In mid-September I participated in a design charrette in Malibu, California to address the reconstruction of houses destroyed in the Woolsey Fire of 2018—which destroyed over 500 homes in this narrow, coastal city made famous by its movie star residents. Most of the houses destroyed by fire weren’t owned by Hollywood movie stars, and their owners are struggling with the prospect of rebuilding with insurance coverage that doesn’t begin to replace what they had.

The charrette was funded by the Malibu Foundation and organized by Bill Browning of Terrapin Bright Green. Bill has led dozens of charrettes over the past several decades, many of them through the Rocky Mountain Institute, and I’ve had the good fortune to work with Bill on a number of those.

But I had never been involved with projects where wildfire is such a pervasive threat, so I spent a lot of time learning about what it takes to build and manage houses that will be less likely to be damaged by wildfire, and I learned a lot more in Malibu. I share some of what I learned here.

While my focus at the charrette was wildfire resilience, the charrette also focused on energy efficiency—since the latest version of Title 24, calling for net-zero-energy performance, takes effect in January 2020—before most of the destroyed houses will be rebuilt.

The guidance below isn’t comprehensive, but it will provide a starting point for what you should be thinking about when building or managing houses in sites vulnerable to wildfire—the so-called wildland-urban interface.

Design Guidance for Wildfire Country

Where to Build

A high priority in thinking about building in parts of the country prone to wildfire is where to build—ideally, you shouldn’t build in places that are most vulnerable to wildfire. But realities are such that keeping development out of vast areas of the country simply isn’t going to happen. So we need to be smart about building sites.

During site selection consider prevalent vegetation types, climate, prevailing winds during the dry season, topography that can channel wildfire in canyons and cut off escape routes, and the risk of accumulated vegetation that provides fuel for a fire.

While it is possible to manage vegetative debris and fuel loading to some extent on one’s property, also consider neighboring properties where you can’t control the vegetation.

Also, be aware that wildfire risks often aren’t limited to the fires themselves. Following large wildfires, soils destabilized by loss of vegetation are highly vulnerable to flooding and mudslides—or debris flows. So, as you consider building sites relative to wildfire vulnerability, also think about topography and whether slopes are steep enough that the soils could become unstable or are configured in a way that floodwater and debris flows could damage a house.

Outdoor Living Space

Outdoor decks are among the leading points of vulnerability in wildfire country. Vegetation often grows beneath decks and, in the dry season, becomes tinder for wildfire. Decks trap wind-blown leaves or cuttings from lawnmowers. Homeowners often use the space beneath decks to store firewood, lumber, or other materials that can ignite in a wildfire. And when decks are built on hillsides—a great place for decks relative to views—upslope fires can spread beneath them and ignite accumulated debris.

From a wildfire resilience standpoint, patios are far preferable to decks. Most patios are made from non-flammable materials, such as flagstone or brick. If having a deck is a must, it should be made from a low-flammability material. CalFire, the Office of the California State Fire Marshall, offers a database of decking materials suggested for wildland urban interface locations (search for “Decking”).

In general, avoid vinyl and other plastic materials for decks, fences, outdoor furniture, play structures, and other outdoor features. Never use recycled railroad ties as landscape features—creosotes wood is not only more flammable than standard wood, but also highly toxic.

Landscape management

Even more important than the choice between decks and patios, the landscape around a house should be managed to minimize fire risk. Accumulated leaves and other dry vegetation should be removed from immediately around the house. Plantings should be selected for low flammability (some plants are far less flammable than others, due to resin content and other factors). Trees should be kept a minimum of ten feet from houses; lower branches should be removed from trees within 30 feet of houses; and trees should be thinned to avoid connected tree canopies within 100 feet of houses.

Flammable objects, like brooms and outdoor furniture, should be removed from decks if a wildfire warning is issued, or before going away during fire season.

Finally, during the dry season in wildfire-prone areas, the vegetation around houses may be sprayed with a nontoxic, phosphate-based chemical treatment to reduce flammability. The treatment of choice is Phos-Chek® whose residual chemicals are similar to a low-concentration phosphate fertilizer. Unless heavy rains occur, Phos-Chek treatments can impart fire resistance for an entire season.

