/18°F Sunny Blue Skies
Farm house: 65°F/53°F One log in evening
Tiny Cottage: 46°F/35°F Roof off most of day
Whey 42°F Tank insulated yesterday after 8 am delivery