Solar Oval One

John Fordice, Architect

Small Cob Series
No permit required - passive solar - small cob buildings

SOLAR OVAL ONE is a compact  passive solar design with a loft which can be an outbuilding for many possible uses.  It has many valuable and green/sustainable features:

  • Potentially able to be constructed without a building permit
  • Cob wall construction 
  • An impervious base wall below the cob for moisture protection of the cob
  • A South facing wall of windows and french door for passive solar heating
  • An earthen floor for solar mass and economy of construction 
  • An interior cob bench for built-in seating and rocket mass heater
  • A built-in desk or kitchenette area with side storage niches
  • A north wall closet for storage space and insulation
  • Small East and West end side windows for views & area lighting
  • A sleeping loft accessible by a built-in ladder
  • Roof framing which needs only minimal small dimension lumber 
  • The roof framing can be easily adapted to your insulation needs
  • A corrugated sheet metal roof
  • The structure is designed to include seismic stability components
  • A pleasing curved design 
  • Low cost if you build it yourself

Building with cob allows the use of local sustainable materials.  In many areas the earth at your site can be used and only water, sand and straw will need to be brought to your site to make your cob.  The cob is mixed right where you are building and stacked up on an impervious foundation. There are no forms needed to make a cob building.  Curving sculptural walls are easily created.  Your imagination is set free when you build with cob.

Floor Plan

THE SMALL COB SERIES is a set of small cob structures designed to not require building permits.  The 2006 International Building Code (IBC), Section 105.2  allows for single story structures of 120 square feet or less inside the exterior walls to be constructed without a building permit .  The IBC is currently in effect in most U.S. locations.    If you desire to create a small cob building in a U.S. location subject to the building code, one of THE SMALL COB SERIES designs may be the answer to your wish.

Solar Oval One is a 120 interior sf. cob design intended to not require a building permit.    Please note that while the Solar Oval One design with the rear extending closet and sleeping loft is a stretch beyond meeting the 120 sf. / single story “no permit” standards, modifying the design to not include the closet and loft should meet the “no permit required” standards.   If you are interested in this design including the closet and loft, it should be verified as acceptable by your local building department.   If you local building official will only consider a strict negative interpretation of  IBC Section 105.2 for this design, you may want to consider SOLAR OVAL ONE without the loft & closet, or another of the designs in the SMALL COB SERIES.

Section looking North


The introduction of  irregular, non rectilinear and curved walls into your building creates a link between you, your building and the natural world. We are imperfect and irregular creatures of  living curves....sensuous, quirky and beautiful. Cob allows this to be expressed in your building.  This way of patterning a building is an important underlying concept in the design of  SOLAR OVAL ONE and the SMALL COB SERIES.

The materials needed to make a cob structure are generally very simple and there are many options:

