2.1.6 Relevant Evidence from Recent California WUI Fires

2.1.6 Relevant Evidence from Recent California WUI Fires

Kasler and Reese (2019) present an analysis by McClatchy of CAL FIRE data and Butte County property records suggesting that 2008 additions to the California Building Code (Chapter 7A) effectively reduced damage to compliant homes built after the code went into effect. The database includes all homes in the Camp Fire perimeter and homes within 100 metres of the perimeter – close enough for embers to spark a new fire (e.g., see ESRI 2020).

Syphard and Keeley (2019) analyze building inspectors’ reports documenting homeowner mitigation practices for more than 40,000 wildfire-exposed structures from 2013 to 2018. They find that “structural characteristics explained more of a difference between survived and destroyed structures than defensible space distance [structure ignition zone in National WUI Guide nomenclature]. The most consistently important structural characteristics – having enclosed eaves, vent screens, and multi-pane windows – were those that potentially prevented wind-borne ember penetration into structures, although multi-pane windows are also known to protect against radiant heat.” Active firefighting sometimes proves crucial, but no single building feature appears to dominate survival. Notably, “while destroyed homes were preferentially included in the study, many ‘fire-safe’ structures, having > 30 metres defensible space or fire-resistant building materials, were destroyed.”

CAL FIRE (2020) provided the project team with a database of 1,065 buildings located within the boundary of the 2018 Camp Fire that had been inspected the year before the fire and immediately afterwards. The database represents a combination of data from CAL FIRE’s Damage Inspection Specialists (DINS) and Defensible Space (DSpace) databases. Only one field from the DINS data is included: whether the structure was destroyed or undamaged. Structures with intermediate levels of damage were omitted. The database contains twelve fields for each house from the DSpace databases, including the use of the structure, roof construction, eaves, vent screens, exterior siding, windows, deck and porch construction, patio cover construction, and fence construction. The database does not contain data about fuel near the structure.

In written and oral communication, Hawks (2020) provided a variety of other valuable information about WUI fires in California between 2013 and 2020. These data are now summarized here. CAL FIRE data shows that 93% of buildings that ignite are destroyed; 5% are affected (meaning repair cost between 1% and 9% of the house replacement cost), and the remaining 2% of buildings experience repair costs that are either minor (10% to 25% of house replacement cost) or major (26% to 50% of house replacement costs) (Steven Hawks, Staff Chief of Wildfire Planning and Engineering Division, CAL FIRE, December 3, 2020, oral commun.).

Figure 4A shows that approximately 51% of the 350 single-family homes built after 2008 within the boundary of the 2018 Camp Fire in Northern California escaped damage. Only 18% of the 12,100 homes built prior to 2008 escaped damage. Manufactured houses burned in nearly equal measure regardless of age.

Figure 4B (Hawks 2020) shows odds ratios (OR) attributable to various building features and defensible space (DSpace). Whiskers in the chart show the 95% confidence intervals (CI) for the odds ratios. Figure 4C illustrates how compliance with the 2008 California Building Code appears to reduce the likelihood of ignition and degree of damage in each of seven WUI fires. Figure 4D aggregates the data from the seven fires (Hawks 2020).

Note that the California Building Code’s requirements and DSpace data map heavily but still imperfectly resemble recommendations of the National WUI Guide. Especially noteworthy is the absence of information about combustible material immediately adjacent to the house, the area that the National WUI Guide refers to as priority zone 1A. Also note the distinction between damage severity and the probability of ignition. Figure 4E shows damage severity conditioned on ignition.

Figure 4: Some summary statistics from CAL FIRE about damage (A) to pre- and post-2008 buildings and (B) by construction feature, (C) and (D) within and near the 2018 Camp Fire perimeter and seven large fires in 2017–2018, and (E) all permanent structures greater than 120 sq ft within the perimeter of fires from 2014 through 2019. (B–E: Hawks 2020)

A

Built 2008 or later

No damage Affected/minor

Destroyed/major

Camp Fire Odds Ratio (OR) for each construction feature by sub-material and DSpace status for structures with a corresponding DSpace and DINS point.

50.6%

8.8%

40.6% Built before 2008

17.7%

3.3%

79.0%

BB

Odds Ratio

C

8

6

4

2

0 Deck/porch elevated Deck/porch on grade DSpace status Eaves Exterior siding Fence Patio/ carport cover

Roof construction Vent screen Window pane

Red points were variables with an OR and 95% CI greater than one, grey point CI fell within one, and blue point OR and 95% CI fell below one.

