low-cost multipurpose minibuilding made with earthbags

7/31/2019 Low-Cost Multipurpose Minibuilding Made With Earthbags

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Low-Cost Multipurpose Minibuilding Made With Earthbags

By Owen Geiger

One of the most practical structures on a small farmstead is a multipurpose garden structure that canserve as a storage shed or cool pantry above ground, or as a root cellar or storm shelter below

ground. You can build this multipurpose structure for about $300 using earthbag construction (bagsfilled with earth and stacked like bricks). And the skills you learn by building the dome will serve youwell if you plan to build a larger earthbag structure or even an earth home.In many cases, no building permit will be needed for this little building, because its below theminimum size required by most building codes (for structures that are not inhabited and not attachedto a residence). But because codes vary by region, check your local building regulations before youbegin.Earthbag structures provide a cool space in summer and an escape from the cold in winter, whichmeans this earthbag dome is well suited for many purposes. Depending on your needs, the mostpractical combination of uses might be a root cellar/cool pantry for daily use and a disaster shelter foremergencies such as tornadoes or hurricanes.The earthbag dome has a natural look and blends in with the land. It has a solid, organic feel just

what youd expect from a design inspired by nature that has been combined with a little moderningenuity and thousands of years of earth- and dome-building wisdom.

Building with Earthbags

No expensive forms or equipment are needed with earthbag building, and the technique is faster andeasier than other earth-building styles including cob, rammed-earth tire construction and adobe(if you have to make your own bricks). Earthbag buildings are more water resistant than those madewith straw bales, making them suitable for earth-bermed and below-grade structures. If your site issusceptible to flooding, earthbag building is one of the best options after all, sandbags have along history of use for flood control. Just use an appropriate fill material, such as gravel, in lower

courses.The cost of building with earthbags varies. You can almost build free if you take the time to scroungethe materials, (used poly bags and barbed wire, recycled wood, and local soil and gravel). Few toolsare required. Below is a middle of the road cost estimate that assumes youll buy the major itemsbut also take time to shop around. For example, if you dont have used polypropylene bags, you canalmost certainly find a local farmer who does.The most significant advantage to building with bags is the simplicity of the process. No special skillsare required: Fill bags with earth and tamp them solid. Building with bags requires a lot of physicaleffort, but brute strength is not necessary because each step can be done a little at a time at acomfortable pace.Earthbag domes are incredibly strong and hold up well in hurricanes, tornadoes and earthquakes.After being plastered or bermed with earth, they are virtually fireproof. A properly constructed

earthbag structure approaches the strength of stone, at a fraction of the cost, labor and necessaryskills.This earthbag shed is 8 feet in diameter inside (about 11 feet outside) and approximately 8 feet high.Plans could be scaled up to create 10- to 16-foot diameter domes. At the top of a larger dome, theearthbag thickness and cantilever (corbel distance) have to be adjusted slightly so its more conical.Any building project can be dangerous, so stay alert while working. Until all the bags are in place andlocked together, theres a risk that they might fall, causing damage or injury.

Earthbag Building Materials and Prices

The cost of building with earthbags varies. Here is a middle of the road cost estimate that assumesyoull buy the major items but also take time to shop around.

Material Quantity Cost (US$)

Polypropylene bags 500 $75

Road base 45 cubic yds. $75


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Gravel 3 tons $15

Four-point barbed wire 400 feet $25

Plaster mesh 17 sq. yds. $10

Lime plaster/sand 7 bags $35

Door, frame and hardware 1 $35

Nails 2 lbs. $5

6 mil plastic 2 12-by-32 ft. 768 sq. ft. $20

Bitumen sealer 1 quart $5

Total $300

(Backfill material not shown, because site conditions will vary.)

