Cross Section of a Straw Bale Landscape Wall/Rubble Trench Foundation

Written by Andrew Morrison

Many people have recently asked me about landscape walls. As a result of those inquiries, I’ve drawn up a cross section of a landscape wall and rubble trench foundation for you to check out below. This is a basic design that can be used in most locations. Some building departments allow for rubble trench foundations within the codes while others are less accustomed to them. Be sure to discuss the potential to use this design before you commit to the design. You may need to make changes to the system or simply educate the building officials around the effectiveness of the rubble trench design.

Sketch of straw bale landscape wall and rubble trench foundation
I’m open to feedback on the design. If you think there’s a better system, let me know. I always like to hear how other people do things. As a builder, I always spent time visiting other contractor’s job sites and talking to them about how they did things. I’ve learned a LOT by talking to others and I continue to learn this way today. One detail I often put into landscape walls in wet climates is a metal wall cap. The caps are custom made by the local metal shops (those who fabricate metal roofing are best) to fit over the top of the finished wall. You need to provide some anchoring points for the caps within the wall, but that’s not shown here.

Simply let in a 2x at the top of the wall so that the bottom edges of the roof cap can hit it during installation. be sure to install the wood nailers before the mesh so that the mesh can lock them tightly in place. Get the wall cap in a color that matches or compliments your plaster and you’ll barely notice it’s there (or you’ll see it as an asset to the design).

One point around landscape walls and moisture. It’s really not that big of a deal if the bales get wet and ultimately rot out. That matters BIG TIME in a house, but a wall is just that, a landscape wall. The bales are not acting as insulation, they’re basically acting as forms for the plaster. Once the mesh is properly installed and you add 1 1/2″ of plaster to each side of the wall (all the way up and over actually), the bales can rot out without the wall collapsing.

Of course, the overall strength of the wall is better with the bales in place, so protect them as best you can. Just don’t loose sleep over water getting in through a failed washer on a screw that attaches your wall cap. The wall will be fine!

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16 Responses

  1. Hi,

    The wall appears to be a conventional strawbale wall.

    My question is how would you keep the wall from being pushed off the rubble trench in a high wind area? It may no happen quickly, but over time the wind force could “walk” the wall off the rubble.


  2. Hi Ted. The mesh that is attached to the toe ups, pulled tight over the bales and then attached to the toe ups on the other side of the wall keep this wall locked in place and attached to the foundation. If the wall were taller and in a high wind environment, then I would use buttress walls to strengthen the “out of plane” direction of the wall. The location and size of those buttress walls would depend on the wind speed and wall height.

  3. Hi Andrew,Ted,

    I have plans to build one myself, and will finish it with a ceramic tiled “mini” cambrel or dual slope roof to protect the bales for rain and snow. In order to do just that I need a top beam which (as I learned on one of your posts) can tie down the bales to the toe ups. That should be enough to protect from strong winds also, don’t you think?

  4. Andrew, now that it’s much too late I’m wondering about rubble-trench foundations for strawbale houses. Are those practical in this earthquake Zone C (or is it D?) area? I specified them when we took the plans to the engineer, but he ignored the idea and specified a conventional concrete foundation — about 25 yards’ worth!…Jerry

  5. They can be if they are designed properly. They need additional details to resist uplift. It can be done for sure, but it takes the right person designing it and a willingness to not take the “standard easy way out.”

  6. Hi Andrew,
    Santa Fe this Spring (2011)! It is just what I want to do and where I am!! Could you email me information on registration? Thanks!

  7. Hi Beth. Thanks for your email. I have some bad news though…the workshop in Santa Fe is not happening as the host’s mother has gotten very ill and so the host will need to focus her efforts on helping her. It’s a sad situation for everyone, most notably the host and her mother. I wish them the best and hope that a quick recovery is possible. I apologize for the misleading information ion the blog post. I will have to remove that. Thanks for alerting me to it.

