Reference
Wildfire Defense Glossary
Definitions for 34 terms used in California wildfire mitigation, home hardening, defensible space, exterior sprinkler systems, and wildfire insurance. Compiled by FireRoofs for Bay Area homeowners.
Fire Science
(7)- Wildland-Urban Interface (WUI)
- The zone where wildland vegetation meets residential or commercial development. California maps WUI areas using Fire Hazard Severity Zones (FHSZ). Homes in WUI zones face elevated wildfire risk and are subject to specific building codes and defensible space requirements.Bay Area WUI communities →
- Ember Intrusion
- The process by which wind-carried burning particles (firebrands) enter a structure through vents, gaps, eave openings, or other vulnerabilities. Embers can travel over a mile ahead of the fire front and cause up to 90% of home ignitions during wildfires.Ember protection guide →
- Radiant Heat Flux
- Thermal energy transferred through electromagnetic radiation from a fire to nearby structures. Measured in kilowatts per square meter (kW/m2). At approximately 12.5 kW/m2, wood can auto-ignite without direct flame contact. Defensible space reduces radiant heat exposure.
- Firebrand
- A burning piece of material (bark, wood, debris) carried by wind during a wildfire. Firebrands are the primary mechanism of spot fires and structure ignitions ahead of the main fire front. Ember-resistant vents and mesh screens block firebrand entry.
- Spot Fire
- A new fire ignited by airborne embers or firebrands landing ahead of the main fire front. Spot fires can occur hundreds of feet to over a mile from the active fire line, making them unpredictable and difficult to suppress.
- Fire Behavior Triangle
- The three factors that determine wildfire behavior: fuel (vegetation type and moisture), weather (wind, temperature, humidity), and topography (slope, aspect, elevation). Understanding these factors helps predict fire spread patterns.
- Slope Effect
- Fire travels faster uphill because rising hot air preheats vegetation above. A fire on a 30% slope can spread twice as fast as on flat ground. Many Bay Area hillside communities face elevated risk due to steep terrain.
Defensible Space
(6)- Defensible Space
- The buffer zone around a structure where vegetation, debris, and combustible materials are managed to slow fire spread and reduce ignition risk. California law (PRC 4291) requires defensible space extending at least 100 feet from structures in SRA and VHFHSZ areas.Defensible space guide →
- Zone 0 (Ember-Resistant Zone)
- The area within 0 to 5 feet of a structure. Requires noncombustible ground surfaces (gravel, concrete, pavers), no combustible fencing attached to the home, removal of all dead vegetation, and fire-resistant materials at the ground-to-wall junction. Adopted in California in 2023 under AB 3074.Defensible space zones →
- Zone 1 (Lean, Clean, Green Zone)
- The area from 5 to 30 feet around a structure. Requires reduced vegetation density, removal of dead plant material, tree limbing to 6 feet above ground, and 10-foot spacing between tree canopies. Combustible items like firewood stacks and propane tanks must be moved outside this zone.
- Zone 2 (Reduced Fuel Zone)
- The area from 30 to 100 feet around a structure. Requires vegetation thinning, grass mowed to 4 inches or less, brush spacing of at least 2x the height of the plants, and removal of ladder fuels that allow ground fire to reach tree canopies.
- Ladder Fuels
- Vegetation arranged vertically that allows fire to climb from ground level into tree canopies. Examples include brush growing beneath trees, vines on trunks, and low-hanging branches. Removing ladder fuels is a key defensible space requirement in Zones 1 and 2.
- PRC 4291
- California Public Resources Code Section 4291. The state law requiring property owners in State Responsibility Areas (SRA) and Very High Fire Hazard Severity Zones (VHFHSZ) to maintain defensible space of at least 100 feet around structures. Enforced by CAL FIRE and local fire agencies.
Home Hardening
(7)- Home Hardening
- The process of upgrading a structure's exterior materials and construction details to resist ignition from embers, radiant heat, and direct flame contact. Includes roof, vents, eaves, siding, windows, doors, decks, and fencing upgrades using fire-resistant or noncombustible materials.Home hardening assessment →
- Chapter 7A (CBC)
- Chapter 7A of the California Building Code. Governs exterior construction materials and methods for buildings in Very High Fire Hazard Severity Zones (VHFHSZ) and State Responsibility Areas (SRA). Covers roofing, exterior walls, eaves, vents, decking, windows, and doors. Applies to new construction and significant remodels.Home hardening products →
- Ember-Resistant Vent
- A vent assembly designed to prevent ember entry into attics, crawlspaces, and wall cavities during a wildfire. Must meet ASTM E2886 (Standard Test Method for Evaluating the Ability of Exterior Vents to Resist the Entry of Embers and Direct Flames). Vulcan Vents is one manufacturer that meets this standard.