Free or Low-cost Items:

2 salvaged 28- to 30-inch tires Boards and plywood to support the tires 7 3-by-32-inch rot resistant wood poles 2 2-gallon heavy duty buckets 1 4-gallon plastic bucket #10 cans optional 1 pound 4d common nails 1 pound 1 1/2-inch galvanized roofing nails 2 string lines (one at 9 1/2 feet, one at 48 inches) 2 wood stakes with screw eyes 2-by-4-by-30-inch tamping board 2-inch-diameter-by-19-inch tamping rod (tree branch) Salvaged rebar for 28-inch pins Small amount of wire 12-inch square of galvanized window screen

Old fishing net Rot-resistant 1-inch saplings Sod (or native grass) Flower seeds and plants as desired

Tools

Tamper(s) Round-nose shovel(s) Hoe/digging tool 13-by-16-inch sheet-metal slider (or old cookie sheet) Knife Hammer Level 10-foot 2-by-4 leveling board Tape measure Fencing pliers Handsaw Trowel Garden hose with spray nozzle 6- or 8-foot stepladder

Basic Information

Interior diameter: 8 feetExterior diameter: approximately 11 feet


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sturdy hoe or similar digging tool to reduce shoveling. Even though shoveling is faster, its easier topull the material into a bucket than to lift the material with a shovel. On the last stroke, a quick flick ofthe hoe will tilt the bucket upright. To keep bags open when filling, use a bucket chute, a funnelmade by cutting the bottom inch off of a 4-gallon bucket.Placing bags: On a dome this size, about two to four courses can be laid each day, depending onthe strength, endurance and skills of the crew. Small, upper courses go a little faster. Pace yourself,take frequent breaks and allow ample time to study the plans. Just one mistake could cause a lot of

extra work.Tamping: We used two tampers: an agile 45-inch one for tamping bag contents during filling, and a55-inch one. Both can be used for tamping walls, but the long-handled version is easier to applymore force. Each one is made with a 6-by-6-by-3/8-inch plate-steel base, 3/8-inch angled steel ribs,and a 1 3/4-inch pipe to hold the wood handle, which is bolted on. You could also buy one for about$10.Leveling: Usually just a few bags per course required a little extra tamping. I attribute this tomeasuring the contents of each bag and placing them with a bit of care.Retaining walls: The walls in front of the dome prevent backfill and excess water from spilling intothe entrance area. They also serve as buttresses and seating. Rebar pins through the bags addreinforcement. You might want to vary the stepped design on the retaining wall slightly and round theplaster edges a little to create a more natural look. Consider using plastic plaster mesh because it

wont rust. Attach the mesh directly to the bags with 1 1/2-inch roofing nails. Plaster with two coats oflime plaster: one part lime to 2 1/2 or three parts of clean builders sand. After the plaster is dry, addplastic sheeting behind the retaining wall for added protection against moisture damage. Add anotherlayer of plastic after most of the backfill is complete. Use bitumen to create a waterproof sealbetween the plastic and the back side of the retaining wall.Arch form: We experimented with old tires to form the door arch, because the time and cost ofbuilding a curved form with plywood didnt make sense. We used wood supports under each side ofthe tires, which were braced side-to-side. This created an opening so we could check the curvaturewith the string line.Door arch: The semicircular Roman door arch is made with 12-inch-long bags filled from above andtamped with an improvised tamping rod (tree branch). Additional tamping from the sides with a 2-by-

4 tamping board creates the necessary taper. You can eyeball the taper, as we did, or set up a stringline.Door: After all the bags have been laid, make a door frame to fit the space. We used 1-by-2-inchrectangular steel tubing for the sides and bottom of the frame, and an 1/8-by-2-inch steel plate bentto an arch for the top. Try to find recycled wood for the door. A curved branch would make a nicerustic handle. We used a cabinet handle. Weld a hasp to the frame if you want a padlock. Make oneor more screened openings in the door for ventilation.Eyebrow (Awning): Use seven peeled, rot-resistant 32-inch wood poles, approximately 3 inches indiameter. Western juniper, a common wood for fence posts in the western United States, is anexcellent choice. Likewise, use 3/4- to 1-inch diameter peeled juniper branches to cover the eyebrow.Add plaster mesh and lime plaster on the top.

