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The type of roof you choose for an earthbag house is important. No matter whether you construct a circular house or a rectangular one, you want to take into account your roof before you build (or be prepared to alter your roof style at the end).
I won’t deny it. When it came to my earthbag roof, as is so often the case with me, I skidded to success by the seat of my pants. It was sheer luck I chose the right type.
Roofing structures exert a lot of pressure onto earthbag walls. This is fine if the pressure is in the right direction because earthbag walls are strong. What you want is the pressure exerted downwards. What you don’t want is it pushing outwards.
The safest roof type for earthbag
If you are a beginner, the safest, most foolproof roof to stick on your earthbag house is a flat or skillion (pitched) roof, utilising horizontal joists running the length of the building. The joists act as a type of grid that pulls the structure together and prevents the walls falling outwards. What’s more, if you’ve built a smallish round house, you won’t even need a bond beam! You could top the joists with a number of materials: Corrugated iron, ply/strand board with a membrane. Any weight on the roof will be distributed by the joists and easily supported by the earthbag walls. The pressure exerted by such a roof is downward.
Steep A-frame roofs will probably need trusses and a bond beam. The trusses pull the roof structure back inwards, so your earthbag walls are not bearing the brunt of the outward pressure.
Here’s nice example of an open gable roof on an earthbag house without trusses, with a wooden bond beam.
The Independent Roof
Another safe roofing option is to create a roof that stands independently of the earthbag structure, as Gautam and Kim did with theirs in India. You might still want to run a few joists across the top of your earthbag house here to lock it together though.
So you want a reciprocal roof?
I don’t blame you, reciprocal roofs are beautiful. But…depending on how they are designed, reciprocal roofs can exert outward pressure on your walls, especially if you're using hefty posts. This doesn’t mean you can never put a reciprocal roof on a round earthbag house, however you need to be (or have access to) a reasonably decent carpenter to do it. If you are adding such a roof to a roundhouse, you will need a decent bond beam, and possibly even some buttresses (depending on the size of your roundhouse). Mustafa the carpenter made this gorgeous one in our workshop in Olympos, but there was some very precise measuring and angling of the beams to ensure the roof wasn’t pushing the walls out.
The link below shows another example of an alternative reciprocal roof on an earthbag house. Notice how this team combined two styles here. While the main structure is reciprocal, a web of rafters has been inserted into each facet of the roof. Those rafters then form the all-important grid through the top of the earthbag wall which locks the whole structure together.
Why you don’t want a compression roof
Compression roofs are typically used for yurts. The design of a compression roof means the strength of the roof structure is derived from the compression between an outer ring or wall, and an inner ring. Such a roof is exerting a lot of outward pressure on your earthbag walls.
If you know what you’re doing, there are of course a whole gamut of other wacky and exciting options for earthbag house roofs, and you'll be able to customize many to work for earthbag. But presumably, if you know all about roof construction, you're not reading this post.
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