If you’re only going to read one post on roofing for earthbag, make it this one. Remember our brave and successful earthbag lovers in Costa Rica? Here’s how they constructed their roof, step by step. Earthbag roofing options are plentiful, but this design is original, strong, and lovely. I’m very grateful to Murat Dirlik for sharing his methodology and photos in such detail in our Facebook group. It demonstrates perfectly how to deal with the issue of outward pressure or “splay” in earthbag roofing options.
Sam and Leora are in the closing phase of their wonderful earthbag home in Costa Rica. The couple wanted a beautiful roof to cover a circular structure. With their building buddy Murat, they decided on an octagonal design. They didn't want any permanent posts inside, and wanted to include a cupola to help move all the hot air out of the space.
“Knowing these factors, splay was obviously a concern, as the roof would put a significant outward pressure on the walls... the direction in which they are weakest,” says site foreman Murat Dirlik, who has documented the build for us in superb detail.
I’m quoting Murat throughout this post, because I really love the way he has shared the details of his thought process as he goes along. It shows how good builders modify and adapt as the mental plan hits physical reality. I’ve never seen anything work in practice exactly how people plan in their minds, and for me this is the most exciting part of construction and off-grid world creation. You have to work with nature, with materials, with time, with money, with people, and with weather. This alchemy creates something wholly unique.
“We are building in Costa Rica, just to put things in context,” says Murat. “It's hot, rainy, and earthquake prone here. Because of these factors we used a lot of rebar in our walls... in a photo or two you can see little x's on the bags, which designate where we drove rebar down into the runs of bags below, roughly a meter or so. The x's are staggered and the rebar runs lower than the preceding ones, to properly integrate them all. Marking them (x is where the top of each rebar ends) was essential to us knowing where the previous ones were placed, since the following line of bags covered all evidence. Of course we used barbed wire as well.”
The Bond Beam
“We have access to a lot of local teak, so we used it for our bond beam instead of cement, which we are avoiding for countless reasons. I was unsure how well this would work, but we had enough slightly curved pieces that we were able to tie them all together in an octagon without jutting past the parameters set by the wall's thickness and its curve. These beams were pinned at opposing angles with rebar down into the walls, and into each other. This was tricky as the bags couldn't be too wet or too dry, but we pulled it off.”
I think Murat’s bond beam is superb, and is exactly how you want to create a bond beam on a circular house. Just a note: It is possible if you are building a small earthbag round house, and adding a roof where the pressure pushes completely down rather than outwards, to omit the bond beam (I did in my Mud Home in Turkey. It has survived many quakes now).
The Pitch of the Roof
The team didn’t want the roof to be over-pitched. In the end the pitch was roughly 5/12. Because of the bond beam, rebar and general structural competence of the house, Murat was right to feel he was off to a good start. “So I got to working on the initial ring for up top,” he says. “I did a mock up on the ground to make the work easier and finished the ring (bolted at joints - perhaps unnecessary with all of the compression enacted by the rafters, but seemed like a good idea) and then did all of the rafter mortising on the ground as well. In retrospect, as much as this allowed for clean work and pretty joints, I think perhaps on this next roof I'll do that work up top just to get things moving faster.”
The Cupola and Rafters
So Murat and the team hoisted the ring up onto some temporary planks, up onto the tower of power, and installed the rafters.
“The rafters are bolted through the ring and down into the teak, and also rebarred down into the walls through the bond beam. You can see in one photo how we had to add little chunks of teak instead of going directly through the bond beam. This was because of the octagon of the bond beam vs the roundness of the walls, as well as the varying thicknesses of our framing. We sort of just went for it, adding stuff where needed and simply making sure that everything was well fastened together with rebar in pretty much every direction we could put it.”
The Extra Tension Cable
“I also decided to put a cable around the whole thing to keep the roof snug... you can see it in the pictures. We basically drove the rebar through all of the rafters leaving about 10cm sticking out, and then ran a cable around the outside of them all, tightening with a turnbuckle, and pounded the rebar down over the cable to pin it in place. I now believe this wasn't necessary, but it is another little back up element that adds to the anti-splay needs.”
Overhangs and Counter-Balancing the Outward Force Exerted by the Roof
“Once the 8 primary rafters were up it seemed pretty stable, but certainly still moved about a bit. I pulled the planks to see what happened because sometimes I just can't help myself, and it stayed put nicely.”
Yay! I always recommend a good kick, pull or shove to check an earthbag house for structural integrity. You can immediately feel if the thing is locked in or not.