Composite Masonry/concrete No deck/porch Wood Composite Masonry/concrete No deck/porch Wood Compliant Non-compliant Enclosed No eaves Unenclosed Combustible Ignition resistant Combustible No fence Non-combustible Combustible No patio cover/carport Non-combustible Asphalt Concrete Metal Tile Wood Mesh screen <= 1/8\” Mesh screen > 1/8\” No vents Unscreened Multi-pane Single pane

CAL FIRE statistics and analysis

Percentage of structures by damage category inside or within 100 metres of the fire perimeter of the seven largest wildfires in 2017 and 2018

D

CAL FIRE statistics and analysis

Sum of damage/destroyed percentages to parcels for the seven largest wildfires (Atlas, Camp, Carr, Nuns, Thomas, Tubbs, Woolsey) in 2017 and 2018

E

Atlas Camp Carr Nuns Thomas Tubs Woolsey

Destroyed (>50%) Major (26-50%) Minor (10-25%) Affected (1-9%) No damage

Pre-2009 Post-2009 100%

75%

50%

25% 2%

0% 50%

46%

Pre-2009 30%

62%

Post-2009 6% Destroyed (>50%) Major (26-50%) Minor (10-25%) Affected (1-9%) No damage

Category of damage Single-family residences Multi-family residences Structure type

Mixed residential commercial

Nonresidential structures Other minor structures Infrastructure Total

% of residential structures damaged and destroyed

% of all structures damaged and destroyed

Destroyed (>50%) 27,185 282 16 956 10,329 17 38,785 92.77% 92.60%

Major (26-50%) 149 7 0 26 143 8 333 0.53% 0.80%

Minor (10-25%) 401 13 3 53 178 3 651 1.40% 1.55%

Affected (1-9%) 1,517 55 2 136 395 12 2,117 5.31% 5.05% Total 29,252 357 21 1,171 11,045 40 41,886 100.00% 100.00%

For example, fire destroyed 93% of the 29,252 single-family dwellings that experienced at least some damage in the fires of 2014–2019, but Figure 4D shows that no more than 54% of houses within fire perimeters of 2017–2018 experienced any damage. Insurers talk about this distinction as frequency (no more than 54% ignition) versus severity (93% total losses conditioned on ignition, 94% expected value of loss conditioned on ignition).

It can help to understand risk through a real-life example. Two personal stories follow that illustrate the value of fire-resistant construction.

Two personal stories from the 2018 camp fire

“I knew we were in the middle of the forest. Why wouldn’t you do everything you could to make it last?”

In November 2018, the deadliest and most destructive wildfire in California’s history ravaged Butte County, California. The Camp Fire burned the densely populated town of Paradise on November 8, leaving at least 85 dead and eventually spanning 240 square miles. It took 17 days to fully contain. CAL FIRE reports that the fire ultimately cost $16.5 billion and destroyed 18,793 structures. By December 12, 2018, FEMA approved $180 million in grants and loans for survivors. Federal and state agencies provided over $27.7 million in grants for home repair, replacement, and rental expenses. The Small Business Administration approved over $140 million in disaster loans for homeowners and businesses.

In an interview with the Sacramento Bee, two Paradise families reported having returned to their homes to find them almost untouched by the fire (Figure 5). Sean and Dawn Herr marvelled at their fortune, visiting their home in March 2019 to find it still standing (Figure 5A, B). They credit its survival to the gravel skirt encircling the house. Built in 2010, their home adheres to the 2008 California Building Code, whose provisions resemble the National WUI Guide in many ways, including fire-resistant roofing, siding, and other features. The code protected 51% of the 350 single-family homes built after 2008 in the Camp Fire’s path, while only 18% of the 12,100 homes constructed prior to the code’s enforcement survived.

5(A)

5(B)

The home of the Carrells, Paradise residents who fled to escape the Camp Fire, also complied with 2008 California Building Code requirements for houses in the wildland-urban interface and survived the fire. “I thought, ‘Oh, well, the house is done,’” said Oney Carrell (Figure 5C). “I knew we were in the middle of the forest. Why wouldn’t you do everything you could to make it last?’”

5(C)

Figure 5. Two houses in Paradise, California, built to comply with fireresistive standards of the 2008 California Building Code, survived the 2018 Camp Fire, while nearby houses burned down at a much higher rate. (A, B) The home of Sean and Dawn Herr, surrounded by its gravel apron. (C) The home of Oney and Donna Carrell. (Hector America, the Sacramento Bee via the Associated Press, reprinted with permission)