Wonderful house construction with lightweight concrete

Exterior of the house. What will one day be the front door is currently bricked up and forms a niche inside. The studio is in the background. Notice that the rebar sticks out to be added later on the second floor.

The following is an excerpt from Home Work: Hand Built Shelter (Shelter Publications, 2004) by longtime MOTHER EARTH NEWS employee Lloyd Kahn. The book contains more than 1,500 photos illustrating various innovative architectural styles and natural building materials that have gained popularity over the past two decades, such as cob, papercrete, bamboo, adobe, straw bales, wooden frames, and earth bags. If you love beautiful, funny, or funky buildings, you will want to own this great book.

Author’s Note: I came across Steve Kornher’s work on the internet. Steve has been building for 30 years, 15 of them in Mexico. He has worked with clay and rammed earth and various types of concrete masonry. He is now “completely in love with light volcanic aggregates”. Here is his report and photos of his latest work.

My wife, Emilia, and I live on a two-acre ranch about 25 minutes from San Miguel de Allende, in the mountains of central Mexico (6,200 feet altitude). Our house is a work in progress: built Mexican style, pay as you go, and let the rebar stick out for future additions. We built the house and the large warehouse over a period of two years with two (sometimes three) bricklayers each. Slow building is a lot more fun and allows you to be more creative because you can think about things and make changes.

The home currently has 110 square feet of interior space with plenty of decks for outdoor living. Most of the first floor is made of adobe. Later additions and roofs made of lightweight volcanic aggregate were added. South windows and an overhang ensure passive solar heat in winter. All roofs are vaulted walls with shell motifs. Mexico has some great masons, and I owe a lot to the knowledgeable masters who helped me figure out how to do this wild and wacky stuff.

I originally came to San Miguel for a two-week ceramic workshop, dealt with potpourri and botanical exports (all legal), then flower and seed production, and now I’m back in construction. I’ve been in the area for 18 years and have lived at Ranchito for eight years.

We have almost an acre here. About half is in flower production for my wife’s business in San Miguel and the other half is mostly made from native plants. It’s a jungle during the rainy season. Between the flowers and natives, I photograph for 400 species.

Former bedroom, now the living room. We keep adding rooms whenever the budget allows – cash building.

Cash construction must be flexible

One of my main goals is low-cost building construction that will take 400 years. To do this in this climate, you need to build self-supporting structures and use masonry adobe bricks, lightweight concrete block, reinforced concrete pillars and other features. The roof is the key to the long life of the building. Therefore it has to be self-supporting, round and curvy – and not flat. Self-supporting vertical walls naturally want to be roundie-curvie. You move on from there and pretty soon everything is roundie-curvie.

If you are starting to think about a durable home, it is best to build it so that it can (likely) be remodeled. You can later hack out a door using adobe and / or lightweight concrete structures. With hard concrete, this is an almost impossible project.

Living room ceiling. This was the first Boveda brick roof that my maestro or I had built – a challenging project.

Roofs for concrete houses

I’ve worked with many different forming systems for different roofs. The largest so far is about 6 by 6 meters. Small roofs can be supported from above during construction, but larger roofs require some center support.

Designing a smart roof shape is key. Barrel vaulted roofs, modified domes, and especially shell shapes (my favorites and actually quite easy to make) are all very compressed. Since the roofs are self-supporting and pour slowly, very little reinforcement is required. Once a lower form shape is made – usually 3/8 inch rebar or welded wire – it can be easily moved in sections to form an identical roof.

All roofs are built with an initial 3/8-inch shell that is poured onto a plaster batten on a metal frame (which will later be removed and reused). This shell stiffens everything for the following pourings of lightweight aggregate and lets you see what the roof will look like. Changes are easy at this point. After the roof is poured (4 to 6 inches thick), you can move the mold again in five or six days for the next roof.

I’m a huge fan of reusable and movable formwork, usually 3/8 inch rebar and / or # 10 or # 6 welded wire. These days, I really enjoy fast, inexpensive barrel vaults.

Build concrete walls

Walls around me are typically 6 to 8 inches thick and made up of the same volcanic aggregate. First, I poured the aggregate into the following shapes: 8: 1 ratio of aggregate to cement by volume for walls; 5-to-1 ratio for roofs. But now blocks are made on site and because they are faster I even use them for roundie-curvie walls.

Books that influenced my construction: The Owner-Built Home by Ken Kern has great alternative ideas for using concrete. A sample language from Christopher Alexander. Every book about Antoni Gaudi – Gaudi from Barcelona is good.

Lightweight concrete stairs at Bonnie and Haden Kayden’s in San Miguel de Allende. We’ve built a railing since then, but it’s not strictly necessary.

House construction with lightweight concrete

Concrete is highly compressed. The best way to take advantage of this property is to build structures that are inherently self-supporting and don’t require a lot of iron reinforcement. Since most of the buildings here in my part of Mexico are made of concrete, it’s easier to let your imagination run wild. Local builders have worked with ferro-cement, wired styrofoam sheets, plastered straw bales and ground crete.

I’ve had the greatest success with lightweight concrete. Lightweight concrete differs from heavy concrete in the use of naturally light materials (aggregates) such as pumice stone (volcanic stone) instead of the sand and gravel used in ordinary concrete mixes. It weighs half that: 50 to 80 pounds per cubic foot.

Not all concrete is ugly, hard, cold and difficult to work with. There is a whole range of lightweight concretes whose density and compressive strength are very similar to wood. They are easy to work with, nailed with common nails, cut with a saw, drilled with woodworking tools, and easily repaired. We believe that ultra-light concrete is one of the most basic bulk solids of the future. “(A sample language)

Some form of suitable aggregate is available almost anywhere in the world. Our locally available aggregate here in San Miguel is a type of pumice stone or slag, called Espumilla or Arenilla in Spanish, Spanish, that we usually mix 8-to-1 with cement for walls or 5-to-1 for roofs.

Most lightweight concrete has a good R-value and is a good insulator for heat and sound. It is used as noise protection in subway stations. It has enormous sculptural possibilities and is ideal for monolithic wall-roof constructions.

I think we need smarter building systems. I am looking for a home that will last for hundreds of years, that is easy to maintain and remodel, and that uses mostly locally available, abundant materials. Lightweight concrete is just the thing.

Photographs by Steve Kornher

Lloyd Kahn is a visionary for sustainable living and the editor of Shelter Publications. He is the author of books on natural buildings, including Home Work, Tiny Homes, Tiny Homes on the Move, Shelter II, Builders of the Pacific Coast, and The Septic System User Guide (all available from the MOTHER EARTH NEWS Store). He lives and builds in Northern California. Follow Lloyd on his blog, Twitter and Facebook and read all of his MOTHER EARTH NEWS posts here.

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Originally published: 06/15/2020 10:11:00 AM

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