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Permit Sonoma › Divisions › Engineering & Construction › Building › Technical Bulletins › B-31 2020 Residential Straw Bale Construction
Building Division

B-31 2020-Current: Residential Straw Bale Construction

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Appendix S Strawbale Construction has been adopted as part of the 2019 California Residential Code.  An architect or engineer is required for strawbale submittals per the California Business and Professions Code.

Version: 04/01/2020

Prescriptive Design

Appendix S allows for a prescriptive design without engineered structural design given that the limitations of the prescriptive design are adhered to.  Refer to Sections AS105, AS106, and R602.10 for these limitations.  Some examples of these limitations are: maximum one story, wall height, building height, amount of solid walls relative to openings for windows and doors, and location of solid walls along the building exterior and within the interior.

Nonstructural walls:  Section AS105.2 allows for nonstructural strawbale walls to be used in combination with structural walls made from wood framed braced panels.  The wood framed braced panels are designed prescriptively per R602.10.

Structural walls:  Section AS106.2 has requirements for strawbale walls when they are load bearing and/or used to withstand wind and seismic loads.

Although the design is prescriptive, an architect or engineer is still required per the California Business and Professions Code.

Engineered Design

Designs that exceed the prescriptive limitations shall be engineered and submitted under an Alternate Methods and Materials application per CBC 104.11.  Straw bale construction requires complete engineering analysis and structural details. Overall, the analysis shall include:

  • A written statement that from the engineer that in their professional opinion, the building elements under their review are equivalent in strength, durability, and seismic resistance of the building to those of a building designed according to the prescriptive provisions of the California Building Code.
  • Analysis of gravity loads, in-plane lateral loads, and out-of-plane loads
  • Connection detailing for vertical and lateral load paths to the foundation
  • Justification for the seismic design coefficients and factors used (i.e. Response Modification Coefficient, Overstrength Factor, Deflection Amplification Factor).
  • Material strengths used in the analysis shall be justified with supporting documentation such as engineering studies/references, code references, and tests.
  • Items requiring special inspection, frequency of inspection, structural observations, testing, etc.  Completion of the Statement of Special Inspections Form.
  • Installation requirements/instructions

When engineered, the non-structural sections of Appendix S still apply (e.g. moisture control, fire resistance, thermal insulation, etc.).

Special Inspection Requirements and Structural Observations

When the structure is engineered, special inspections shall be conducted during construction as noted in the Statement of Special Inspections Form.  At the close-in inspection, the building inspector shall be provided with the structural observation report, which shall address deficiencies or shall clearly state that the structural items meet the requirements of the applicable codes and the approved plans.

 

Permit Sonoma Building Inspection Requirements

In addition to the Building Inspection Procedures outlined in CNI-004.  The following inspections shall take place:

  1. The building inspector shall inspect the straw bale walls at the following times (AS105.7):
    1. After the foundation is poured and before the bales are placed; Sill Plate Anchors
    2. During the lay up of the straw bales; Pins
    3. At the close-in inspection, before any lath or wall finish is installed.
  2. At the lath inspection, the building inspector shall be provided with a report showing the results of moisture testing of the bales done on the day of the lath inspection. The building inspector will select the bales to be tested. Testing shall be done with a probe capable of being inserted into the center of the bales. The moisture content of the bale shall not exceed 20% of the total weight of the bale. (AS103.5)
  3. All lath (placement and attachment) shall be inspected by the building inspector before being covered.
  4. A final inspection shall be performed by the building inspector.

Commercial Strawbale Construction

All commercial construction shall be submitted with an Alternate Materials and Methods Application. Appendix S only exists in the California Residential Code and there is no analogous section in the California Building Code.  In addition, to the structural requirements in the Engineered Design Section above, the Alternate Materials and Methods Application for commercial construction shall also address compliance with non-structural items such as:  moisture control, fire-resistive construction, insulation values, long-term weathering, plumbing/electrical/mechanical installation within walls, etc.  The Guidelines for Straw-Bale Structures, Health and Safety Code Sections 18944.30 to 18944.41 can still apply.

References

Appendix S California Residential Code 2019

Health and Safety Code Section 18944.30 to 18944.401