Introduction: How to Build an Earthbag Roundhouse
Note: If you’re new to earthbag building, please read the introductory Step-by-Step Earthbag Building Instructable first. Also, my new Earthbag Building Guide and Earthbag Building DVD are now available. Stay up-to-date on all the latest earthbag news by following our Natural Building Blog.
We built this earthbag roundhouse in 2010 as part of an earthbag workshop in Thailand, and finished it later that summer. Roundhouses are perhaps the simplest, fastest, easiest earthbag structure to build. We’re extremely pleased with the results, especially in terms of strength and cost. This is one of the strongest structures I’ve ever worked on in my 30-plus year construction career. The main impression is one of incredible fortresslike strength - massive walls with no sway. I’m sure it could easily withstand a direct hit by a speeding vehicle. This is no exaggeration. There’s been at least one incident where a drunk driver hit an earthbag wall and only chipped the plaster. (The vehicle was totaled.) Earthbags also excel at withstanding floods, hurricanes, tornadoes and earthquakes. Engineered plans are now available for whatever conditions you face. Earthbags are even bullet resistant, as explained in our highly popular blog post where compressed earth withstood 50 cal “BMG” 661 grain Full Metal Jacket rounds. Bullet Resistance of Compressed Earth
The other key advantage of earthbag is cost. For our roundhouse, we wanted things to look nice, of course, but we didn't want to spend a fortune. The final cost came out to $11.50/square foot. Most stick-built houses are $100/sq. ft. and up, so this roundhouse demonstrates how anyone can build their own home even on a very tight budget. We used a few basic, low cost methods to class up the roundhouse: rounded window and door openings (free), nice colors (no extra cost), curved bathroom wall and buttress (no extra cost to create curves), exposed wood and thatch roof (dirt cheap), earthen plaster on the interior (really dirt cheap), and lots of beautiful old windows for views, ventilation and to add a sense of spaciousness. In summary, build small -- just what you need, use simple shapes, pay with cash, and add on later if needed.
Basic project information:
18’ exterior diameter; 15’ interior diameter; 177 sq. ft. interior floor space; total cost of materials: $2,045, which is about $11.50/square foot
The following instructions assume you have cleared and leveled the site, removed topsoil, positioned fill soil around the building site to minimize work, dug a trench to stable subsoil, buried any utilities, put about 12” of gravel in the trench, and added a center pole with stringline to measure the radius. Bags or tubes can be used. We demonstrate bags, because they’re often available recycled for very low cost. My YouTube Channel has short video clips that show each step of construction.
Step 1: Earthbag Foundation
Earthbag foundations – gravel-filled bags or stabilized bags – offer many advantages over reinforced concrete foundations and work well with many types of sustainable buildings. In particular, they are low-cost, fast and easy to build, require no cement (a major expense and cause of global climate change), and require no forms or expensive equipment. In cold climates you can use lava rock or pumice to create an insulated foundation. This one simple step can save you thousands of dollars over building with concrete, and cut your energy costs.
Typical earthbag foundations are made with poly bags (double-bagged for strength) filled with gravel. Aggregates are preferred for foundations because they will readily drain away any moisture and prevent wicking into the wall system. Some prefer to use stabilized soil in earthbag foundations, seeing it as a longer lasting solution. The jury is still out, but it appears poly bags kept out of sunlight can last hundreds of years, so gravel-filled bags should last at least a lifetime.
Simply fill the bags in place with gravel. Stitch the ends closed or fold the bag end over. Butt each bag tight against the previous bag. Tamp the bags flat after each course is complete. Add two strands of 4-point barbed wire between each course. Add courses of gravel-filled bags until you’re at least 6” above the risk of moisture damage.
Step 2: Door Thresholds and Door Bucks
You’ll want durable thresholds at entry doors. I recommend concrete or stone for this purpose. In this method, finished floor height (F.F.) is the same height as the threshold. In other words, the threshold and floor will be exactly the same height. To build the threshold this way, pour a 2”-3” concrete threshold, sloped slightly for drainage, on top of the earthbags at each entry door location,. You can add concrete pigment to make it any color you want. Allow adequate time for drying (about one week, depending on the temperature), and then set the door buck on the threshold and brace securely in position.
Step 3: Stacking Soil-filled Bags
At this point you can continue stacking courses of bags for the remainder of the walls, but now the bags are filled with soil. (Although there are numerous alternative fill materials. See our Earthbag Building Blog for full details.) Most subsoil is suitable for earthbag construction. Subsoil is the clay/aggregate soil that’s below the topsoil. You could dig your own, but you can buy it very inexpensively and save many days of hard work. You can also buy an engineered mix called road base, which contains the ideal ratio of clay to aggregates, no mixing required.