“Secondary rafters, cross ties, etc. went up next. I tried to design this roof to have almost as much weight outside as inside, to put more of a downward pressure on the walls.”
You pro Murat! Totally correct thinking. This is exactly how our carpenter in Turkey formed the massive reciprocal roof on a workshop build we did there.
“The soffits are about 120 cm... we needed big overhangs anyway because of the rains here, so they counterbalance fairly well. To add to this I put the thick ends of the teak at the bottom, where the extra weight also furthered my plan. Finally, the cross ties that carry the secondary rafters are as low down the primary rafters as possible (in terms of the acceptable span for the ceiling tablilla) to put less roof weight and material on the part of its structure that is within the walls. Any of this make sense? Hope so!”
Crystal clear to me, and very sound reasoning.
“I would say we ended up with maybe a hopeful 35% of the roof's weight outside of the walls? So nowhere near half, but still nicely counterbalances and eliminates a decent bit of that outward push.”
“Then we put down the 3/8 tongue and groove ceiling boards, the insulation, and the 2x2 nailers for the metal roofing. Notice the purlins for the metal are fastened to little 2x2 cleats instead of directly to the ceiling boards. This was to create a larger air space between the ceiling and the metal in order to move more air through the roof and out the top. With every step the roof became more and more sturdy, and once the metal went down I felt like we had totally pulled it off. It is one big monolithic hunka chunka that seems to not want to go anywhere.”
If I could say one thing about sensing intuitively whether your earthbag home is structurally sound, it would be that it should feel like one massive monolithic hunka chunka that has no intention of moving anywhere:))
“Many of the fasteners used in putting a house together are really just needed to hold things in place while you build, as once all is integrated things aren't going anywhere regardless of the fasteners," concludes Murat.
Yup. Once everything is locked in, the downward pressure of the roof, and the interlocking strength of the earthbag structure, make it incredibly strong.
Once again, many thanks to Murat Dirlik for his superb documenting of this roof construction. If you want a beautiful natural house built, you can connect with Murat on Facebook.
My 4 beginners’ ground rules for getting your earthbag roof right:
1. Is your roof structure pushing outwards or downwards? If it’s only downwards you have little to worry about and may, on small circular structures, get away without a bond beam. You always need a bond beam if you have straight walls.
2. If your roof pushes outwards as well as downwards, you need a bond beam, and possibly buttresses/rebar. In addition you need either trusses (to pull the roof back inwards and thus exert some pressure in the opposite direction), or counter-balancing roof weight with overhangs outside of the walls (as Murat did).
3. Your rafters need to be firmly anchored to your roof. You can do this by driving rebar into the bond beam (as Murat did), or by nailing the rafters to pillows and nailing or rebarring the pillows into your earthbags (as I did).
4. If you live in a place prone to high winds that could potentially lift off your roof (I’ve seen this happen, be warned), you can run ropes through your earthbag walls and cinch your rafters down.
For my beginners’ guide to roofs for earthbag houses, go here.
Do you enjoy The Mud Home?
If you find these articles inspiring or useful please consider joining us on Patreon. Your support pays for the running of this website, my virtual help, and my sustenance. A big thank you to all The Mud Sustainers, and everyone chipping in and keeping these posts and articles coming.
16/11/2018 01:19:54 pm
A quick calculation shows that with your wall diameter at about 71% of the roof diameter The area inside the walls will roughly equal that outside the walls. This should lead to the stresses going straight down through the walls.
18/11/2018 12:52:32 pm
Loved the detailed description. Looking for more of such Informative articles.
Atulya K Bingham
19/11/2018 08:35:30 pm
Have you signed up for the earthbag email series? http://www.themudhome.com/earthbag-pdf.html
Leave a Reply.
Many thanks to the Mud Sustainers supporting this site!
Do you find The Mud Home valuable? Please consider supporting the blog on Patreon. For as little as $2 a month (not even a coffee where I'm from), you can join the club.
BENEFITS FOR PATRONS INCLUDE:
Email priority, private Facebook group, review copies of my books, sneak previews of courses and books, Q and As, priority for courses and more.
Atulya K Bingham
"Beautifully written and inspiring." The Owner Builder Magazine
If you want the step by step guide of how I built my house, sign up for the PDF.
WHY NOT? IT'S FREE!
All the Mud Home How-to posts have been compiled into a PDF package with 75 articles and over 200 photos. You can still buy it now, and enjoy lifetime access to all the updates.
“Entranced! Be inspired by one who’s lived and breathed dirt.”
Kim Fraser, Get Rugged