By Arnie Valdez

Arnie Valdez, a professor or architecture and owner of southwestern Colorado’s Rezolana Farm, was one of the first licensed hemp growers in the state. Valdez and his team created a field-to-shelter sustainable demonstration casita with homegrown and hand-decorticated hemp and clay to make adobe bricks.

This article documents the process of building an adobe structure that utilizes traditional adobes and hemp infused compressed earth blocks (CEB) for a vaulted roof. The combination of adobe and compressed earth blocks resulted in a structure that is mostly built out of earth/hemp with small amounts of wood for window and door frames and a couple of beams. As a result the total embodied energy is kept to a minimum compared to more conventional construction.

Read Valdez’s report below on the project, including costs and materials.

Rezolana Farm started growing Industrial Hemp in 2014 with the support of Fibershed, as per the experimental clause of the 2014 Farm Bill authorizing States to grow hemp. From 2014 through 2018 small acreages of hemp were planted and harvested throughout different plots of the farm property. Each year of planting, yielded hemp stalks, that were cured and used for small scale experiments manufacturing compressed earth blocks (CEB) and traditional adobe bricks infused with hemp fiber and hurd. A small decorticator was built and used for processing the hemp stalks. The stalks are run through the decorticator to separate the fiber from the hurd or inner layer of the stalks.

*Figure 1. 2017 Rezolana Farm hemp crop*

Eventually enough hurd was accumulated for use in experimenting with building construction technologies. In 2017 a series of test blocks were produced using a Cinva Ram Press. Various combinations of hurd, clay, sand, lime and cement were used to manufacture variations of compressed block configurations. The process was documented in a paper submitted to Earth USA 2017 Conference held in Santa Fe New Mexico. The findings have been read by many scholars and there has been an interest in seeing how the blocks could be utilized in a structure or small building.

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Finally, in 2020 there was an opportunity to coordinate with Fibershed to sponsor a workshop and build a small 100 square foot building on the concrete slab of a defunct windmill base. Prior to the launching of the project, an individual from southern Colorado expressed interest in learning how to build with adobe. The timing was good, so I acquired an adobe apprentice for the duration of the summer. With his help and enthusiasm for earth building he dedicated his time towards the construction of the Casita.

Foundation & Site Preparation

The foundation for the building is an existing ten foot square by ten foot deep concrete slab. The slab has been sitting idle since a windmill tower was decommissioned decades ago. Otherwise a perimeter poured concrete foundation would have been required. Site preparation included clearing and grubbing around the slab edges (Figure 3). The east side required some excavation since the grade elevation was about two foot higher than the slab.

Adobe Walls

The basic shell or walls require around 500 adobes or earth blocks. Calculations for the roof show that around 400 Compressed Earth Blocks (CEB) are needed to build the vaulted roof. A local rancher had a stockpile of adobes left over from a project many years. The 10” by 14” by 4” adobes were eroding, and when contacted about selling them he offered them for twenty five cents a piece. Normally they would cost upwards of a dollar and would need to be transported from New Mexico. Having the adobes at an affordable price and locally available was a great incentive for the project. Over the course of several days the adobes were hauled in a trailer about ten miles to the building site. A total of about 600 adobes were hauled. The piles of rubble/broken adobes were hauled and used to make mortar for laying the adobes.

The basic composition for the traditional adobes is about 75% sand, 25% clay and straw mix poured onto wooden forms and sun dried. The mortar for laying the blocks is recycled adobes that are crushed and screened to provide a fine mortar mix. Using the adobe mortar mix ensures that the mortar and block are totally compatible for the construction of the wall.

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A ten inch wall thickness was selected. This is the minimum wall thickness recommended by the New Mexico State Adobe Building Code. The NM code was used as a guide for the construction of the building, even though there is no existing building code in Costilla County. The adobe code insures that the structure meets basic standards that address public safety and welfare of the occupants. The main features that are covered in the code are wall thickness, window and door lintels and frame nailers, and ring or bond beam. The construction of the vaulted roof follows the best practices of traditional middle-eastern construction techniques.

Vaulted Roof System

A vaulted system was selected for the roof to illustrate, that the building is built of mostly earth construction while utilizing small amounts of wood for the door, and window frames and two cross beams. The curve of the vault is a catenary curve based on drooping a chain over the span. The chain was lowered to three feet, the proposed height of the vault. While the chain was hanging at the desired curve, it is then traced on a sheet of plywood. The curve is cut with a jig saw. Two sections were required, one for each of the gable ends. The two ends of the curves were connected with two by fours and covered with 1/8” Masonite sheets. This form will be used as a guide for laying the CEB’s (Figure 5).

