...practical information on natural building materials & techniques...
volume 2, issue 1
January 2013

designing natural buildings & teaching
hands-on workshop for over 15 years

Simple, Dry Footers that Outperform 
Concrete (and cost less)

...understanding rubble trench footers

by Sigi Koko

The foundation for any structure takes all the weight from the building above and distributes that weight/load to the ground below.  It holds the building up and prevents uneven settling.  The footer is the part that extends below the ground.  Most commonly, that footer is formed in concrete, which requires digging a ditch wider than required structurally, installing forms, adding in steel rebar and then concrete, and backfilling the ditch after the forms are removed.  The problem with this system is that it requires a extra site disturbance and relies on a large amount of concrete, which is a huge greenhouse gas contributor.

Rubble trench footers offer an alternative that outperforms the standard concrete footer, limits site disturbance to the building footprint, and greatly reduces greenhouse gas contributions.

What is a Rubble Trench Footer

A rubble trench is simply a continuous trench footer around the structural perimeter, dug as deeply as the ground's winter freezing point, and filled with stone.  A structural (usually reinforced concrete) grade beam (a beam that rests on the ground) is poured on top of the stone-filled trench.  The concrete distributes the structural loads of the building across the surface area of the trench below.

  1. This type of foundation uniquely provides both structural bearing as well as water drainage in a single foundation system.  Eliminated redundancy means lower cost.
  2. Water is the single biggest culprit for foundation failures (erosion, freeze thaw heaving, etc.).  A rubble trench is a drain!  Which means that all water is taken away from where it could cause a problem.
  3. The trench you dig is the form for the stone, so no overdigging is needed (as with a concrete footer).  This means you only disturb the ground directly under the building, no additional area.
  4. A rubble trench footer uses a fraction of the total concrete, usually 25% compared to conventional construction.  The manufacture of every 1 pound of cement contributes 1.25 pounds of greenhouse gasto the atmosphere.  Less concrete means less greenhouse gas contributions.
  5. Less excavation and less concrete means less cost!
That's right...when installed correctly, a rubble trench footer results in a higher-performing, more durable, more eco-friendly, and lower cost foundation footer.  Win-win-win-win!

How's it done?

1.     Dig a trench to frost depth.  Slope the bottom of the trench to a spur that slopes to daylight or a dry well.  The width required for the trench is determined by structural loads & the specific soil bearing capacity. 
2.    Line the trench with filter fabric to prevent silting-in between the stones in the footer over time.  If the cavities between the stones fill in, the footer will not drain.
3.    Fill the trench with stone flush to original ground level using 1-1/2" clean gravel and tamp every vertical foot of fill.  Hand tamping works just fine...no need for a pneumtic tamper.  Tamping locks the stones together to provide strong bearing that won't shift over time. 
4.    Coat your formwork for the grade beam with biodegradable oil to ensure easy release of the form for potential reuse.  Any vegetable oil works well.  I use wood that can then be reused for the floor or roof structure later...no waste.
5.    Set formwork and reinforcing bars where your grade beam will be located.  (As with any formwork, be sure that you brace the forms well so they stay in place when the weight of the concrete is added.)
6.    Pour concrete grade beam.  The grade beam can be a discrete structural element around the entire perimeter of the structure, or can be integrated into the thickened perimeter of a slab-on-grade foundation.
Building Permits
Rubble trench foundations meet the requirements and the intent of U.S. building codes, however, since this system is not specifically identified in current codes, acceptance is provided on a case-by-case basis.  Since this puts permit approval at the discretion of individual permitting officials, it is recommended to intiate a dialog prior to submitting for a building permit.  This provides opportunity to inform and educate permitting staff on the benefits and merits of rubble trench foundation systems.  You can bring a copy of the article linked under the Recommended Reads in the sidebar of this newsletter.  Stamped structural drawings are highly recommended.

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What's Happening

What we're up to at Down to Earth Design...

If you are on facebook, check out our daily postings of info & inspiration, like this one:

You always want to use good quality strawbales to build your walls: dry, tight, and clean.  read more...

Low Hanging Fruit

small habits that make a big difference...

Use washable napkins & towels

What's the big deal?
Most paper towels and paper napkins are made from virgin fibers, mostly from freshly felled trees.  That's right, we cut down trees to wipe our faces.  Not only that, most paper goods involve bleaching to get that pearly white, and that bleach ends up in local ecosystems.  Plus, the whole point is disposability, meaning one use and throw away.  That disposability contributes to nearly 30% of landfill waste coming from paper goods.

The solution: cloth!
You can buy or make cloth napkins and kitchen towels.  I recommend cotton over synthetic fabrics, since cotton is far more absorbant.  Cloth napkins & towels save you money (by reducing your paper goods purchases ), help the planet (by reducing unecessary resource use), and add so so little to your wash loads.  Plus you can add color & fun to your life!

For more info: CLICK HERE

For instructions on how to make your own: CLICK HERE

Recommended Reads

Article on Rubble Trench Foundations in the Building Safety Journal by Sigi Koko

This is a link to an article that I wrote in 2003 for the Building Safety Journal (for building permit officials)

click the cover above and scroll to page 20 of the pdf


Download a 2-page version of this article:

Click here to download our 2-sided trifold brochure on Rubble Trench Foundations

My approach to design...
Sigi Koko is the principal designer at Down to Earth Design, which she founded in 1998 to help her clients manifest their dreams of living in a natural, healthy home.  She translates each client’s vision into a unique building design that reflects their personality and lifestyle, while responding to the surrounding landscape and climate.  Sigi’s uniquely collaborative design process provides a high level of information and support that encourages her clients to engage fully throughout design and construction.  Sigi also teaches natural building workshops that empower her clients to contribute creatively during the construction of their own home.

All of Sigi's projects are designed to function in synchronicity with their environment.  Each building relates to seasonal cycles of sun, wind, and rain to provide natural heating and cooling primarily from passive (free!) sources.  Her clients enjoy an average 75% reduction in total energy usage compared to conventional buildings.  She uses a palette of building materials that ensure healthy indoor spaces and minimal environmental impact.

For more articles like this, visit us online at www.buildnaturally.com and contact us if you have a natural building topic you would like to see covered.

Thank you for reading!

Down to Earth Design

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