Drainage for Earthbag Buildings
Questions answered by Kelly Hart

Q: I was wondering when you start laying the bags is there any footer under them or do you start laying them directly on the ground? Also, how deep do you dig to start laying them?

A: Earthbags don't really need a conventional foundation; you can fill the first course or two with gravel to keep water from wicking upward. And you can dig the first course into the ground a few inches to make sure that it has a good "toe-hold". If your soil drains poorly and there is a danger of frost upheaval, then it might be a good idea to dig a deeper rubble trench, and possibly install a French drain, to guard against this.

Q: Can you give me any information on the type of footer I would need for an earthbag building? What about a stemwall if you are building in a wet climate?

A: There are several options for making a foundation for an earthbag building, depending on the type of soil and climate in your locality. One of the simplest is to make a "rubble trench foundation", which is described here and here. A frost-protected foundation is described here.

The need for a stem wall depends more on the building design than on the climate; the foundations described above would work fine in wet climates.

Q: I am interested in building an earthbag home for my mother and stepfather however we are kind of stumped by the foundation portion. We can get up to 2 weeks of rain at once and then go 2 months without any rain, the soil is kind of unpredictable.  I assume we would need to build up the foundation so it will not gather water but should we dig below ground level and use small rocks in the bags for the first 3 rows? 

A: I would recommend digging below frost level with a trench that is as wide as the bags and filling this with enough rubble or drain rock so that the first course of bags will be embedded several inches below grade level. And yes, filling the first few courses of bags with gravel is a good idea to assure that moisture does not wick upward. If it seems likely that this rubble trench will fill with water during heavy rains, then a French drain arrangement, where a perforated pipe embedded at the bottom of the foundation can collect the water and run it off to some "daylight" location away from the building, might be a good idea.

Q: Does anyone install any kind of land drains from the foundation trench?

A: Depending the soil type and the contours of the grade at the site, this might be recommended. The soil at our site is pure sand and drains quite readily, so we didn't bother with drains.

Q: As I thought about your recommendations concerning the rubble footing/sand bag stem wall, something occurred to me that I wanted to check with you. You mentioned it was best to place a layer of large (4 inch) rocks at the bottom of the trench and then layer(s) of progressively smaller gravel until just above grade - then start the bags. At first I thought this was just for drainage, but then it occurred to me that tamping these layers would wedge progressively smaller stones between larger stones, thus stabilizing and distributing the load in a horizontal as well as vertical plane. Does that make sense or am I inferring something that wasn't intended?

A: I think you are absolutely right in your thinking about the distribution of aggregate for a foundation...this is what Mother Nature would like.

Q: You built a house with a sand foundation. How is this done? Do you have some details on foundations?

A: Actually I built an earthbag house that happens to sit on a huge bank of sand, since that is the native soil at this site. Earthbag structures do not really need a traditional concrete foundation, since the bags themselves become a sort of foundation. Earthbags can also be used as a foundation for other sorts of buildings, such as strawbale buildings. The sand is nice because it drains readily and therefore does not present a problem of frost upheaval, as would more expansive soils. One way to deal with these kinds of soils is to utilize a rubble trench foundation.


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