Options for Finishing the Earthbag Dome

Plaster the entire dome. This is the fastest, easiest option to protect the dome if all you need is atool shed or basic shelter. Mesh isnt required, although its recommended on the areas previouslymentioned.Create a living roof with backfill and plants. A living roof is especially appealing and worth theextra effort. This option is best approached as a labor of love because it is time-consuming. Smallterraces slow water runoff and make working on the dome much easier. We chose a drought-resistant sod thats used by the highway department, and getting it established was still trickybecause water runs off the steep areas quite rapidly. Our plan is to use the sod to anchor the soil,then gradually add lots of plants and flowers for a more natural look and the pleasure of harvestingfresh herbs.First, cover the entire dome with 6-mil black plastic by overlapping two 12-foot wide pieces. Addbackfill in layers at a 45-degree slope by walking on it and tamping lightly. After the backfill is nearly


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up to the desired level, cover the entire surface (including the backfill) with 6-mil black plastic. Add athird layer of plastic on the top, down to the eyebrow level.Next, add poly fishnet over the entire dome to give the soil and roots something to grab onto. Weused poly shade screen on the top, but we discovered fishnet works better and is less expensive.Starting at the bottom, add a 3- to 4-inch layer of soil on top of the fishnet. Randomly shoveling earthcould leave thin spots. Instead, work methodically to cover one area at a time. Tamp as you go tokeep soil in place. The top of the dome requires extra care. We made an 8-inch wall around the

sides and back of the dome with lots of stomping.The steep areas on either side of the eyebrow and the area near the top call for a different method.Use your hands to create a miniature earth wall with a flat ledge on top. A soil with high clay contentworks best in these areas. After final shaping and raking, add more fishnet to the top quarter of thedome to help hold the sod in place. Then add sod and/or plants. Youll need about 70 square yards.We used 2-inch-long pieces of split bamboo to hold the sod in position until the roots becameestablished. One sprinkler on the top will water the entire living roof if you have good water pressure.

Step-by-step Building Schedule

Day 1: Use temporary stakes to mark building location. Remove and stockpile topsoil (to cover the

dome later), then level the building site. Stockpile road base around the dome site in a continuous U-shaped pile about 4-feet high by 8-feet wide at the base.Day 2: Place a permanent stake to mark the center of the dome, and attach a string line to measure4 feet to where the inside edge of the first course (row) of bags will be. Measure and stake out wherethe retaining walls will go. Make a rubble trench foundation (trench filled with gravel) at least severalinches larger than the width of your bags, digging down to solid mineral soil (often a sand/gravel/clayconsistency). A rubble trench foundation eliminates the need to go below frost line.Make the rubble trench foundation under the retaining walls at the same time. Fill the trench with 4-to 6-inch layers of tamped 3/4-inch crushed aggregate until 6 inches below finish grade (the top ofwhere the soil was before you dug the foundation). A special mix of aggregates, 5/8-inch minus,also works well. Place the first course of gravel-filled earthbags, double-bagged for extra strength, inthe trench. It takes three 2-gallon buckets of gravel to fill a bag. To avoid folded bag ends protruding

from the wall, simply fold the ends toward the middle of the bag and then tuck them underneath astheyre placed. The top of the first course will be right at finish grade. Tamp the course after all bagsare in place. Then lay one strand of barbed wire down the center (two strands if the dome is morethan 14 feet in diameter), held in place with bricks or rocks. (You can see the barbed wire on the firstcourse of bags in the image gallery.)Overlap the ends of the barbed wire about 12 inches and fasten them together with tie wire. Repeatthe process for the second course, overlapping each joint and tamping each course after the bagsare in place. Starting with the second course, fill each bag on the wall on a sheet metal slider. Thisenables bags to be adjusted. Lift the bag slightly and remove the slider with a quick pull after the bagis in its final position. The first two courses of bags are complete circles (plus the retaining wallportions) bonded with barbed wire. Succeeding courses start at either side of the door opening.Looking down from above, bag ends abut each other at an angle. Its easy to create these angled

ends with a few whacks of a 2-by-4. Also, remember to check both string lines after each course hasbeen laid.Day 3: Lay the first three and a half courses of bags with road base. For our size bags (18-by-30inches), we used four buckets of road base per bag. The length of full bags with the end folded undershould be 18 inches. Lightly tamp the contents two to three times after adding each bucketful. Fullbags measure about 8 inches high and compact to about 4 to 5 inches high. Its easy to adjust thesize of bags by adding or subtracting one or two shovels of road base.For the first few courses, create the vertical dome curvature by lining up each course with the insideof the previous course. After tamping, the bags will expand and create a gradual curve toward thecenter of the dome.The retaining wall seating area is 15 inches above finish grade, which consists of one course ofgravel-filled bags above grade and two courses of road-base-filled bags. The retaining wall is fourbags long.Insert the first two steel door anchors between courses 3 and 4. Earthbags expand horizontally asthey are tamped and will throw off the door-opening size unless it is adequately braced. To prevent


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this problem, build a sturdy door form that supports the tires and includes braces to resist thehorizontal thrust. Use two 28-inch tires wired together to form the arch.Day 4: Lay the next two and a half courses (we had a short crew this day). Increase corbel size toabout 1 1/2 inches. Insert the second set of steel door anchors between courses 6 and 7. Eachcourse of the retaining wall steps in about 8 inches. This works out nicely because this is about thelength of half a bag, meaning each course on the top of the retaining wall is half a bag shorter thanthe previous one, which creates a stair stepped effect.

