From Soil to Structure: Earthen Building Codes and Standards

The list below highlights key earthen building codes and standards. If it feels like a lot to digest, remember that sorting through these requirements is usually the designer’s or engineer’s job. Hire a qualified professional who can navigate the details and ensure your project stays safe, well built, and fully code compliant.

design to building flowchartUnderstanding and applying earthen building codes and standards turns the question “Can it be permitted?” into a confident yes, carrying your build from drawing board to finished home.

Earthen Building Codes and Standards plus other Natural Building Codes & Developments

Extruded Earth Specimen

Every state and county adopts different versions of building codes, which may vary by the year of adoption or through local amendments. This means that even though the codes are based on national standards like the IBC or IRC, the specific requirements in your area could be different. It's important to check with your local building authority to ensure you're following the most current and applicable codes for your project. If a local jurisdiction has not adopted any of the following appendixes, an engineer can still reference them as an alternative design method. The building official has the final say and can approve it if the engineer can show the design meets the required safety and performance standards.

  1. Cob Construction (Monolithic Adobe): 2021 & 2024 International Residential Code (IRC) Appendix AU provides standards for building with cob, a natural material made from clay, sand, and straw. The appendix covers design, construction methods, and structural requirements for cob buildings, including the use of cob blocks or bricks. 2024 IRC: The appendix was renumbered to Appendix BK.
  2. The 2021 New Mexico Earthen Building Code (NMAC 14.7.4). This code specifically addresses adobe, rammed earth, and other earthen construction in New Mexico. It’s one of the few codes in the U.S. that directly applies to CEBs and ensures structural and safety guidelines for earthen materials.
  3. The 2021 New Mexico Historic Earthen Buildings Code is a specialized set of regulations designed to preserve and maintain historic earthen structures, including those built with materials like adobe, rammed earth, and other earthen masonry. The code is tailored to New Mexico, a state with a rich architectural history deeply rooted in adobe construction, especially for historic and cultural structures.
  4. Light Straw-Clay Construction: 2015 & 2018: the International Residential Code (IRC) Appendix R. 2021 IRC: Appendix AR. 2024 IRC: Appendix BI. Provides guidelines for using light straw-clay as a non-bearing building material and wall infill system.
  5. Strawbale Construction: 2015 & 2018: the International Residential Code (IRC) Appendix S. 2021 IRC: Appendix AS. 2024 IRC: Appendix BJ. Provides prescriptive and performance-based requirements for using baled straw as a building material.
  6. Hemp-Lime (Hempcrete) is officially included in the 2024 International Residential Code (IRC) under Appendix BL, which allows it to be used as a non-load-bearing wall infill system in one- and two-family homes and townhouses, especially in areas with low seismic risk. For regions with higher seismic activity or taller buildings, engineered designs are required. This inclusion simplifies the permitting process for residential hempcrete projects, although local jurisdictions must adopt the appendix. While hempcrete is not yet included in the 2024 International Building Code (IBC) for commercial structures, efforts are ongoing to add it in future code cycle.
  7. Section R403.5 of the International Residential Code (IRC) addresses Crushed Stone Footings, specifically allowing for rubble or gravel trenches as an alternative to traditional concrete footings. The use of crushed stone can enhance drainage around the foundation, reducing the risk of water-related issues.
  8. Local Amendments: Many jurisdictions adapt the IBC and IRC with local amendments. For instance, counties with historic use of earthen materials may have specific regulations for earth blocks based on local climate, freeze/thaw concerns, and building traditions. To find out what amendments a particular county has adopted, check with the local building department or review their published building codes and ordinances, which are often available on the county's official website.
  9. Alternative Compliance: In regions where earth blocks are less commonly used, their approval may require adherence to local building code amendments or compliance through the Alternative Materials, Design, and Methods of Construction and Equipment provisions outlined in the International Building Code (IBC) and International Residential Code (IRC). These provisions allow the use of non-standard materials, such as compressed earth blocks, provided their safety and effectiveness are demonstrated through engineering design and testing. An AMMR request (Alternative Materials, Methods, and Requests) is a formal process defined under Section 104.11 of the IBC. This request is submitted to a building official to seek approval for materials, designs, or construction methods not explicitly covered by the building code. The submission must include evidence, such as engineering analyses, test results, and supporting documentation, to demonstrate that the proposed alternative meets or exceeds the intent of the building code in terms of safety, performance, and compliance.
  10. ASTM E2392/E2392M-23: Standard Guide for Design of Earthen Wall Building Systems  serves as a guide for the design of earthen wall building systems, covering technical requirements for systems like adobe, rammed earth, and cob. While it is not directly referenced in the IRC or IBC, it provides valuable guidance for ensuring the safety and performance of earthen building materials, particularly when seeking alternative design approvals.
  11. ASTM E2393 - Standard Guide for Earth Construction focuses on testing methods, including performance evaluations, and ensuring that materials and construction practices meet building regulations.  
  12. 2020 New Zealand Standards: NZS4297 Engineering Design of Earth Buildings; NZS4298 Materials and Construction of Earth Buildings; NZS4299 Non-Specific Design of Earth Buildings.
  13. The International Building Code (IBC) Chapter 21 Section 2109 Empirical Design of Adobe Masonry: This section provides design guidelines for adobe masonry construction, which can also be applied to compressed earth blocks, especially regarding wall thickness, height, and structural safety.  Note: The 2024 IBC update emphasizes limits on the use of empirical methods based on factors such as wind speed and seismic activity, reinforcing the need for engineered solutions in certain conditions.
  14. The 2018 International Residential Code (IRC) Section R606.12 covers specific guidelines for adobe masonry. However, adobe construction is not explicitly mentioned in the 2021 IRC.
  15. TMS 402-16 Building Code Requirements for Masonry Structures provides guidelines for traditional masonry materials like concrete and clay units but does not specifically address compressed earth block (CEB) construction.
  16. It's important to note that Appendix A, which previously addressed empirical design, has been removed from the 2022 edition of TMS 402. As a result, IBC Section 2109.1 (which references Appendix A for adobe masonry design) now points to content that no longer exists in the current TMS standard. This creates a gap for jurisdictions that adopt the 2022 edition of TMS 402 but still rely on IBC Section 2109. To bridge this gap, jurisdictions may choose to continue referencing TMS 402-16 (or earlier editions) to preserve empirical design provisions for earthen masonry, rely on engineered design methods in TMS 402-22, or apply the remaining prescriptive requirements in IBC Section 2109 where applicable.
  17. The Earthen Modular Masonry Committee (EMMC) of The Masonry Society (TMS) is actively working to develop a reference standard for the design of earthen masonry wall systems, including adobe and compressed earth block. This initiative aims to establish prescriptive design criteria for earthen masonry, facilitating its integration into building codes and allowing for the retirement of references to the older TMS 402-16 standard. 
  18. The International Energy Conservation Code (IECC) provides standards to improve building energy efficiency by defining the required insulation levels for various types of walls, including mass walls. This distinction is important for earth block homes, as mass walls, with their unique thermal properties, typically have lower R-value requirements compared to other wall types.
Mass Wall Callout in R Value ChartIECC R-value requirements for mass walls by climate zone are an essential benchmark when working with earthen building codes and standards. Source: 2024 International Energy Conservation Code (IECC), Table R402.1.2.