*Figure 5. Wooden Form for Vaulted Roof*

The vaulted roof is built of 12 inch long by 6 inch wide compressed earth blocks. The blocks are manufactured using a Cinva Ram Press. A dry mix is prepared consisting of sand, clay, hemp hurd and sometimes small amounts of lime and Portland cement(Figure 6). The mix is poured into the press hopper and compressed by hand using a long lever handle. The CEB’s are stacked on pallets to cure for about a week. For the roof vault construction, the blocks will be laid sideways providing a six inch thick roof system that spans over two side of the building.

*Figure 6. Mix for compressed earth blocks*

Figure 7. Cinva Ram Press

Manufacturing 400 CEB’s will require a considerable amount of labor to secure and process raw materials of sand, clay, hemp hurd and stabilizers. Onsite farm labor, coupled with two workshops was the main workforce in the making of the roof bricks. Two persons can make about a hundred blocks a day under ideal conditions, however, more typically, fifty blocks a day is a reasonable goal.

Construction Process

Walls and Bond Beam

Prior to beginning of the wall construction, the site was organized so that the materials are available for immediate use. The blocks were stacked close to the foundation, a mortar mixing tub to receive the crumbled adobes for soaking and converting to mud mortar. A water supply is set up with a 55 gallon plastic barrel. Four by four wood columns are erected at each corner. They are set plumb and will serve as guides for a string to keep the courses aligned and in building the corners. The corners are first erected to several layers using the 10” by 14” adobes. Once the corners are laid, the walls are infilled between the corners. Typically three to four courses are laid daily. This allows the walls to set up before adding additional layers on the next day. The door frame is set in place and braced so that the adobes can be laid to the wood frame (Figure 8).

*Figure 8. Corner Posts, Door Frame, Adobe Courses*

At the third course a wooden block known as a gringo block is laid next to the frame so that the frame can be attached to the wall. Three gringo blocks are used on each side of the frame. The window frames are also attached to the wall with gringo blocks. Once the walls are raised to the height of the door and window frames, the next step is to pour a concrete ring beam around the wall perimeter. The ring bean or bond beam is required as per the NM Code to be 6 inches thick and be reinforced with ½” rebar. Wooden forms attached to the walls contain the concrete mix until it sets, then they can be removed. The sand and gravel mix for the concrete was secured from a local gravel pit and mixed by hand in a wheel barrel. The bond beam was cantilevered 4” beyond the north and south sides. This ledge allows water from the roof to drain away from the wall. The base of the concrete bond beam serves as the ledge to support the CEB barrel vault roof. On the interior side of the bond beam, ½” bolts are imbedded on the sides supporting the vault. The bolts will secure two 4” by 6” wood beams that are used to stabilize and counteract the horizontal thrust exerted on the support walls by the weight of the vault. The estimated weight of the vault is about 4,000 pounds or about a ton of weight pushing down on the 10” adobe walls. After the bond beam is poured and cured, the forms are removed and the two wood beams are then bolted to the walls with angled brackets (Figure 9).

*Figure 9. Bond Beam & 4” X 6” Cross Beams*

Vaulted Roof Construction

The manufacture of the CEB’s begin concurrently with the wall construction. As the blocks were made they are stacked on pallets for transport to the site. The CEB manufacturing site was set up about 500 feet from the building site. Set up for the block making site required stock piles of sand, clay, water, hemp hurd and lime stabilizer. The sand was locally available while the clay had to be purchased from a supplier forty miles away. The block making process is rather tedious since the sand and clay have to be screened to fine particles for use in preparing the mix for the Cinva Ram Press. Two persons working for six hours on the site can process and make fifty to seventy five blocks daily.

Knowing that the blocks are going to require labor and time two workshops were sponsored so that the participants could see the building in progress, witness and assist in the making of CEB’s for the roof vault. One workshop was held in early August. Interested participants were invited to attend as an exchange of labor for the knowledge of basic adobe construction and roof vault construction (Figure 10). Over 100 blocks were made during the workshop consisting of six participants. The mix of the ingredients varied as the process started and later refined as the block results were revealed. A typical mix consists of two parts sand, one part clay, ½ part hemp, and ½ type S Lime. As the blocks cured observations are made to detect crumbling, cracks and overall integrity. Some of these blocks were used to demonstrate how the vault would be constructed in a sample course layout.