Day 5: Lay the next two courses, increasing the corbel to about 2 inches. Insert the third set of steeldoor anchors between courses 9 and 10. Be sure to align the anchors vertically so the door will beplumb. The bottom anchor aligns with the inside wall; the top anchor aligns with the exterior of thedome; and the middle anchor is in between.The top of the retaining wall is course 10, which is also the same level the door arch begins. Thedistance from the bottom of the door opening to the beginning of the arch is about 48 inches. Westood on plastic crates for courses 9 and 10 (or you can use ladders). We started filling bags bystanding on top of the wall with course 11. The first door-arch bag (12-inch tall tapered bags thatform the arch) starts at course 11. Fill arch bags from above, tamping the contents in place with a 2-inch round stick or equivalent.Arch bags taper towards the center of the tires. I just eyeballed the alignment by standing back a fewfeet, but you could create a more accurate angle with a string line from the center point. (For the

string line option, youll need to screw a piece of wood across the center of the tire in order to attachthe string line.)Connect the arch bags to the rest of the dome with barbed wire. Embed the first two eyebrow poleson top of course 11. They are placed approximately 29 inches on either side of the door opening,perpendicular to the door. The first two poles protrude about 9 inches. The next course of earthbagswill mold themselves over the poles, securely locking them in position.Day 6: Use a ladder while laying the next two courses. Continue adding arch bags around the tiresas you go. Embed the next two poles on top of course 12. They are placed 16 inches on either sideof the door opening, perpendicular to the door. The second two poles protrude about 13 inches.Note: Poles do not protrude into the interior of the dome.Day 7: Finish the arch, lay the next four courses, and add the final three poles. The arch is the most

difficult part of the project, so take extra care and allow ample time. Remove the tire form after layingtwo courses above the arch. Unless you plan to plaster the entire dome, you could start addingbackfill around the dome at this time, except next to the retaining walls, which get plastered later.Adding some backfill as we progressed on the dome worked well for us. Its safer (not so far to fall)and easier to hand up buckets. However, we only added about 3 feet at this point so we could wrapthe finished dome in plastic. Create a slope around the dome using 4- to 6-inch layers of backfill,tamping each layer some to reduce future settling.Day 8: Finish the top of the dome by laying the last five courses. As the courses become smaller inradius, angle bag ends more sharply. Use partial bags (two to three buckets per bag) on the top fewcourses. The top course is actually one bag formed into a rounded shape.Time to celebrate! But be extra careful working on top of the dome. As on any construction project,there is always a risk of injury. One could lose their balance working around barbed wire, bricks and

tools in a small space, especially if fatigued.Day 9: Build the door frame to fit the opening and install the door. Youll need a welder and metalcutter.Day 10: Nail juniper branches or equivalent on the poles to create the eyebrow. (The branchescreate decking to support mesh and plaster.) Add plaster mesh on both sides of the retaining wallsand around the door opening and threshold with galvanized roofing nails. On the back side of theretaining walls, extend the mesh about 48 inches onto the dome as a shield for water runoff from theeyebrow. Also add mesh on top of the eyebrow.Day 11: Plaster the eyebrow and retaining walls with lime plaster. At this point, you could plaster theentire dome if you dont want a living roof.Day 12: Sand and varnish the door. Wood stain is optional; however, in our case it added a desirablecontrast and rustic look.

Day 13: Apply the finishing coat of plaster to exposed areas (areas other than the back of theretaining walls) after the first coat has cured. Curing time will vary depending on temperature andweather conditions. We used a rough texture and rounded edges to create an aged, rustic look. Mixin your choice of stucco color.


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Day 14 onward: Add plastic as described above and finish adding backfill. Remember to addterraces to slow runoff.

