Earthbag domes and arches replace roof timber with curved masonry-style construction: bag courses step inward until they close at the top, and every part of the curve is in compression, which earth handles well. Small domes of 2-3 m diameter are inherently stable, while domes of 4 m and more need engineering knowledge or a carefully chosen profile.
Part of Earthbag Building
Domes and arches are curved forms that carry load by compression, built from earthbag courses laid in a shrinking circle or over a temporary form.
A Curve Turns Weight Into Squeezing
Packed earth is strong when squeezed and weak when pulled. A dome or arch is shaped so that the weight of the structure presses its parts together along the curve, which is why earthbag buildings can dispense with roof beams. The structure pushes outward at its base, and managing that outward push is the main design task; see Dome Building for the general principle and Arch Construction for arches.
What You Need
- A circular footing on a foundation like the one in Foundation and First Courses.
- A way to set out the curve: a stake and cord for a dome, a temporary form for an arch.
- Enough base thickness or buttressing for the outward push.
Method: Choose The Form And Size
Step 1: Choose the dome diameter. Domes of 2-3 m are inherently stable. Domes of 4 m or more require engineering knowledge or a shallow profile (height less than half the diameter), as the parent article puts it. The step arithmetic in Corbelled Dome Technique shows that flattening the profile makes corbelling harder, not easier.
Step 2: Learn the courses. Each course is shifted slightly inward; see Corbelled Dome Technique.
Step 3: Set out the curve with a cord and stake; see Compass and Form Guides.
Step 4: Add buttressing or extra base thickness for domes larger than 3 m diameter; see Buttressing and Reinforcement.
Step 5: Use pointed arches over openings. Gothic (pointed) arches are stronger than round arches in earthbag construction.
Common Mistakes
- Building large before building small. Practise on a small dome first.
- Ignoring the outward push at the base. The dome spreads.
- Removing arch forms early.
Safety
A partly built dome can fall
Until a dome closes, its upper courses are held only by friction and wire. Brace overhanging courses and keep people out from under them.
What’s Next
- Learn next: Corbelled Dome Technique — how the courses step inward
- Combine with: Catenary Arch — a method for finding a pure-compression curve