In recent years, several U.S. states have adopted new wildfire codes for construction in wildland-urban interface (WUI) areas. These codes determine which building materials may be specified for exterior siding, and understanding definitions such as Class A siding is more important than ever.
Class A siding material meets the highest possible fire resistance rating. While many new codes require ignition-resistant siding, factors such as the test method and treatment process used to achieve Class A are critical to code compliance.
Key Takeaways
- To qualify as Class A siding, materials must pass an ASTM surface burning test with low flame spread and smoke development.
- Materials may achieve Class A flame spread via pressure treatment, surface coating, or naturally.
- The Colorado Wildfire Resiliency Code requires exterior wall coverings be noncombustible, ignition-resistant, or fire-retardant-treated wood (FRTW).
- The Class A rating alone is not sufficient for siding material under the CWRC.
Class A Fire Rating
Class A siding meets the highest possible classification for fire resistance. To qualify as Class A, materials must pass an ASTM surface burning test and meet performance requirements for flame spread and smoke development.
ASTM Surface Burning Test
In the standard ASTM E84 test, also known as the Steiner Tunnel Test, a building material is exposed to a controlled surface burn for a period of 10 minutes. The ASTM E2768 test is an extended version of the same test and lasts for 30 minutes.
When verifying Class A siding test results, be sure to check the duration of the test, as some building codes like the Colorado Wildfire Resiliency Code require the extended version.
Flame Spread vs. Smoke Development
Both the ASTM E84 and ASTM E2768 tests measure the flame spread and smoke development of a material. In order to achieve Class A, the material must exhibit a flame spread index (FSI) between zero and 25 and a smoke development index (SDI) between zero and 450. Materials are classified as Class A, Class B, or Class C, with Class C material having the lowest fire resistance.
While a material must technically meet flame spread and smoke development requirements to achieve a full Class A rating, some wildfire and WUI codes only require materials with Class A flame spread.
How Materials Achieve Class A
Siding materials can achieve Class A in three ways:
- Natural Ignition Resistance
Some materials don’t require modification to achieve Class A flame spread. For example, certain tropical hardwood species like Red Balau are naturally ignition-resistant due to their high density and tight cellular structure.
- Surface Coating
Surface-applied coatings can help siding materials meet Class A flame spread per ASTM E84. These coatings are designed to repel fire.
The downside with surface-applied coatings is longevity. These coatings need to be re-applied over time, and intense weathering can cause them to degrade rapidly. Additionally, siding materials with surface-applied Class A coatings don’t always meet wildfire code requirements.
- Pressure Treatment (FRTW)
Although most natural wood species such as cedar or hemlock are naturally combustible, wood materials may still achieve Class A flame spread through pressure treatment. This process involves impregnating wood materials with fire-resistant chemicals, which permanently alters the material’s chemical structure and surface burning characteristics.
Fire-retardant-treated wood (FRTW) meets Class A flame spread per ASTM E2768 as well as additional requirements for extended weathering.
The Colorado Wildfire Resiliency Code
The Colorado Wildfire Resiliency Code is the first statewide building code for construction in wildland-urban interface (WUI) areas. The CWRC includes new minimum requirements for structure hardening and defensible space.
CWRC Siding Requirements
Under Class 2 structure hardening, which applies in Moderate and High Fire Intensity areas on the CWRC map, exterior wall coverings (siding) must be one of the following materials:
- Noncombustible material
- Ignition-resistant material
- Fire-retardant-treated wood (FRTW)
The Class A rating alone is not enough for siding material to meet CWRC requirements. Ignition-resistant material and FRTW both require 30-minute surface burning tests, and FRTW must be pressure treated.
Local Code Interpretations
When submitting project plans for permit, be sure to check with your local building department and verify which siding materials they allow. Code official interpretations of the CWRC may vary from county to county, and some jurisdictions may adopt stricter requirements.
Summary: Know Your Materials
Test standards for building materials can be confusing. For projects in wildfire-prone areas, be sure to review applicable local building codes, surface burning performance standards, and material requirements. The Class A rating alone may not be sufficient.
With a bit of due diligence, design and construction professionals can confidently specify fire-resistant siding materials that resist flame spread and provide long-term protection for residents.
Explore Wood Source Class A wood siding solutions on our Fire Resistant Wood page.