Earthbag Building Tips

As with any building project, examine the site carefully. For example, dont build in a low areaprone to flooding.

For domes built into a hillside, dig a swale (drainage channel) above the structure to directrunoff. If possible, install a French drain to divert any excess water.

Moisture content of fill material (road base or local soil) should be around 20 percent. If itstoo dry, it requires excessive tamping. If its too wet, water will ooze out of the bags. Make aball in your hand. It should be moist enough to create a firm clump, but not leave a lot of mudon your hand. Test by dropping the ball from shoulder height. It shouldnt shatter (if it does,its too dry), but only crack or break into a few large pieces.

Pleated bags are less common, but also available. These bags produce a neater appearancebecause they dont have protruding (bulging) corners. But protruding corners arent aproblem just smack the corners with a hammer before plastering. Our dome is coveredwith earth, so this was not an issue at all. You also can pin protruding corners with nails,

which is what we did as the radius decreased on upper courses to help keep bags moreevenly aligned. Align bag ends with the center of the dome by whacking them with a 2-by-4 to obtain the

proper angle. We used our feet to do this on lower courses, but as you go higher, the radiusdecreases in size and youll need to create sharper, more accurate angles.

You could increase ventilation and daylight with sections of pipe placed between courses,sloped at a downward angle to keep water out and screened to block insects and other pests.

Vertical arc measurements (maintaining a 112-inch radius) are a little tricky. It takes sometrial and error to discover how much the bags will expand horizontally after tamping. Weincreased the corbel size every few courses from about 1 inch on lower courses until itreached about 5 inches on upper courses.

Be creative and look around for salvaged materials. We found cheap hardwood furniture

spindles for the eyebrow poles. We used a metal chisel and steel plate to cut barbed wire. Its quick and easy. Retaining walls: They flare out to create extra clearance in front. Add three 1/2-by-28-inch

rebar pins on each side of the door, driven in at an angle, to reinforce the junction betweenthe dome and retaining wall. In addition, add about seven rebar pins of the same sizevertically in each retaining wall for extra reinforcement. If you choose to backfill around thedome, dont add soil behind the retaining walls until the plaster has cured. After the plaster isdry, add plastic sheeting behind the retaining wall for added protection against moisturedamage. Bitumen works well for sealing the plastic to the retaining wall. It works as anadhesive and hardens in a few minutes.

Build wood supports under the tires that form the top of the door arch. Use 2-by-4s sheathedwith plywood. Then add 2-by-4 braces between them, top and bottom, to keep the sides

parallel. Door: Door anchors are made with pieces of scrap metal welded together in a T shape. The

long part of the T sits flat between the bags. Improvise some spikes out of 1/8-inch steel rodsto hold the anchors in place. The doorstop (what the door closes against) is an 1/8-inch steelrod welded to the frame. Use scrap plywood to shield the bag walls when welding the frameto the door anchors. Note: some steel shims will likely be needed between the door frameand anchors. Check the door opening for alignment as soon as the arch form is removed. Anyproblem bulges around the door opening can be tamped into submission (moisten slightly ifnecessary) and the slack taken out of the bag with a nail.

Owen Geiger is the co-developer ofEarthbagbuilding.com, a site that promotes earthbag building, andDirector of the Geiger Research Institute of Sustainable Building.
http://www.earthbagbuilding.com/http://www.earthbagbuilding.com/


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Brute strength is not required to build with earthbags. Heres a tip to make it even easier: Fill

buckets using a hoe (instead of a shovel).

Use earthbags to make a studio, garden shed, chicken coop or root/storm cellar no building

permit required!


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The earthbag dome is built on a rubble foundation.

The footprint for the earthbag dome is an 11-foot circle, plus retaining walls.


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Earthbag construction works well for building curves. Its also fireproof when plastered or bermed

with earth.

A string tied to a stake will help as a guide to maintain the correct curve of the dome as you stack

bags.


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Posts held between layers of earthbags support a structure to protect the door. Mesh on the

retaining walls will hold plaster securely in place.

The retaining walls in front of the dome prevent backfill and excess water from spilling into the

entrance area.


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Front elevation of the earthbag minibuilding.

Illustrations of above-ground, earth-bermed, recessed and underground versions.

low-cost multipurpose minibuilding made with earthbags

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