From Soil to Structure: Earthen Building Codes and Standards

If you're looking to understand earthen building codes and standards, the list below can serve as a helpful reference. Keep in mind, it's typically the engineer's responsibility to navigate and choose the right codes, so you won’t need to dive into all the details yourself. It’s all about making sure your home is safe, sound, and up to code!

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

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.

  1. 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.
  2. 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.
  3. 2021 & 2024 International Residential Code (IRC) Appendix AU: Cob Construction 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.
  4. 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.
  5. 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.
  6. Since 2015, the International Residential Code (IRC) addresses Light Straw-Clay Construction in Appendix R. If a local jurisdiction has not adopted this appendix, an engineer can still reference it 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.
  7. Since 2015, the International Residential Code (IRC) addresses Strawbale Construction in Appendix S. If a local jurisdiction has not adopted this appendix, an engineer can still reference it 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.
  8. 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.
  9. 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.
  10. 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.
  11. Alternative Compliance – In areas where earth blocks are less common, they may be approved under local building code amendments or through the Alternative Materials, Design, and Methods of Construction and Equipment provisions of the IBC and IRC. These provisions allow for non-standard materials like compressed earth blocks, provided that an engineer demonstrates their safety and effectiveness through design and testing.
  12. 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.
  13. The International Energy Conservation Code (IECC) includes specific guidelines for energy efficiency, and in its R-value chart, it designates a column for '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.
  14. 2020 New Zealand Standards: NZS4297 Engineering Design of Earth Buildings; NZS4298 Materials and Construction of Earth Buildings; NZS4299 Non-Specific Design of Earth Buildings.
  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. To bridge this gap, The Masonry Society’s Earthen Modular Masonry Committee (EMMC) is developing standards tailored to earthen materials, including CEBs. These efforts aim to integrate CEBs into mainstream masonry practices, ensuring they meet structural requirements while maintaining the unique benefits of earthen construction.
R Value Chart Mass Wall Column

16. 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. 

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.

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.