  • The foundation serves to support the building and connects it to the ground.  In areas of low seismic activity. the foundation can be a simple rubble filled trench   In areas of high seismic activity, a continuous reinforced concrete foundation below all the walls is recommended.
  • The Base Wall serves to elevate the bottom of the cob wall and separate it from both ground moisture and splash back erosion from roof runoff.  Typically the material used is either stone or urbanite (broken concrete slab).  Base walls can be either dry laid or mortared.  Mortared construction is more stable than dry laid and is recommended. In areas of low seismic activity lime sand mortar can be used.  Portland cement mortar is recommended in areas of high seismic activity.
  • The Cob is a mixture of soil, water, sand / aggregate and fiber.  The soil needs to be clay bearing and in many cases can come directly from the construction site.  If the local soil is too sandy / silty and does not contain sufficient clay, a clay rich admix soil will need to be imported to the site.  Fresh water is best.  The sand / aggregate is best if it is angular, well graded and of 1/4” and smaller particle sizes.  The fiber is typically  wheat or rice straw.  Other fibers may be used subject to testing of samples.  The mixing occurs at the site and can be either manual foot / tarp mixing or machine mixed.  Considerable labor can be saved with machine mixing and cement mixers, mortar mixers, rototillers, tractors, bobcats, and silage mixers have all been successfully employed.
  • Including Seismic stability components in the construction is recommended in areas of high seismic activity.  They consist of: a continuous reinforced concrete foundation; vertical smooth #3 rebar foundation / wall / bond beam ties; a flexible wood bond beam at the top of the cob wall; and a wood roof diaphragm.  These components serve to tie together and keep in place the various parts of the structure if the building is fractured during an earthquake.
  • Windows and Doors can be a major cost if purchased new.  A few trips to local building material recyclers to find the windows and doors you need will help keep down the cost of your building.  Windows and doors are typically installed in wood frames (bucks) installed in the cob.  Glass without frames can also be installed directly in the cob to create windows.  Dual glazed glass is recommended for it's energy saving benefit.
  • The framing for your building is most easily accomplished with wood. Standard dimension lumber is readily available and allows for quick easy construction.  The use of sound recycled lumber is also possible and can reduce the cost of the materials.   If you have access to a forest area, small diameter trees are an exciting and beautiful way to create the wood portions of your building.  A higher level of carpentry skills is required to build with wood harvested from nature, but the result can be both low cost and stunningly beautiful.  
  • Cob is excellent for passive solar design due to it's heat storing thermal mass capacity. Cob is not a good insulator.  In temperate climates, the thermal mass of uninsulated cob walls works well to store the daytime passive solar heat gain and release it back into the interior at night.  In cold climates, wall insulation is needed, particularly on the non-solar North side of the building.   This insulation can be accomplished by constructing hybrid walls at those non-solar areas.   A wider foundation is constructed along the North perimeter and the exterior of the wall is wrapped with straw bales which are then plastered with a thin layer of cob.
  • Heat loss and gain thru the roof needs to be controlled in all US climates.  The roof can be insulated either within the roof structure, or above the roof.   In steep roof designs, this is best accomplished with a commercial fill insulation, of which there are numerous choices.  For those desiring a more natural & sustainable material, sheep's wool is a possibility.  The designs of the Small Cob Series allow for the depth of the roof framing to be varied to accommodate the insulation depth your climate demands.  In designs with less steep roofs, the use of a living roof can be used to control heat gain / loss.  The roof is covered with a layer straw or soil over a waterproof membrane. This layer is seeded with grasses and flowering plants which are allowed to grow.  The thermal mass or insulation of the living roof layer you choose serves to retard the thermal transmission thru the roof.   The result is a beautiful roof which changes with the seasons.  
  • Earthen floors provide thermal mass and are an important part of the passive solar design.  Earthen floors are both economical and beautiful.  When properly constructed and maintained, they are surprisingly durable.   Proper site drainage away from your building is important part of the successful use of earthen floors. 
  • The cost of creating a small cob building can vary greatly. If you have an experienced cob builder provide both the labor and materials to construct it for you, the cost should be about the same as a simple stick framed building of similar size.   This cost will vary depending on your location.  If you are considering this path, it is important to find a good builder in your area and develop a sound estimate of construction.  In 2009 in the San Francisco Bay Area, construction can easily run $200 per sf. This would give a potential cost of $24000. 
    Another possibility is to have a builder do the foundation and roof construction, while you build the cob walls.  Cob is easy to build, but  foundations and roof framing are more technical work which will benefit from expertise. This is an economic middle middle road by which both you & your building could benefit.
    Building your cob structure yourself is by far the most economical. If you can provide the labor to build, and are resourceful in your acquisition of materials, it may be possible to create your cob building for 1/10 th of what  it would cost to have a builder do the construction. It's a big project, but cob is forgiving, and can be done slowly.  Do it this way, take you time, stay sane and create a beautiful and inexpensive building.
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Review Plans* - PDF digital file

Review Plans* - 3 Sets of Prints

Extra Set of Prints
(only available with the purchase of above)

Construction Plans** - PDF digital file

Construction Plans** - 3 Sets of Prints

Extra Set of Prints
(only available with the purchase of above)


Plans are available as print sets or .pdf files.  Prints will be mailed to you and .pdf files will be sent via e-mail.

* REVIEW PLANS consist of : a floor plan, two elevations, two sections, and one or two perspective views.  These are appropriate for review with your local Building Department  to establish if you will be allowed a “no permit required” status for your project.  If your Small Cob Series building is to be an accessory structure on a property with an already existing building (a house or ?), it is important that you also review the project with your local Planning (Zoning) Department.  A site plan showing the proposed location on the property will most likely be needed for this review.  

**CONSTRUCTION PLANS will include all the plans, elevations, sections and details you will need to construct your cob structure.   

SITE PLANS AND CUSTOM CHANGES to these plans are available for a fee. If you are interested in this service,  please contact DreamGreenHomes with your request for an estimate of the fee involved. Click this link for Contact



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I specifically disclaim any warranty, either expressed or implied, concerning the information on these pages. Neither I nor any of the designer/architects associated with this site will have liability for loss, damage, or injury, resulting from the use of any information found on this, or any other page at this site. Kelly Hart, Hartworks, Inc.

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