Rammed Earth Construction
Categories: Building Methods
When you build a sand castle, you pack dirt into buckets that will form the sand castle's structure. You might even have fancy buckets for creating turrets, brickwork and domes. Then comes the hard part -- flipping the bucket and removing the form so that what remains is a structure fit for a king, or at the very least, a sand crab.
Rammed earth homes are like building sand castles without that tricky step of flipping the bucket. Instead, the form of the home, which is usually a plywood structure that provides the outline of a wall, is already in place. A cross-grade, or mix, of soils is rammed into the walls, either by hand or machine. When everything is packed tightly, the forms are removed, and what's left is a solid, stable wall. Builders ram and repeat until the entire house is built.
This method of building has a long history. Parts of the Great Wall of China were constructed using the technique and are still standing more than 2,000 years later. Examples of rammed earth buildings are all over Europe. French and German immigrants brought this style of building with them to the United States, and now churches, homes and other buildings made from rammed earth can be found from New York to Florida.
After the U.S. Department of Agriculture issued instructions for how to build a rammed earth home in 1926, they became popular with poor farmers during the Depression. The method faded away after World War II, but in the 1970s, builders in the western United States resurrected rammed earth. These homes increasingly can be found in Arizona, New Mexico, California and Colorado [source: Fatsis]. Rammed earth construction also is extremely popular in Australia, where wood for construction is limited and expensive. About 20 percent of new homes in the western part of the country are constructed this way [source:Kennedy].
But what are the benefits of living in a rammed earth home? Are they subject to the same dangers as a sand castle on a beach -- water, erosion and time? How do you build one?
Sedimentary rocks are formed over thousands of years, as layers of soil are carried to a settlement bed by water and compacted by each subsequent layer of sediment. Building a rammed earth home is like creating sedimentary rock but very, very quickly. The number of rammed earth construction crews is growing, but a house can also be completed by an owner without professional help.
In the days of building the Great Wall of China, people just rammed some earth and called it a day. Walls either stood the test of time, or they didn't. These days, we know more about the properties of soils. Rammed earth requires a cross-grade of soils, but the mix shouldn't have too much clay; excess clay will cause the walls to shrink and crack.
Can you go outside and just start digging? Well, you might want to create a test sample and take it to a geotechnical lab first. You can find a geotechnical lab by contacting agriculture departments at local universities or by searching for geotechnical engineering services in the phone book. The lab can test the sample's compressive strength. The sample should be able to withstand pressure of 300 pounds per square inch (PSI); this is the standard in earthen material building codes [source: Branch].
Rammed earth builder Quentin Branch estimates that he uses natural soil from the building site about 50 percent of the time [source: Branch]. Rock quarries also sell reject fill for a few dollars per ton. The mixture should also contain water, but no more than about 10 percent [source: McMeekin].
While earthen mixtures can be mixed by hand, it's more common to use a tractor or a tiller. Once the earth is mixed, however, it needs to go straight into the walls before it hardens and dries. For this reason, rammed earth construction is done on-site. Before mixing, builders need to assemble a set of wooden forms. These are like the buckets of sand castle construction, except they're rectangular plywood forms rather than plastic buckets. Two end pieces are the width of the wall, two other pieces are the length of each wall segment, and everything is held together by pipe clamps that can be loosened or tightened as the form is moved.
Next, about 4 inches to 6 inches (10 cm to 15 cm) of soil is shoveled into the form at a time [source:Easton]. Ramming can either be done by hand or by machine. A hand rammer should be about 15 pounds (6.8 kg), and it's usually a piece of steel attached to a pole [source: Easton]. Professional rammed earth builders use machines that are hooked up to air compressors. The air pressure does the work of tamping down the earth. When you first start ramming dirt, it makes a dull sound, but when it's adequately tamped, it will change to a ringing sound. Once the earth is ringing, then you can add the next 4 to 6 inches of soil. It is possible to ram too much, which will cause stress on the wooden form.
