Rubble Trenches and Lime Mortars to Avoid Cement for Construction Purposes

The most important part of any home is its foundation. While roofs can be replaced and walls can be knocked over and raised once again, a bad foundation is often impossible to fix. In the modern day construction industry, almost every house sits on a cement slab foundation that is buried beneath the frost line underneath the walls of your home. While cement certainly has high compression strength, there are numerous ecological concerns regarding the cement and concrete industry.

Some estimates find that the cement industry is responsible for close to 3% of all carbon dioxide emissions. Furthermore, many scientists that believe that human impact on the planet has led us into the Anthropocene, a new geological era defined by human influence over the functioning of the planet as a whole, cite the fact that we humans have produced enough cement or concrete to cover the entirety of the earth´s surface with a thin layer of cement.

While cement might be necessary for a number of uses, is the traditional concrete slab the only way to build a solid and sturdy foundation for your home? Below we look at two different techniques that allow natural builders to reduce their dependence on concrete through using natural and more ecological materials.

The Rubble Trench

There are essentially two parts to any home foundation, the footing, and the stem wall. The footing is the part of the foundation underneath the soil line that goes at least as deep as the frost line for your region. This will ensure that your home is sitting on a solid foundation that won’t shift during the continuous freezing and thawing of the ground. The stem wall is the above ground part of the foundation that raises your walls off of the ground to avoid direct contact with moisture.

While concrete slabs have become the standard for footings in the conventional construction industry, the famous architect Frank Lloyd Wright popularized the use of the rubble trench foundation. The rubble trench foundation is simply a ditch dug down to below the frost line that is subsequently filled in with small rocks and large gravel. Once the trench is filled in up to the ground level, it is covered some sort of material to prevent the trench from filling in with dirt and other debris and the stem wall of the house is built directly on top of the gravel filled trench.

The rubble trench offers the same strength as a concrete footer while also offering drainage capacities. The trench should be dug slightly off contour to drain to light. This ensures that any water that enters the rubble trench will flow away from your home.

Step by Step Directions to Build a Rubble Trench for a Cob or Adobe Home

1.) Dig a trench 2 feet wide and down to the frost line. The extension office can tell you your frost line.

2.) Use a Bunyip water level or A-Frame level to slightly slope the trench towards daylight. You will have to open a trench connecting to the rubble trench that leads towards an escape.

3.) Fill the trench with 2 inches of gravel, then lay 4-inch perforated pipe and drain to fresh air. Fill gravel to within 2 to 3 inches of the top of the trench. Tamp every 6 inches.

4.) Finally, cover your completed rubble trench with landscape cloth or some other sort of fabric or impermeable material to prevent the trench from filling in.

Lime Mortars

Cement and concrete aren´t only used for footings but are also usually used for building the stem wall, either out of cement cinder blocks or as solid cement. If you are lucky enough to have a plentiful supply of rocks on your land, you can build your own rock stem wall (on top of your rubble trench) without the use of any cement. To tie the rocks together in your stem wall, you can use a simple lime mortar that can be made by following this simple recipe:

– 1 part lime putty (lime putty is made by mixing two parts hydrated lime and 1 part water in plastic buckets and letting to sit while covered for several weeks)
– 3 parts coarse sand
– ¼ part grog (ground up brick)
– Water as needed

This mortar will take a little bit longer to “set” than a traditional cement mortar, but there are numerous ecological advantages to using lime mortars. Firstly and most importantly, lime mortars will reabsorb the carbon dioxide released during manufacturing as it sets. Through a chemical process, the lime mortar will reabsorb carbon dioxide, essentially turning into limestone once again as it cures as a mortar holding your rock stem wall together. Whereas cement releases large amounts of CO2 into the atmosphere, lime will reabsorb the CO2 created during its manufacture to make it essentially carbon neutral.

Furthermore, lime mortars are biodegradable. Lime will convert back to its raw form—calcium carbonate, and can even be removed, re-wet and re-mixed. Cement, however, is a one-time material that usually ends up occupying space in landfills. Lime mortars are also compatible with earthen construction techniques. Lime has similar properties as earth and it will expand and contract at the same rate as earth. It also shares earth’s characteristics of being soft and breathable.

Lastly, there are several health benefits associated with choosing lime mortars over cement mortars. Lime can absorb and release moisture helping to prevent condensation, the main cause of fungi and molds in homes. Lime is highly alkaline and therefore naturally antibacterial, antifungal and antiseptic.

Tobias Roberts

After working in the development industry for over a decade, Tobias decided it was time to stop advising Central American farmers how to do things if he didn´t have a piece of land to live coherently with what he taught. Together with his family he runs a small agro-forestry farm, tourism cooperative, and natural building collective in the mountains of El Salvador.


  1. Good article. One small prickly point… this statement: “… the fact that we humans have produced enough cement or concrete to cover the entirety of the earth´s surface with a thin layer of cement”

    Technically, I could make a single bucket of cement and there are enough molecules in that single bucket that I could make a 1 molecule thin layer of cement that would cover the planet. So that argument is quite meaningless, because you just have to make the layer thin enough and you can accomplish this with practically any amount of cement.

    I only mention this because people in the environmentally conscious movement tend to overstate things, which turns others off and makes them not listen or care about everything else we say that is factual. We have to be really concerned about things we say, so that we don’t turn people away, and we get them to listen to what the real concerns and issues are. We need to be really careful about grandstanding – which is what that statement is.

  2. For rubble trench foundations, broken pieces of concrete can be used as “large rocks,” reducing cost and keeping concrete out of landfills.
    Another way to reduce material use if with a shallow-frost-protected-foundation (SFPF). Search for that term or try buildipedia.

  3. The idea that lime is carbon neutral is a myth. If lime is burned on site with wood, AND the wood grows back, AND the all other raw materials were transported without fossil fuels, then I think it would be safe to say it’s carbon neutral. Examination of the lime burning process reveals multiple points of carbon release. Bag lime is burned transported and packaged with fossil fuels. That is an interesting point, but the more pressing question is why we so readily accept and repeat information that we read when, in this case, a very cursory examination would reveal that it is not true. If using lime is to be more than a symbolic gesture, it should be produced as locally as possible, or at least buy wood burned lime. I have videos on YouTube on how to burn lime with wood. Burning lime on or near building sites using local materials is how it was usually done in the old days. It’s not very hard.

    1. Steven, what is your YouTube channel? I’d like to learn more about wood burned lime (sourced locally and burned on site). Thank you! Dee

  4. Why would you put the landscape cloth AFTER filling the trench with gravel? wouldn’t it need to line the trench that is THEN filled with gravel? Doing so would prevent the soil around from seeping into the gravel and blocking the drain action of the foundation, otherwise I don’t understand how that would work. Also I don’t understand how you can lead the drain pipe “to fresh air”… wouldn’t you need a very specific type of site to do so?

    Thank you in advance for your answers!


    1. Federica,
      I have lined both the trench and then cover the top, by draping the cloth over the top. This keeps sediments from any direction. Then placing a stem wall on top.
      As to the drain pipe to fresh air, yes that needs a slope to happen, but you can make a pit, and add a sump pump for sites without enough slope.
      I am experimenting with small scall foundations, before settling in one for our natural build home.
      Looking up rubble trench, or gravel foundations, and you will find a lot out there.
      Hope this helps. Good luck.

  5. Why the war on carbon dioxide, which is plant food? There is no CO2 driven climate change on this planet. And definitely no warming.

    Thank you for the recipe, very useful. I plan on doing a rubble trench here in the desert, i am looking for a solution for a barrier

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