Adobe bricks are made from a simple mixture of sand, clay, water, and straw, shaped in wooden molds and dried in the sun. No kiln, no special equipment. The process has been used for thousands of years and still works beautifully today, whether you’re building a garden wall or an entire house. The key to good adobe is getting your soil proportions right before you ever pour a mold.
Test Your Soil First
Not all dirt makes good adobe. You need soil that’s at least 50 percent sand, with clay making up no more than about 30 percent. Too much clay and your bricks will crack as they dry. Too little and they won’t hold together at all. The sweet spot falls into three soil types: loamy sand (70 to 85 percent sand, up to 15 percent clay), sandy loam (50 to 70 percent sand, 15 to 20 percent clay), or sandy clay loam (50 to 70 percent sand, 20 to 30 percent clay).
The simplest way to find out what you’re working with is a jar test. Fill a clear, straight-sided jar one-third full with sifted soil (remove rocks, roots, and debris first). Add clean water to near the top, cap it, and shake hard until everything is a uniform slurry. Then set it on a level surface and wait. After one minute, mark the layer that has settled at the bottom. That’s your sand. After two hours, mark the next layer up. That’s silt. After 48 hours, mark the final settled layer. That’s clay. Measure each layer’s height and calculate the percentages against the total. If your sand fraction is below 50 percent, you can amend the soil by mixing in clean sand until you hit the right ratio.
Mixing the Adobe
Once you have suitable soil, break up any clumps and remove stones larger than a marble. Combine the soil with water in a shallow pit, wheelbarrow, or on a tarp until you get a thick, workable mud, roughly the consistency of stiff cookie dough. Traditionally this was done by hand or by foot, and stomping the mix in a shallow pit still works well for small batches. For larger projects, a mortar mixer saves considerable labor.
Add straw or dried grass to the wet mix. Straw serves two purposes: it reinforces the brick (acting like rebar in concrete, giving it tensile strength the clay alone lacks) and it reduces shrinkage cracking as the brick dries. A good rule of thumb from the USDA Forest Service is roughly one and a half bales of straw per thousand finished bricks. Cut or break the straw into lengths of four to eight inches so it distributes evenly through the mud. Mix until the straw is thoroughly incorporated.
Building Your Molds
Adobe molds, traditionally called “adoberos,” are simple open-topped wooden frames with no bottom. You press mud into the form, lift the form away, and leave the brick on the ground to dry. Standard adobe bricks are 4 inches thick, 8 to 12 inches wide, and 16 to 20 inches long, though the most common traditional size is 4 by 8 by 18 inches. You can make molds from scrap lumber (2x4s work well for the 4-inch depth), screwed or nailed together at the corners.
A practical tip: line the inside faces of wooden molds with sheet metal or coat them with a thin film of oil or sand before each use. Mud sticks tenaciously to bare wood, making it hard to lift the form cleanly. Metal-lined forms release much more easily. Build at least two or three molds so you can keep working while previous bricks set up enough that you won’t accidentally step on them.
Molding the Bricks
Choose a flat, dry area of ground as your drying yard. Lightly sand or dust the ground surface so bricks won’t bond to it. Wet the mold, set it on the ground, and shovel or fork the mud mix into the form. Pack it firmly into the corners and strike the top level with a board or the back of a shovel. Then lift the mold straight up. The brick should hold its shape. If it slumps badly, your mix is too wet. If it cracks or crumbles as you remove the mold, it’s too dry or has too little clay.
Space bricks a few inches apart so air circulates around them. In a good production rhythm, one or two people can turn out 50 to 100 bricks in a day.
Drying and Curing
This is the stage that requires the most patience. Leave freshly molded bricks untouched for at least three days before handling them at all. After that initial set, turn each brick on its side so the bottom face can dry. Rotate them periodically over the following weeks so all surfaces cure evenly.
Full curing takes three to four weeks, depending on temperature, humidity, and sun exposure. Bricks made in hot, dry weather (the ideal conditions) cure faster than those made in cool or humid climates. Don’t rush it. Bricks that haven’t fully cured will be weak and crumbly. If rain threatens during the drying period, cover them with tarps or plastic sheeting, but allow airflow underneath so moisture can still escape.
Preventing Cracks
Cracking is the most common problem in adobe brick-making, and it almost always traces back to one of three causes: too much clay in the mix, too much water, or drying too fast. Adobe bricks never permanently harden the way kiln-fired bricks do. They constantly shrink and swell with changes in moisture content, and this is especially dramatic during initial drying when a brick goes from saturated to dry.
If your test bricks crack, try adding more sand to the mix. Even a 10 percent increase in sand content can make a noticeable difference. Adding more straw also helps by distributing shrinkage forces throughout the brick instead of concentrating them at the surface. In very hot, windy conditions, bricks can dry too quickly on the outside while still wet inside, creating surface cracks. Loosely covering them with burlap or light shade cloth for the first day or two slows surface evaporation enough to let the interior catch up.
Stabilized vs. Traditional Adobe
Traditional adobe bricks contain only soil, water, and straw. They’re breathable, easy to repair, and have been used successfully for centuries, but they remain sensitive to water. Their strength drops as moisture content rises, and prolonged exposure to rain or ground moisture will erode them over time. This is why traditional adobe buildings need protective plaster coats and good roof overhangs.
Stabilized adobe bricks have a small amount of Portland cement, lime, or asphalt emulsion mixed into the mud before molding. This makes the finished brick more water-resistant and durable. Under New Mexico’s earthen building code, a stabilized brick is one where a dried four-inch cube gains no more than 2.5 percent of its weight when placed on a water-saturated surface for seven days. Stabilized bricks are a good choice for new construction in wetter climates or for walls that will see more weather exposure.
One important caution: don’t mix the two types. Cement-based mortars are incompatible with traditional unstabilized adobe because they expand and contract at different rates. Using cement mortar with soft, traditional bricks actually accelerates the bricks’ deterioration because the mortar is stronger than the brick itself. If you’re making traditional adobe, use a mud-based mortar. If you’re making stabilized bricks, cement or lime mortar is appropriate.
Meeting Building Codes
If you plan to build a permitted structure with your adobe bricks, check your local building codes. New Mexico has the most detailed earthen building standards in the United States, and its requirements are a useful benchmark even if you live elsewhere. Under the 2021 New Mexico Earthen Building Materials Code (still in effect through 2026), cured adobe bricks must have an average compressive strength of at least 300 pounds per square inch, with no individual brick in a sample of five falling below 250 psi. Well-made adobe with good soil proportions typically meets this threshold without difficulty.
For a simple backyard project like a garden wall, raised bed, or pizza oven base, building permits generally aren’t required. But for load-bearing walls or habitable structures, you’ll need to verify that your bricks meet local strength requirements. Some jurisdictions require you to submit sample bricks for testing before construction begins.