Once a section of wall has been constructed, the form is dismantled and moved to the next section, and the process begins again. Windows and doors are installed by ramming earth around separate forms that serve as placeholders. Once the earth has been cured (or set) around the placeholder, the form is removed and the earth will hold without it.
On the next page, we'll look at some of the benefits of building this way.
Benefits of Rammed Earth Construction
When you approach a rammed earth home, you might notice faint lines on the outside. These lines show where each level of earth was rammed. You also might notice that the exterior isn't all one color. It wouldn't be jarringly different, but there might be some slightly lighter or darker places on the wall. The owner could have covered up this imperfect wall with stucco or tile, but most rammed earth owners seem to prefer the unusual look of the house.
When you step inside the house, you'll probably note how thick the walls are; rammed earth walls usually run between 18 inches and 24 inches (46 cm and 61 cm) [source: Easton]. The thick walls add to the home's general feeling of quiet, warmth and comfort. One homeowner compared entering her rammed earth walls to a hug from a loved one [source: Lund]. But these walls provide many other benefits as well.
For one, rammed earth walls contain excellent thermal mass. If you're visiting a rammed earth home in summer, you'll find the home to be nice and cool during the daytime, without the aid of an air conditioner or fan. If you're staying overnight, the home will begin to warm up as it cools down outside. This is due to thethermal flywheel effect. The walls hold in warmth and exude it about 12 hours later.
It's not entirely the walls' doing. Rammed earth home design should factor in the natural elements that will affect the warming and cooling properties of the home. Passive solar design takes into account the sun'sdifferent positions throughout the year. For example, in the winter, southern-facing windows welcome the sun, while overhangs will shade these windows in the summer [source: Branch]. When done right, a rammed earth home will use only one-third as much energy as a conventional home, saving on energy bills [source: Whipple]. In cold climates, insulation can be added to rammed earth walls to improve their warmth.
The thick walls of rammed earth homes are also extremely fire-resistant because there are no flammable components in the earth. In addition, everything has been packed so tightly that there's little chance of combustion. InAustralia, rammed earth walls have achieved the highest fire ranking available, withstanding a fire for four hours [source: Sirewall]. The walls are also rodent-resistant because they don't offer any food sources for insects or other vermin.
When completed, rammed earth walls can be left just as they are, or they can be finished with plasters, paints or siding. If left unfinished, the earth provides a natural, breathable wall, in comparison to artificial sidings with chemicals.
Other than the walls, you might not even be able to tell you're in a rammed earth home because in most other ways, it resembles a conventionally built house. The foundation, roof, electricity, plumbing and decorative features are installed in the same way as other houses.
On the next page, we'll take a look at the disadvantages of rammed earth construction.
Challenges of Rammed Earth Construction
Mixing soils, building wall forms, ramming earth inches at a time…as you might guess, one of the main challenges of rammed earth construction is that it's very labor-intensive. In African and Middle Eastern countries, where labor is cheap, rammed earth construction is common and practical. In the United States, however, crews doing the same work can charge a pretty penny. Rammed earth builder David Easton estimates that a wall system in a rammed earth home can cost about 30 percent to 50 percent more than a conventional wood-frame house, which means the entire home might cost about 5 percent to 15 percent more [source: Fatsis].
In desert climates where the thermal mass of the walls can provide the sole means of heating and cooling, these construction costs might be a worthwhile investment. In other climates, additional insulation or energy sources might be needed for heating and cooling, which will further increase the costs. In rainy climates, for example, a rammed earth home might need additional insulation, as well as a larger roof, so that the overhangs protect the earthen walls. Rammed earth in rain-prone areas is acceptable, but the house site should be selected so that the home is protected as much as possible from the elements. Additional design features, such as good gutters and drainage systems that draw water away from the house, help protect against weather.
Like most alternative building methods, rammed earth homes might take more work to meet with approval by building officials, bankers and insurers. In the southwestern United States, there are several experienced builders and contractors, and rammed earth is starting to appear in building codes in that area. Outside of the Southwest, it's generally not mentioned and will require more research and persistence on the part of the person trying to get a construction loan, a mortgage or home insurance.