I am often asked if a straw bale house can be built over a basement. The answer is always yes. A raised floor system, either over a basement or crawlspace, is no different than building a second floor on a bale home. Interior footings will likely be necessary to carry the load of the floor system. In some cases, you may need to step these down into a lower sectional basement for continued support. Once they are poured and in place, you can build your stem walls. You can use poured concrete or block, depending on your desired finish result.
With your stem walls in place, simply add your floor system to the top of the wall. Be sure to account for the dead weight of the bale walls when calculating your floor joist size and spacing and when considering how many interior beams or girders will be required to split the load across the span of the house. In some cases, you may need to build a secondary bearing wall directly below the interior face of the bale walls to transfer that load off of the joists. When necessary, this actually creates a great place for running plumbing and electrical in a finished basement.
About the Author
Andrew Morison is a specialist in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs. You can check these out at www.LearnStrawBale.com..
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January 12th, 2009 at 1:09 pm
Great site! In regard to a raised floor, I have designed conventional homes with the stem wall having an “inverted brick ledge” along the top that accepts the joist (you have to use treated between the joist and concrete). This method might save some money on flooring systems since the bulk if not all of the load of the standing bales is on the stem wall not the floor.
What do you think?
January 13th, 2009 at 7:30 am
This sounds great to me. I have done similar systems here over the years in conventional. I can see that it would work for bale as well. The ideal situation is to limit the amount of concrete used in the stem wall as well as limiting the size of the joists. If I have to choose one, I always choose to increase joist size (or reduce spacing) before increasing the amount of concrete used.
October 6th, 2009 at 3:18 pm
Would it be possible to extend the floor joist (length) beyond the stem wall enough to center the bale’s over the stem wall?
October 26th, 2009 at 4:06 pm
Yes, that would work with the right engineering (i.e. size of floor joists).
February 20th, 2010 at 9:16 am
Have you tried, or seen, insulated concrete forms used for a basement wall in a straw bale house? We would really like to have some unheated storage under part of the house.
February 22nd, 2010 at 12:43 pm
Yes David. They work fine under bale buildings. When using them as a basement floor or foundation, simply build your bale walls on a deck (floor system) that ties into the foundation walls. There’s more about this on my blog, just do some poking around and you should be able to find it. I think a keyword of “basements” would be a good search term.
March 11th, 2010 at 12:35 pm
This is a tremendous article, im glad I stumbled onto it. Ill be back later to check out other posts that you have on your blog.
March 14th, 2010 at 9:05 am
Andrew on straw bale and basements. Is there any reason why one couldn’t dig the hole for the basement pour the footer. Have a perforated pipe and pea gravel set up that drains into a cistern around the outside of the footer. Build load bearing walls plaster them put a thick water proof coating on the outside plaster. Then place pea gravel along the wall as you back fill. Also with a paver walk way around the base of the house and good over hangs as details. Do you feel this would work without any problems or what problems could occur?
March 16th, 2010 at 5:05 pm
I’d be concerned about two things. First, you can’t drain to a cistern in most areas because of the risk of clogs and backflow/flooding. You may be able to work something with your local building department to design a proper system for this idea.
Second concern is the use of bales below grade. Even though you have a good idea of creating water proofing and the gravel would be good to create some relief of moisture, I think you could still have problems with the overall concept.
April 15th, 2010 at 11:44 am
Andrew,
What about the new trend of using Shipping Containers as structural components of buildings, I was wondering if these could be used to form the basement then the straw bale home be build above it. The shipping containers would be utilized not only as support and load bearing but the space inside them (rectangle formed by them) would act as a basement?
April 23rd, 2010 at 9:42 am
That sounds interesting. I don’t know how much load those containers are rated to hold above them. If they fit the bill and aren’t subject to rust out, that could be cool.
October 18th, 2010 at 5:06 am
I’m curious about integrating strawbale and shipping containers. If the bales are not load bearing, can they be stacked (and plastered) around the exterior of the containers, which would serve as the internal frame? The context is in an arid environment.
October 18th, 2010 at 6:38 am
Hi Elzi. Yes, that would work. There would obviously be some issues related top moisture that would need to be dealt with, even in an arid climate, but the bales could most definitely be used to wrap the container.
Andrew