WUI-compliant exterior wood in California: what architects and contractors need to know

​Recent wildfire activity across California, including the devastating fires of recent years, has reshaped the conversation around exterior building materials. For architects and contractors working in or near Fire Hazard Severity Zones (FHSZ) and Wildland-Urban Interface (WUI) designated areas, the question is no longer whether fire resistance matters. It is about which materials can meet code requirements while still delivering the aesthetic and performance qualities that clients expect.

Real wood has long been considered incompatible with WUI construction, but that is changing. At Black’s Farmwood, we have been focused on sourcing exterior wood solutions that can meet WUI requirements without sacrificing design intent. This post outlines what is available, what the certifications mean, and what questions should be asked before specifying exterior wood in fire-prone areas.

A note before we begin: building codes, fire-hazard designations, and compliance requirements vary by jurisdiction throughout California. The information below is intended as a general resource. Always verify specifications and compliance with your local building department, CAL FIRE, and the Authority Having Jurisdiction (AHJ) before specifying any material for a WUI or FHSZ project.

Understanding the regulatory landscape:
California’s fire safety framework for buildings in high-risk areas underwent a significant reorganization effective January 1, 2026. Requirements that were previously housed in Chapter 7A of the California Building Code have been consolidated into the California Wildland-Urban Interface Code (CWUIC), Title 24 Part 7. The underlying technical standards remain largely unchanged, but the way they are referenced has. Architects and contractors should reference the CWUIC rather than Chapter 7A going forward.
The CWUIC applies to new construction, additions, and exterior alterations in State Responsibility Areas (SRA) and in Local Responsibility Areas (LRA) designated as Very High Fire Hazard Severity Zones. Materials used for exterior siding, decking, and cladding in these areas are generally required to be ignition-resistant, a standard that untreated wood typically cannot meet.
Fire Hazard Severity Zones are classified as Moderate, High, or Very High. For most WUI compliance purposes, the Very High designation is the critical threshold, though local jurisdictions may impose additional requirements. Updated maps released in recent years have significantly expanded designated areas across the state, meaning many projects that previously fell outside these requirements are now subject to them.
The key certification to look for is a Class A fire rating, which represents the highest level of fire resistance under ASTM standards. For wood products, the relevant test is often ASTM E2768, an extended 30-minute E84 surface burning test conducted after accelerated weathering per ASTM D2898. Products that pass this test following weathering demonstrate sustained fire resistance under real-world conditions, not just in ideal laboratory settings.
It is worth noting that many products marketed as “California WUI compliant” have only passed the standard 10-minute E84 test. The extended 30-minute test after weathering is the higher bar, and the one most relevant to ignition-resistant performance under the CWUIC.
CAL FIRE maintains a list of approved materials through the Office of the State Fire Marshal (OSFM) Building Materials Listing (BML) program. Products with BML listings have been evaluated for use in California’s fire zones. BML listing is voluntary for most exterior materials. Products not on the list may still comply if documentation and test results are submitted to and accepted by the AHJ. Final confirmation of compliance for any specific project should always come from the AHJ.

What matters beyond the product:
In many WUI applications, compliance is tied to the full wall assembly, not just the exterior material. This includes the underlying sheathing, WRB, furring, and installation method.
A Class A-rated product does not automatically create a compliant assembly. Approval ultimately rests with the Authority Having Jurisdiction.
We recommend that architects and contractors confirm assembly compliance directly with their local building department or fire official.

What we carry for WUI projects:
Black’s Farmwood currently offers several product options for WUI and fire-zone construction. Each has different performance characteristics, aesthetics, and compliance profiles.

Saferwood (Class A fire-retardant treated wood)
Produced by Chemco, Inc. using its proprietary Thermex-FR fire-retardant, Saferwood is our most established WUI product. Thermex-FR is pressure-treated into the wood and kiln-bonded at the molecular level. It is not a surface coating, and the protection is designed to be long-lasting for the life of the wood. The treatment is non-leachable, non-toxic, and non-hazardous per independent third-party testing.
Saferwood carries a Class A extended 30-minute rating and is listed by the CAL FIRE State Fire Marshal. At Black’s Farmwood, our primary Saferwood species include thermally modified vertical-grain hemlock and STK western red cedar, available in profiles such as nickel-gap and tongue-and-groove. Douglas fir is also available.
Saferwood remains one of the most direct paths to achieving WUI compliance while maintaining a true wood aesthetic.

