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How Do You Build a Rubble Trench and Rock Stem Wall with Lime Mortar?
While most construction teams will have lots of experience with cement footers and concrete foundations, building with lime mortars is much less practiced. Lime mortars are best suited for foundations constructed from solid rock. Instead of digging a trench down to your frost line and filling it with solid concrete, some alternative builders return to the rubble trench, an alternative type of foundation used widely by the famous architect Frank Lloyd Wright. The rubble trench is simply a lined trench dug to below the frost line filled with gravel. This type of footer offers a structurally sound foundation for your home while also helping to improve drainage underneath your house. The trench is usually dug off contour so that excess water can move through the gravel and away from the walls of your home.
On top of a rubble trench, a solid rock stem wall works exceptionally well with lime mortars. It is a much more durable, natural, and aesthetically pleasing option for a home foundation. According to the American Society of Home Inspectors article, “lime mortar sets in several hours, but the final hardening is a prolonged process as the lime re-forms into calcium carbonate. This can take months or years to achieve maximum strength. The result is a mortar that expands slightly as it sets (making shrinkage cracks unlikely) and relatively stable through temperature changes. Lime mortar is also porous enough to allow water vapor to migrate out of the masonry and water-soluble enough to enable it to redeposit itself and ‘heal’ hairline cracks that appear due to settling or other stresses.”
The rock or stone used in this type of natural foundation can be held together with an essential lime mortar made from lime putty (two parts hydrated lime and one part water), four parts coarse sand, ¼ part grog (ground up brick), and water.
An essential element of sustainable building practices is rediscovering the wisdom that was found in vernacular building techniques. While smart home technologies, more energy-efficient home appliances, and modern-day building techniques can improve the energy efficiency and overall sustainability of a house, older procedures and practices that have been used for centuries also have a place in more sustainable homes. For example, replacing our dependence on cement with lime-based mortars is one way to reduce the carbon footprint of our homes.
Tobias Roberts
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.