Key Points of the IBC 2021 Chapter 21, Section 2109: Empirical Design of Adobe Masonry

The 2021 International Building Code, specifically section 2109, covers the empirical design of adobe masonry, providing detailed regulations for its construction. Below is only a brief summary that may omit important details. For comprehensive information, please consult the official 2021 IBC, available through resources such as the ICC Digital Codes Library.

Limitations and Necessary Engineering

  • Empirical design is restricted by limitations in Section A.1.2 of TMS 402. For structures exceeding these limits, alternative engineered designs are required. 
  • Adobe buildings are generally limited to one story unless designed by a registered professional for two stories.

Material Properties and Construction

1. Unstabilized Adobe 

  • Units should have a compressive strength of at least 300 psi, with individual units not dropping below 250 psi.
  • The average modulus of rupture is set at 50 psi, with the minimum allowed at 35 psi.
  • A moisture content cap of 4% by weight is required.
  • Units should not have more than three shrinkage cracks, nor any crack over 3 inches long or 1/8 inch wide.

2. Stabilized Adobe

  • Soil for stabilized adobe should be compatible with stabilizing chemicals.
  • Absorption requirements dictate a maximum of 2.5% moisture absorption by weight over seven days.
Earth Block Section Detail

Structural and Construction Details

  • Walls need to be mounted on foundations rising at least 6 inches above the ground and made of non-adobe materials.
  • Exterior walls require a minimum thickness of 10 inches, and interior load-bearing walls require 8 inches, with the unsupported wall height not exceeding tenfold their thickness. 
  • Adobe walls need lateral support at certain intervals.
  • Parapet walls must be waterproofed.
  • Paints and plasters must conform to specified standards in thickness and material types, with stipulations on the types of plaster suitable for adobe walls. 
  • Exterior walls require weather-protective finishes, particularly unstabilized adobe walls.

These guidelines ensure that adobe masonry is constructed to meet safety and durability standards appropriate for the climate and resources available.

Want to help advance earthen building codes and standards? It's an important step toward making sustainable construction more mainstream. Dive deeper by reading Demystifying Code Development for Earthen Construction in the United States by Anne Galer and Ben Loescher, The Earthbuilders' Guild.