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.
The new west roof as seen from the inside looking up through the internal scaffolding, trusses and support structure.
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
Snow and SHORTS??????
Patti, in my defense there was no wind. :)
I’ve been following your blog about the tiny cottage but haven’t commented until today. I’m impressed with the work you’ve put into it. Can’t wait to see the finished cottage. Hope you’ll be showing pictures of the inside when it’s complete.
I’m impressed with the work you’ve put into the building. Can’t wait to see the finished cottage. Hope you’ll be showing pictures of the inside when it’s complete.
If you ever find my architecture a bit odd then go here for some really fun stuff! :)
It’s coming along wonderfully Walter.
and shorts? a sign of a true New Englander, well done. ;)
Walter how do you ever get so much stuff done in a day! You inspire me. I want to build a house!
I'm trying to research how to build a ferrocement roof for a 30' round barn with a reciprocal roof. It will actually be about 50' after I add the 10' shed roof all the way around the main structure. so the ferrocement roof will go over that. This type of structure is normally covered with a pond liner and hay for a living roof. Of course, the hay becomes only 4-6 inches of compost after a year or so. I live in Arkansas, so we don't get a lot of snow.
I'm curious how many layers of lath you used and did you put them on one at a time, or wire and cement both on at once. And would you mind sharing your cement recipe?
Thanks for those links. So, if you omit the lime, it's basically a typical 1:3 ferrocement mix, with pva added? How much would you estimate the cost per s.f. per layer to be? Over at the ferrocement.com site, it said about $6/s.f, I think, but I'm not sure if that was for each layer or what.
I'm reconsidering my design to be a low barrel vault to house goats, but I'd like it to be strong enough for them to climb on and help wear down their hooves. And if I make it for this purpose, the roof span would probably need to be shorter to make it strong enough for the goats' weight. I wonder where I can find figures for the load capacity of one layer of ferrocement.
Hmm… $/sq-ft. The $6 seems high at first thought for just a layer or even the whole roof. That would make our cottage roof $2,112. The total cottage construction to date is about $7,000 so I don't think the roof is that expensive. Is it just the roof layer you're thinking of or building the whole structure?
For the whole structure our cottage is ~$7,000/252 sq-ft = $27/sq-ft. That is from the bedrock up including the pad of crushed stone, insulation, concrete slab, walls, roof, insulation all around, windows, door, plumbing, electric, etc. Some, like most of the windows, are scavenged which keeps costs down.
I would go with an arch for strength to deal with the goats walking on it. On a pig house that is very similar to what you're describing I have driven our tractor over it fully loaded (about 9,000 lbs) and two of our 1,000 gallon tanks sit on top of its roof now – about 17,000 lbs plus the platform. Our cottage roof has had several feet of snow on it. I like arches.
Most excellent work. You guys are increadible.
Walter, Thank you for sharing so much. I am planning on building my own home in Northern Mn starting next summer. Vaults are going to be a part of the structure. I am curious about the engineering you have done. Where are you deriving your figures from? The ferro-cement roof here is amazing. So thin. I see you mentioned that some day you will berm earth up and over it. That is a tremendous load. How are you figuring out the loads for the roof itself and also the thrust loads? I saw mention of buttresses but haven’t seen them. Also ribs in the roof structure but didn’t see those. How has the structure performed so far, any spreading of the walls? Has the roof developed any cracks of note. I assume the wire lath and mesh would hold it together but snow can be heavy.
No cracks. Since it is a barrel vault it is innately a very strong structure even though it is very thin at only 1.5″. The welded wire mesh was only to hold the lath up during the application of the wet concrete. The reinforcement consists primarily of the lath and the fibers in the concrete matrix. There are two hoops of #4 rebar at the one third points along the length of the roof. I put those in as insurance but in retrospect don’t think they are necessary. The buttresses are interior which is why you might not have recognized them. They are the partition walls inside the building which have tensors across them.
Regarding the earth berm to come, realize that the 1.5″ shell of the current roof is just an initial hard coating. I am planning to add a insulating concrete coating and then another hard shell above that. This creates a laminated arched beam to carry very heavy loads while reducing heat loss with a structural material that is not subject to decay.