Thermory + Saferwood (Thermex-FR)
Thermory USA has partnered with Saferwood by Chemco to introduce fire-retardant treatment options within their thermally modified wood product lines. Through this collaboration, Thermory + Saferwood products are intended to deliver ignition-resistant, Class A performance for exterior applications in wildfire-prone environments.
Select Thermory products are available with Thermex-FR treatment, though profiles, availability, and specific applications may vary. Not all Thermory products are treated, and the use of fire-retardant protection is dependent on the product configuration and project requirements.
We recommend confirming current product options, testing documentation, and availability based on your specific project and timeline.

FSC-certified Ipe and Cumaru (naturally dense hardwoods)
Ipe and Cumaru are among the densest naturally occurring hardwoods available for exterior use. Their density and natural oil content contribute to inherent fire resistance without chemical treatment. Both species have achieved Class A ratings under ASTM E84 testing and have been listed by the California Office of the State Fire Marshal.
Black’s Farmwood is Chain of Custody FSC-certified, meaning our Ipe and Cumaru carry full FSC documentation, a meaningful credential for LEED projects and sustainability-focused clients.
While these species can achieve Class A ratings, their acceptance in WUI applications should be confirmed with the AHJ, as requirements may extend beyond surface burning characteristics.

Accoya
Accoya is an acetylated wood produced from sustainably sourced radiata pine. The acetylation process transforms the wood’s cell structure, dramatically improving dimensional stability, durability, and resistance to moisture and decay. Accoya is FSC-certified and carries strong sustainability credentials.
Its fire resistance characteristics should be verified for specific WUI applications and jurisdictions. We recommend requesting current technical documentation directly from Accsys and confirming suitability with the AHJ.
Accoya may still play a role in WUI-adjacent applications where durability and stability are prioritized alongside broader fire-safe design strategies.

Questions to ask before specifying exterior wood for a WUI project:
When evaluating any exterior wood product for a fire zone application, consider the following:
• Does the product carry a Class A rating after accelerated weathering, not just the standard 10-minute E84 test
• Is it listed by CAL FIRE OSFM or supported by documentation for AHJ approval
• Is the treatment integral to the wood or surface-applied
• What installation details affect long-term performance
• Has the AHJ approved it for this specific project

​Why real wood is back in the conversation:
For years, architects and contractors defaulted to fiber-cement, composite, or steel cladding. Not because they preferred them, but because compliant wood options were limited. That constraint is starting to change.
The combination of fire-retardant treatment technology, thermally modified wood science, and the natural properties of dense hardwoods has created a new category of exterior materials that can meet California’s most demanding fire zone requirements while retaining the warmth, texture, and performance of real wood.
Common mistakes we see
• Assuming a product rating equals assembly compliance
• Not confirming approval with the AHJ early
• Overlooking how weathering affects fire performance
• Relying on outdated or incomplete testing documentation

At Black’s Farmwood, we have been sourcing and supplying specialty and reclaimed wood for nearly 30 years. The emergence of WUI-compliant wood products is one of the most important shifts we have seen in that time, both for the industry and for the clients we serve. We are committed to staying current on certifications, code changes, and product availability so that the architects and contractors we work with have accurate, up-to-date information when it matters most.

​To discuss any of these products for an upcoming project, reach out to us directly. Our showroom in Petaluma operates by appointment.

​Disclaimer: Black’s Farmwood provides this information as a general educational resource. Product certifications, code compliance, and fire zone designations are subject to change and vary by jurisdiction. This content does not constitute professional code compliance advice. Always consult your local building department, CAL FIRE, and the Authority Having Jurisdiction for project-specific guidance.