    I am just about to secure a deposit for a workshop in Colorado, so hopefully that is something close enough to you to replace the Santa Fe class. It’s not a landscape wall class, but a full house workshop. I am also trying to find a way to do a landscape wall workshop in Montana. The timing is the hard part right now. Stay tuned to for updates on workshop dates and locations.

  8. I’m not an engineer, but I seriously doubt it. Deeper foundations are typically due to frost lines. That said, if you need to get below the clay to noexpansive soils, it may be necessary. You are best to discuss it with a local soils engineer or ask builders in your area what a standard foundation is. Good luck.

  9. If when you form up to pour concrete you fasten 2x4s to the inside top of the forms you will incorporate a ledge in the concrete. Once concrete is cured you can anchor 2x4s into the ledge area to serve as toe ups instead of 4x4s and placing rebar verticly @ the approximate location of the center of the bales in the concrete will provide lateral support to the wall in addition to the wire mesh stretched over the top.

  10. There are two main issues with constructing the wall this way.

    1. The toe ups would likely be flush to the top of the concrete, meaning there is no way to lift the bales up off the concrete to provide a solid capillary break. If the toe ups were somehow held up above the top of the concrete, there is still the question of attaching the toe ups from the side as opposed to from the top. The strength of doing so is not as good as it would be if the toe ups are anchored down from the top.

    2. Rebar does not do a good job of providing strength to the wall. The mesh is much stronger and better at its job. The inclusion of rebar would be more time consuming and expensive, but would yield little benefit. If you want to add strength, you can include buttress walls every ten feet or so or you can add a structural frame. I would only do either of those if the wall was exceptionally tall (over 5 bales tall) and/or if there are high winds in the area.

  11. If the soils hold together well enough while digging the trench then the rubble solution will work. You will need to provide adequate drainage to keep water out of the trench, but that is always the case. I will note that I am not an engineer, so my thoughts are merely my own. To be certain, you should connect with an engineer licensed in your area who can verify that it will indeed work.

  12. Hi Andrew, I have talked with you briefly before about my building project here in central Quebec Canada. I will be digging my trench for my flotting slab, to be cast in three18ft sections soon,my building is 52x28ft it will be bermed on the north and 30%east &west. I have a well drained sand & gravel site.The trench will be 30″wide and 4ft. deep with a sock covered french drain all around under compacted gravel.Does the trench start at exterior perimeter where all 30″ will be on the inside, or should the slab be centered over the trench?Would you use 1/4inch or 3/4″gravel for filling & compacting? I will have an 18″ slope around the perimeter and 8″thick slab over the rest.Do you think I will need an extra footing , or just 3/4 rebar where I plan to build a 4ftx2ft x 28ft high mass chiminey from cement block infilled with rock &sand around the flue tiles? the cement block will be stuccoed afterwards.Looking forward to your thoughts on this! Thanks in advance, Tino

  13. Hi Tino. Great to hear you are getting started. Your questions are a bit too engineer oriented for me to give you solid answers. I can give you my opinion, but I strongly recommend that you talk to an engineer or designer/architect in your area to get solid answers as there are so many variables I can’t included from here.

    1. I would personally start the trench at the edge of the slab, but be sure to include insulation to break up any thermal bridging that might happen if the concrete is solid to the exposed climate.
    2. I always recommend 3/4″ minus crushed rock as a backfill. Be sure it is angular, and damp when compacting and only install/compact in 6″ lifts. If you add more than 6″ to the trench at a time, the compaction will only effect the top 4-6″ and the rest will be loose gravel forever, causing settling in your foundation.
    3. I would expect an extra footing will be necessary under the chimney. That’s a lot of weight.
    4. In terms of rebar in the foundation, you will only likely need #4 (1/2″) rebar in a grid pattern of 18″ on center. I don’t think you need 3/4″. Again, this is a question for a local engineer to answer but #4 rebar is pretty standard.

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