As to engineering, I don’t do engineering for others, just for myself, and as such I just report here what I have done. It is important for anyone else to do their own engineering or hire a professional engineer. Since I am not licensed to do engineering for you I don’t want to present that sort of information and get accused of practicing engineering for other people without a license. One must be licensed in each state and locality – with the internet it is impossible for me to do that, of course.
I have some ideas for improvements over what we have done on the cottage, experience is a grand teacher, but those will have to wait for a future article.
Here in Alabama I am applying the final coat of ferrocement on the exterior of my house. I wondered if the cold nights which are sometimes around freezing, would be a problem for my curing cement. I cover it with canvas and sometimes plastic. The temp only gets that low for an hour or so at night about 3 times per week lately. The days are in the 60 degree range. I have the walls done and about 1/2 of the roof done. It looks as though you were working cement in much colder weather than it is here. How did that turn out?
I built the entire structure of ferrocement then sprayed it with urethane foam insulation. Now I am putting on another ferrocement layer. This layer is not quite as thick as the structural layer. I tried to get 3 inches of insulation on the entire structure. It is that or more in some places. That or a bit less in others. It is difficult to get an even layer when the entire surface is curved in a variety of directions. It is probably difficult to get spray foam even under the best conditions.
It has been a pleasant experience and experiment. It has a lot of character and it seems to be strong enough to last a few generations.
My question for you is about waterproofing. Do you experience any leaking through the ferrocement roof? Did you put any sort of coating on the roof?
I have a water collection system connected to my house so I am afraid of most coatings. I hope the cement will be the last thing that goes on the roof so that nothing can get into my domestic water system.
Great job on your fc house. You were fortunate to have a family to help on the project. It has taken me a long time to get this far with mine but I had to work a JOB most of the time to be able to continue the house work when I had a day here and there.
Yes, we’ve done concrete down below 0°F here in the mountains of Vermont. I try to avoid that and when I must pour in cold weather I use an accelerant such as calcium nitrate also marketed as Polarset. Avoid the chloride based accelerants. While less expensive they will corrode steel. You may also be interested in learning about basalt reinforcement for use with ferrocement. We’ve been using that for two years and I’m quite pleased with it.
Your temperatures are much warmer than our fall pouring weather, in fact you’re closer to our summer weather. The burlap bags and any other insulation plus a layer of plastic will help greatly with curing. Give it a little extra time too.
For water proofing we do several things:
1) design so water drains both structurally and around the building;
2) use dense cement mixes which make the water not penetrate as much;
3) admixes like Aquaron;
4) surfactants to reduce water for denser concrete;
5) coatings like polyurea and polyurethane;
6) shedding layers; and
7) a membrane on top – billboard tarps are great for this and can be above the insulation layer below the final ferrocement top layer.
Making the top cement layer thicker helps make it last a long time. Having a high stone content helps too. Fear not to mix techniques. We use molding, ferrocement (be it basalt, fiber nylon, stainless steel or plain steel), RC all together – each has their right places in the mix of construction.
If you put up photos of your house, send me a link. I would love to see them.
Enjoy your new house!
Walter, thanks for posting your info .
Would an inner layer of papercrete be a good insulator?
It should adhere to the cement well and is basically similar to work with.
Also adding a bit of strength.
I would not consider it a good insulator because it is organic and will rot out quickly.
Wondering how the roof is holding up now that it’s 10 years(+) later?
Would you have done anything differently?
The roof is doing great. It is currently 1.5″ of concrete in a barrel vault, two layers of Foil-Bubble-Bubble-Foil and topped a billboard top. I’m amazed with how well the building performs thermally using just 0.75 cord of wood a year to heat it. My next step is to add more insulation to the roof and a top coat of concrete that will be then covered with sand and dirt for planting so as to have a living roof.
If I were to do it differently I would make the cottage 2′ taller, 2′ longer and 2′ wider. It would not have cost much more for a lot more internal space. I would also have done more built-in’s for shelves in the walls.