- Class A Roof
- The highest fire resistance rating for roofing materials, as classified by ASTM E108. Class A roofing withstands severe fire exposure including burning brands, radiant heat, and intermittent flame. Materials include concrete tile, clay tile, metal roofing, and certain asphalt fiberglass shingles.
- Ignition-Resistant Construction
- Building methods and materials that reduce the likelihood of a structure igniting from wildfire exposure. Includes noncombustible siding (fiber cement, stucco), tempered or dual-pane windows, enclosed eaves, and noncombustible decking. Does not mean fireproof.
- BML (Building Materials Listing)
- The California State Fire Marshal's listing of building materials approved for use in WUI areas. Products with a BML number have been tested and certified to meet specific fire resistance standards for their category (roofing, siding, decking, vents, etc.).BML-listed products guide →
- Open Eave
- An eave construction where rafter tails and roof sheathing are visible from below. Open eaves create gaps where embers can accumulate and ignite. Enclosing open eaves with noncombustible material (fiber cement soffit panels) is a common home hardening retrofit.
Active Defense
(5)- Exterior Wildfire Sprinkler System
- An automated system that saturates a structure's roof, eaves, walls, and perimeter with water (and optionally Class A foam) to prevent ignition during a wildfire. Unlike interior fire sprinklers, these systems defend against external fire exposure from embers and radiant heat.Exterior sprinkler systems →
- Class A Foam
- A biodegradable firefighting foam approved by the U.S. Forest Service that reduces water's surface tension, allowing it to penetrate deeper into combustible materials. When mixed into an exterior sprinkler system, Class A foam clings to surfaces longer than water alone and creates a more durable protective barrier.Class A foam explained →
- Dual Detection
- A wildfire detection approach that combines two independent sensor types, typically satellite monitoring and on-property cameras. Satellite monitoring detects fires within a wide radius, while cameras confirm the threat at the property level. Dual detection reduces false activations.How it works →
- Perimeter Sprinkler
- A ground-level sprinkler positioned along a property's boundary to wet surrounding vegetation, creating a moisture buffer between the structure and approaching fire. Perimeter sprinklers complement roof and eave sprinklers by addressing vegetation ignition risk.
- Structure Saturation
- The process of thoroughly wetting a building's exterior surfaces (roof, walls, eaves) before a fire arrives. Increasing the moisture content of building materials raises their ignition temperature and makes them harder to ignite from embers or radiant heat.
Insurance
(9)- FAIR Plan
- The California Fair Access to Insurance Requirements plan. The state-mandated insurer of last resort for property owners unable to find coverage in the private market. Currently caps dwelling coverage at $3 million. Offers up to 16.4% wildfire portion premium discounts for qualifying mitigation measures.FAIR Plan and insurance →
- CDI Regulation 2644.9
- A California Department of Insurance regulation requiring all admitted property insurers that use wildfire risk factors in pricing to offer premium discounts for documented wildfire mitigation measures. Identifies 12 specific measures across three categories: community designations, defensible space, and home hardening.Insurance discounts guide →
- Safer from Wildfires
- The California Department of Insurance framework establishing standards for wildfire mitigation discounts. Created under CDI Regulation 2644.9, it defines 12 qualifying mitigation measures that insurers must recognize when setting premiums for properties in wildfire-prone areas.
- AB 38 (Wildfire Disclosure)
- California Assembly Bill 38 requiring sellers of property in high or very high fire hazard severity zones to provide documentation of defensible space compliance to buyers. Also requires a defensible space advisory for properties in SRA.
- AB 888 (Safe Homes Act)
- The California Safe Homes Act establishing the Safe Homes Grant Program within the Department of Insurance. Provides grants to qualifying homeowners for fire-safe roof replacement and Zone Zero mitigation measures.
- SB 429 (Wildfire Catastrophe Model)
- California Senate Bill 429 establishing the state's first public wildfire catastrophe model. Gives consumers transparency into the wildfire risk algorithms insurers use to set rates. Effective January 1, 2026.
- Fire Hazard Severity Zone (FHSZ)
- A geographic classification assigned by the California Office of the State Fire Marshal based on vegetation, topography, climate, and historical fire data. Zones are classified as Moderate, High, or Very High. FHSZ designation determines building code requirements and insurance risk classification.Check your fire zone →
- Evidence Packet
- A documentation package prepared by FireRoofs at system commissioning. Includes system specifications, material data sheets, commissioning test results, progress photography, regulatory alignment matrix mapped to CDI Regulation 2644.9, and licensed contractor verification. Designed for direct submission to insurance carriers or brokers.
- E&S Carrier (Excess and Surplus)
- Specialty insurance carriers not regulated in the same way as admitted (standard) insurers. E&S carriers serve high-risk or hard-to-place properties that standard carriers decline to insure. They often offer more flexible underwriting but at higher premiums.
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