Earthbag Building
Earthbag construction (also called superadobe) uses bags or tubes filled with earth, stacked like giant bricks, with barbed wire between courses for tensile reinforcement. It produces incredibly strong, earthquake-resistant, bullet-resistant structures using the most abundant building material on the planet: dirt.
The technique was developed by architect Nader Khalili and has been used for emergency shelters, permanent homes, military bunkers, and even structures approved by building codes. An earthbag building can be constructed by unskilled laborers with minimal tools.
Materials
Bags
The standard material is polypropylene bags (woven plastic grain bags, sandbags, or feed bags). These are ubiquitous in agricultural and industrial areas.
- Grain bags (50kg size): ~45×75cm when empty. Most common, easiest to source
- Continuous tubing (superadobe): Tubular polypropylene mesh, laid continuously rather than in individual bags. Faster but harder to source post-collapse
- Burlap/hessian bags: Natural fiber alternative. Biodegradable, which is actually fine—the bags are only needed during construction. Once the earth sets and plaster is applied, the bags can rot away and the wall stands on its own
UV warning: Polypropylene bags degrade rapidly in sunlight (weeks to months). Walls must be plastered or covered before the bags deteriorate. Unplastered earthbag walls left in sun will fail within 1-2 years.
Fill Material
The ideal fill is a mix of approximately 70% sand/gravel and 30% clay (a “sandy clay” or “clayey sand”). This ratio produces bags that compact firmly and set hard.
Testing your soil:
- Jar test — shake soil and water in a jar, let settle 24 hours. Observe sand/silt/clay layers
- Ball test — wet a handful and squeeze into a ball. If it holds its shape but crumbles when poked, you’re close to the right mix
- Ribbon test — push wet soil between thumb and finger. A ribbon 3-5cm long before breaking indicates good clay content for earthbag
Too sandy (ribbon breaks immediately): Add clay or cement (5-10% by volume). Too clayey (ribbon exceeds 8cm): Add sand or gravel.
The first 2-3 courses should be filled with gravel only (no clay/soil) to prevent moisture wicking into upper courses. This acts as your stem wall and moisture break.
Barbed Wire
Two strands of 4-point barbed wire are laid between every course. This serves the same function as rebar in concrete—it provides tensile strength that earth alone lacks. Without barbed wire, earthbag walls can slide apart under lateral loads (wind, earthquake).
Salvage barbed wire from fence lines, farms, and agricultural supply stores. If unavailable, alternatives include:
- Thorny branches (blackberry, hawthorn) crushed flat
- Wire mesh or chicken wire strips
- Sharpened bamboo strips woven between courses
Foundation
Earthbag buildings are heavy. A single-story 5×5m building with 40cm-thick walls weighs 20-30 tonnes. The foundation must distribute this weight.
Rubble trench foundation:
- Dig trench to below frost depth, 50-60cm wide (wider than wall bags)
- Lay perforated drain pipe at bottom, sloped to outlet
- Fill with compacted gravel/rubble in 15cm lifts
- Top with 2-3 courses of gravel-filled bags — these serve as the stem wall and moisture break
Wall Construction
Bag Filling
Set up a filling station: a form (bucket with bottom cut out, or short pipe section) holds the bag open. One person holds the bag, another scoops fill material in. Fill to about 80% of bag volume—the bag must be closable and not overstuffed.
Close bags by folding the top over and placing the fold down against the previous course. The weight of subsequent courses locks the fold closed.
Alternatively, for continuous work, don’t close bags at all—fold the open end under the next bag in the same course. A continuous row of overlapping bags acts almost like tube construction.
Placement & Tamping
Place the filled bag on the wall, fold side down. Tamp vigorously with a hand tamper (a flat plate on a handle—a piece of plywood nailed to a broom handle works). Tamp until the bag is flat-topped, approximately 10-12cm high and the width spreads to fill the wall thickness.
Each course should be level. Check with a string line stretched between corner points. Shim or tamp individual bags as needed.
Running bond: Offset each course by half a bag length, just like bricks. Vertical joints should never align between courses.
Barbed Wire
After tamping each course flat and level, lay two strands of barbed wire along the full length of the wall before placing the next course. The barbs grip into the bags above and below, locking courses together.
At corners, bend the wire around the corner and extend at least 50cm along each wall face. This ties the corners together structurally.
Openings
Frame doors and windows with timber box frames as the wall goes up. An arch form (a temporary wooden semicircle) supports the bags above openings until the wall is complete and the arch is self-supporting. Gothic (pointed) arches are stronger than round arches for earthbag construction.
Domes
Earthbag domes eliminate the need for timber roof framing entirely—the bag courses simply continue inward, each course corbelled (shifted slightly inward) until they close at the top.
The compass method:
- Drive a stake at the dome’s center point
- Attach a cord to the stake, length equal to the desired dome radius
- Attach a plumb bob or level to the cord’s end
- For each course, shorten the cord by the bag width minus the corbel offset (typically 1.5-3cm per course)
- The cord guides exact bag placement, ensuring a smooth curve
Maximum corbel: No more than 1/3 of the bag width per course. More than this risks collapse before the dome closes.
Buttressing: For domes larger than 3m diameter, external buttresses or additional thick courses at the base prevent the dome from spreading. Earth-berming the lower third of a dome provides excellent buttressing plus thermal mass.
Small domes (2-3m diameter) are inherently stable. Large domes (4m+) require engineering knowledge or should use a shallow dome profile (height less than half the diameter).
Plastering
Plaster serves three critical functions:
- UV protection — prevents bag degradation
- Weather resistance — keeps rain out of the earth fill
- Structural integration — binds all bags into a monolithic wall
Interior: Earth plaster (same mix as straw-bale-construction interior plaster). Apply in three coats.
Exterior: Lime plaster or cement-lime plaster. In regions with minimal rain, earth plaster with a high proportion of cement or lime stabilizer can work externally.
Application to earthbag walls: Earthbag walls have a rougher texture than straw bales, which gives plaster excellent grip. The concave spaces between bag courses act like built-in scratch-coat keys. Apply plaster directly to the bags—no mesh is usually necessary.
Structural Performance
- Compressive strength: Earthbag walls have been tested to support 3-4 stories of load
- Earthquake resistance: Domes are inherently earthquake-resistant. Straight walls with barbed wire reinforcement have survived simulated seismic events
- Bullet resistance: A 40cm earthbag wall stops most small arms fire. Military applications use earthbags extensively
- Flood resistance: Unlike most natural building methods, earthbag walls below the water line survive brief flooding if allowed to dry afterward
Common Mistakes
- No barbed wire — the single most critical error. Without it, walls have minimal tensile strength
- Overfilling bags — overstuffed bags don’t tamp flat and create unstable courses
- No gravel base courses — moisture wicks into earth-filled bags at ground level
- Delaying plaster — UV degrades bags within weeks. Plan plaster immediately after wall completion
- Too-steep corbel on domes — if a course overhangs more than 1/3 of bag width, the dome may collapse during construction