This is what we’ve been saying all along but especially this week with all the news about the Spokane wildfires. So let’s explore wildfires a bit….
(Click here or just scroll down to go to the section on the difference between building with “fire resistant” materials vs R327 materials meant for Wildfire Urban Interface areas like where the Floral Hills Terrace subdivision would be built.)
Does adding a high density housing complex with 22 buildings, 40 household units, 116 bedrooms, accommodating 120-200 additional residents and 80 to 100 more cars to the low density neighborhood cause a higher fire risk just by virtue of the number of people and cars?
Yes it does! It’s one of the reasons the Springfield Fire Department is choosing to separate from the Eugene Fire Department. Eugene is focused on building more and more high density housing and Springfield notes how the resources to fight fires in high density subdivisions takes away from their ability to fight fires in the neighborhoods around Springfield, a city which is not pursuing those types of subdivisions. So let’s explore this a bit, especially how it’s related to the Wildfire Urban Interface where the Floral Hill Terrace subdivision would be located.
Research points to a firm YES — high density builds increase fire risk just by virtue of the number of people and cars! It’s not a popular answer in the current “build build build environment” of affordable housing developers but it is validated. Even if individual buildings are well-constructed, adding 120–200 residents, 80–100 vehicles, and 22 high-density structures to a low-density Wildland-Urban Interface (WUI) neighborhood directly increases wildfire risk during high fire danger weather.
This elevated risk is driven by three primary human and operational mechanisms:
1. Increase in Human Ignition Sources
Roughly 85% to 90% of all wildfires are caused by human activity. Adding 120–200 residents directly increases the statistical probability of localized accidental ignitions during extreme red-flag conditions, including:
- Vehicle-Related Sparks: Hot exhaust pipes or catalytic converters contacting dry roadside grass, flat tires running on rims, or towing chains dragging.
- Outdoor Equipment & Power Usage: Mowers, weed trimmers, outdoor power tools, outdoor grills, and increased demand on local electrical infrastructure.
- Human Error: Improperly discarded smoking materials, outdoor fire pits, or improper storage of combustible household goods / fuel canisters.
2. Vehicle Fuel Mass and Fire Intensity
Vehicles themselves act as concentrated, highly volatile fuel loads.
- An influx of 80–100 additional cars parked in close proximity to structures introduces significant quantities of gasoline, lithium-ion/lead batteries, rubber tires, and synthetic plastics.
- When a vehicle ignites in a dense parking lot or driveway during wind-driven ember storms, it produces intense radiant heat, toxic heavy smoke, and long-lasting flame intensity that can bridge gaps between structures or ignite nearby dry vegetation.
3. Evacuation Bottlenecks and Emergency Access Conflict
During high fire danger weather, wind-driven WUI fires often necessitate rapid, immediate evacuations.
- Egress Roadway Chokepoints: Low-density WUI neighborhoods are frequently served by limited, narrow access roads or single-entry/exit corridors. Adding 80–100 vehicles simultaneously attempting to evacuate can create gridlock, stranding residents in hazardous ember zones.
- Impeding First Responders: High vehicle traffic or street-parked cars narrow available roadway clearance, directly hindering incoming fire engines and emergency crews trying to access the fire perimeter.
Summary
While high-density housing reduces regional land footprint, at the localized neighborhood level in a WUI zone, adding dozens of households and nearly a hundred vehicles creates a higher concentration of potential ignition sources, heavy secondary fuel loads, and critical evacuation chokepoints.
Would a high density housing complex in a Wildfire Urban Interface zone increase fire risk during high fire danger weather IF IT IS BUILT WITH FIRE RESISTANT MATERIALS, BUT NOT UP TO THE R327 Wildfire Urban Interface guidelines?
YES, BUILDING A HIGH-DENSITY COMPLEX IN A WILDFIRE URBAN INTERFACE (WUI) ZONE WITHOUT ADHERING TO SPECIALIZED WUI ZONE GUIDELINES (LIKE R327) STILL INCREASES WILDFIRE RISK, EVEN IF STANDARD “FIRE-RESISTANT” MATERIALS ARE USED.
While general fire-resistant materials help protect against internal building fires or direct radiant heat, standard building practices often miss the specific vulnerabilities that cause structure loss during major WUI events—chiefly, wind-driven embers.
Why General “Fire Resistance” Isn’t Enough
Standard fire-resistive construction (such as 1-hour fire-rated drywall or standard ignition-resistant siding) primarily protects against structure-to-structure thermal radiation. However, WUI-specific codes like R327 (Wildfire Hazard Mitigation) target the primary way homes actually ignite during high-wind fire danger weather: ember intrusion and micro-ignitions.
If a structure is built without full WUI code compliance, key systemic vulnerabilities remain:
1. Vent Vulnerability (The Primary Ignition Pathway)
- Standard Code: Conventional building vents allow attic airflow but have mesh openings large enough (1/4 inch) to pull in blowing embers during high winds.
- R327 Standard: Mandates ember-resistant or specialized fine-mesh (1/16″–1/8″) corrosion-resistant vents.
- The Danger: In high-density developments, embers blowing through unhardened attic or soffit vents ignite interior roof structures, bypassing fire-resistant exterior walls entirely.
2. Eaves, Soffits, and Roof Gaps
- Standard Code: Often permits open rafter tails, combustible soffits, or standard roof flashing.
- R327 Standard: Requires fully enclosed soffits with non-combustible or ignition-resistant backing, plus bird-stops or sealed gap closures along tile and metal roof edges.
- The Danger: Wind-driven embers lodge under roof tiles or inside open eave pockets, igniting structural framing behind fire-rated exterior siding.
3. Glass Thermal Shock and Door Gaps
- Standard Code: Standard dual-pane or annealed glass windows can shatter rapidly when subjected to intense wind-driven heat.
- R327 Standard: Requires tempered glass on exterior windows and doors, along with tight perimeter seals.
- The Danger: Broken windows allow embers direct entry into apartment units, igniting interior furnishings.
How Density Amplifies the Non-Compliance Risk
When structures are non-compliant with WUI home-hardening standards, high density converts localized ignitions into rapid community conflagrations:
- Short Structure Separation Distance (SSD): High density naturally places structures closer together. If a single unit ignites through an unhardened attic vent or window, radiant heat and massive localized ember output rapidly ignite neighboring buildings.
- Combustible Attachments: Decks, balconies, and fences on high-density buildings create uninterrupted “fuel bridges” between units unless built strictly to WUI exterior projections standards.
- Compounded Evacuation Risk: Higher unit counts increase localized vehicle traffic. During high fire danger weather, an ignition within a non-code-compliant high-density complex can trigger emergency evacuations that gridlock narrow access roads, hampering emergency response.
Summary
| Building Aspect | Standard “Fire-Resistant” Construction | Full WUI Code (R327 Compliance) | Fire Risk Impact during High Winds |
| Attic & Soffit Vents | Standard mesh (allows embers) | Specialized ember-resistant baffle vents | High Risk: Interior roof ignition from blowing embers. |
| Eaves & Roof Edges | Open or standard soffit materials | Fully enclosed, non-combustible/ignition-resistant | High Risk: Embers trapped under eaves igniting framing. |
| Windows | Standard double-pane glass | Tempered safety glass on all exterior openings | Moderate-High Risk: Glass cracking opens interior to embers. |
| Overall Impact | Protects against radiant heat, vulnerable to embers |