Step 4: Measuring the Radius
Measure the radius of each course of bags to maintain an accurate circle. Tie a piece of rope to the center pole and use it to check the radius to every bag. Always keep the rope level for accurate results.
Step 5: Tamping and Leveling
Tamp earthbags solid and level after each course is complete. Tamp the high points first. Then evenly tamp the entire wall several times as you continually move the tamper. This last step avoids creating low spots. And if you use the same number of buckets of soil in each bag, and are careful placing the bags, the courses are almost self-leveling. Consistency is the key. Use the same process for each bag. Remember to place two strands of 4-point barbed wire between every course.
Step 6: Metal or Wood Anchors
Add metal or wood anchors every few courses to secure door and window bucks to the earthbags. Most people use wood anchors. But in areas with termites, it’s best to use anchors made of galvanized sheet metal. They’re inexpensive, strong, easy to make from scrap metal, resistant to decay and, because they’re thin, they don’t take up space between bags like wood anchors do. In most cases, sheet metal anchors are preferred over wood anchors. Simply nail into the earthbags with galvanized nails and screw into the backside of the bucks.
Step 7: Anchors for Electrical Boxes
Most earthbag builders use some type of improvised wood or plywood anchor to attach electrical boxes. I’ve found the simplest solution to be 3” wood poles. A rot resistant variety is best. Embed the poles between courses wherever you want an outlet or switch. If you forget or want to add extra boxes, sharpen the end of the pole and drive it into the wall with a small sledgehammer before the soil gets too hard. Use an electrical box as a depth gauge. You want the front of the box to protrude about 1-1/4” beyond the earthbags to match the plaster. After the walls are built, screw boxes to the stakes with two screws and staple or nail the wiring in the recesses between bags. Complete all the rough electrical work before plastering.
Step 8: Window Bucks
Set window bucks on the wall when you reach windowsill height. You can pre-build the bucks while your threshold is drying. Use standard carpentry techniques to brace the bucks square, and to keep the buck plumb and level. We use double 2x4 bucks instead of wide lumber, and we use poles for braces instead of milled lumber to cut costs. It’s often a good idea to pound ½” rebar down through the bags on either side of door and window openings if you have lots of openings like we do.
Step 9: Bond Beam
It’s easiest to build a bond beam immediately above windows and doors so you don’t have to build lintels. This method saves hundreds of dollars and one whole step. Reinforced concrete or wood bond beams are both acceptable. You’ll want to read up on these building steps since the subject is too involved to summarize in a few sentences. We used concrete due to the prevalence of termites in our area. We used two layers of ¼” plywood as forms, ½”rebar pins pounded in the tops of walls, and ½” horizontal rebar. Pour the concrete nonstop so you don’t have any joints. Embed hurricane ties in the concrete for securing roof poles. Allow to dry several days before stripping the forms and building the roof. Any voids in the concrete can be patched when you plaster.
Step 10: Build the Roof
Build the roof after the bond beam is finished. Any type of roof is possible, and space doesn’t allow a lengthy discussion. We used a metal compression ring with radiating wood poles covered in thatch. Create large roof overhangs of approximately 30”-36” to protect walls from moisture damage. The exact size will depend on your climate.
The roundhouse has a super strong feeling due to the concrete bond beam, thick walls, sturdy poles and round shape. You could probably have a dozen workers on the roof with no visible effect. The roof frame went together almost like clockwork. The compression ring worked perfectly. The roof was all bolted together in about 4 hours, in part because the poles were precut and prefinished in advance.
Thatching is usually very slow and laborious, but we thatched our roundhouse in just one day and for only $100 using pre-made thatch panels. That's why I love thatch panels - they're very inexpensive and all the tedious work has already been done. And they work great on round (conical) and organic shaped roofs because they're flexible. Just bend them into position and nail in place. We installed the panels using 8" spacing. Plan on rethatching every 3-5 years or so, depending on the quality of your thatch. Some thatch will last 10 years or even longer.
In cold climates you’ll want a thick roof with lots of insulation. This can be achieved with trusses or TJIs (engineered truss joists). Wide dimension lumber could also be used, however, this requires cutting old growth forest.
Step 11: Install Doors and Windows
Use standard carpentry techniques to install doors and windows. This boils down to simply screwing the door and window frames to the bucks so they are plumb and level. Add shims as necessary. Remove the doors after they are fitted to prevent damage. Mask off windows with plastic to reduce cleanup later.
Step 12: Cement Plaster on Exterior Walls
Here's a brief summary of the cement plastering process we used on the exterior of our earthbag roundhouse:
Step 1: Put cement between the bags in the recesses.
Step 2: Add thin coat of cement over bags.
Step 3: Add more cement.
Step 4: Add more cement.
Step 5: Trowel the plaster smooth and use a sponge float to smooth out the finish.
As you can see, the process is quite simple, although labor intensive. It boils down to adding a little plaster at a time. Leave each coat rough so the next coat will better adhere. No plaster mesh is needed - the plaster sticks to the earthbags, no problem. Resist the temptation to overwork the plaster or you’ll lose adhesion to the bags. The total process took about five days of work for two workers spread out over a little more than week. A third worker helped the last day. Use best practices in your area. Always prevent plaster from freezing. Keep plaster moist and allow to dry slowly for best results.
Step 13: Earthen Plaster on Interior Walls
Earthen plaster is lowest cost, easiest type of plaster to work with. Many people find earthen plaster very enjoyable work. They’re often pleasantly surprised at how incredibly simple it is. After all, the main process is basically smearing mud on the walls. Follow one of the free recipes on the Internet. Experiment with small patches to help determine the optimum mix for your materials. Add more sand or less water if the plaster cracks. Add more clay if the plaster doesn’t stick to the walls.
The first step is to fill the recesses between bags and apply plaster around doors and windows. Plaster mesh is recommended around all windows and doors to help prevent cracking. You can apply earthen plaster with your hands or with a hawk and trowel. Allow each coat to thoroughly dry before adding additional coats. Leave each coat rough so the next one will more readily bond. Add more coats of plaster until your walls are finished. Smooth the final coat until you’re satisfied with the results.
2nd photo: Here you can see the natural beauty of earthen plaster that's been sculpted to gracefully curve around window and door openings. This not only looks good, but also lets in more light and improves the view. I want to emphasize how this simple concept totally transformed our roundhouse from a bunch of bags into a thing of beauty. Click here for more details about rounded corners.
Step 14: Finishing Details
At this point you can finish your electrical and install plumbing fixtures. You can also install doors, trim, shelving and cabinets, and then stain and varnish any woodwork, and paint your home. But plastered earthbag walls don’t require painting or trim. Many builders plaster right up to windows, doors and floors. This is another major time and cost saving advantage of building with bags.
Step 15: Interior View
Finishing details turn a house into a home. It's what people really notice. If you want a really nice home, be sure to plan accordingly and give this some extra thought. Finishing work does require extra time and effort, but the results are definitely worthwhile.
Interior view showing natural color of earthen plaster, recycled windows and yellow concrete floor. For the floor, we troweled natural iron oxide pigment on the surface before the concrete set up. The pigment is sprinkled by hand and troweled into the top ¼" as the concrete sets up. A little more color was added later during final troweling to even out the color. We were careful to moisten the sand and gravel under the floor to slow drying and prevent cracking.
Step 16: Exterior View
Step 17: Conclusion
It boggles the mind how flimsy stick-built houses meet code and yet some people have doubts about earthbag building. (And even more hilarious is how trailer houses are somehow considered safe and adequate.) Let's briefly compare stick frame and earthbag houses: A strong person could readily kick their way through pressed board siding and sheetrock in about one or two minutes. The same person would probably be struggling an hour later trying to penetrate an earthbag wall, even with a sledge hammer, pick, shovel and crow bar.
What type of house would you rather live in? One that is thick, quiet, energy efficient, non-flammable, non-toxic and dirt cheap? Or one made of sticks and toxic materials that burn like crazy, offgass toxic fumes, leak energy and cost a small fortune. Earthbag is inexpensive enough that you don’t need a mortgage. Instead of spending 30 years paying off a mortgage, build super inexpensively and have far more time to spend with your family and friends, and help build community. This also lessens the influence of big banks, which are wrecking society with their greed.
Remember, over 70% of Americans can’t afford a contractor built home built to code, and that was before the current housing crisis. And since earthbag building is far faster and less labor intensive than adobe, cob and rammed earth, it’s one of the best of all possible sustainable housing options.
And as a final word, I want to say anyone can do this. It’s as basic as filling and tamping bags of dirt and covering with plaster. Just take some time to do the necessary background research and maybe start out by building a small storage shed to refine your techniques. Almost everything you need to know is free on our websites.