Building foundation

Relative to fire risk, slab-on-grade and full basement foundations are preferable to pier foundations or crawl spaces. Vegetation can grow beneath a house built on piers, and dead leaves and other flammable debris can easily collect there. Crawl spaces may also accumulate debris, though not as easily, and embers may be able to get through vents and ignite that debris. Embers may also enter a house through basement vents, but it is usually easier to close basement vents than crawl space vents.

Vents for crawl spaces (and full basements) should be screened with maximum 1/8^th-inch metal screening, and as an added safety feature, install a manual closure. Sealing off vents can be on the pre-evacuation checklist prior to evacuating a house during a wildfire alert.

Structural building system

Concrete and masonry materials provide the greatest fire resistance. To minimize fire risk consider these construction systems, but for energy performance and sustainability reasons, also consider insulation options. Cladding a masonry or concrete building with an exterior insulation and finish system (EIFS) can eliminate that fire resistance, since expanded polystyrene (EPS), the most common insulation with EIFS, is highly flammable. Insulating on the interior provides far better fire resistance.

Autoclaved aerated concrete (AAC) may be an ideal building system in wildfire-prone regions. It offers excellent fire resistance as well as modest integral insulation—the insulation value may be adequate in those climates most prone to wildfire, such as coastal California. See the article, “AAC: An Ideal Material for Resilient Buildings,” for more on this material.

With wood-frame construction, consider brick facing, cement stucco, or fiber-cement siding to improve fire resistance. If a vented rainscreen (an airspace behind the exterior siding) is used in the wall assembly (recommended for moisture management reasons), the top and bottom openings into that air space should be screened with maximum 1/8^th-inch screening to keep most airborne embers out.

Windows

Install insulated (double- or tripleglazed windows) not single-glazed widows. Tempered glass is more resistant to impact and heat than standard glass and is recommended in wildfire country. For maximum fire resistance, specialty fire-rated glass can be specified, such as Pilkington Pyrostop, but there will be a considerable upcharge for this.

As for window frames, thermally broken metal and pultruded fiberglass are better in fire conditions than wood or vinyl. Both vinyl and wood can burn, and vinyl will deform at high temperatures, potentially allowing the window assembly to totally fail, providing an entry point for embers and fire. A reasonably affordable option is the Marvin Integrity product line, in either Essential and Elevate collections.

Roofs

Because ember entry through soffit and ridge vents is a leading cause of house destruction in wildfires, good arguments can be made for building unvented roofs in wildfire country. Due to moisture condensation concerns, however, extreme caution must be taken in building unvented roofs. Achieving a robust air barrier in such a system to keep indoor air out of the roof assembly is essential in unvented roofs; even small quality control problems can lead to significant problems.

With more common vented roofs in wildfire country, it is extremely important to include ember-excluding soffit, ridge, and gable-end attic vents. Maximum 1/8^th-inch screening should be used for such vents, or specially designed ember-excluding vents should be installed. It may also be possible to devise manual closures for roof vents (this will be easier with gable-end attic vents than for soffit and ridge vents). With manual closures, sealing those can be on a pre-evacuation checklist for homeowners.

As for roofing, metal, slate, concrete, and tile are typically better than wood or asphalt shingles, though some asphalt-like fiberglass shingles offer fairly good fire resistance. A non-flammable roof underlayment, such as gypsum or perlite, can help protect a building exposed to the extreme heat of a wildfire and is recommended over standard plywood or oriented strandboard (OSB). With the latter roof sheathing options, specifying fire-rated materials will improve fire resistance.

With tile roofing, avoid barrel tiles or S-tiles, because air spaces at the eaves can allow debris accumulation or bird nesting and ember entry. Even if bird stops have been installed on such tile roofs, these often fall out over time, eliminating that protection.

Heating, Ventilation, and Air Conditioning

With mechanical ventilation—recommended in all homes—install manual closures on air inlet and exhaust ports. These can be closed as part of a pre-evacuation checklist during wildfire alerts; the ventilation system should also be turned off. Closing off these ports will protect against ember entry.

With wildfires becoming more common, regional air quality problems from smoke are increasingly common—even in areas far from active wildfires. Filtering out such smoke is becoming a more and more important function of ventilation systems. Discuss air filtration with an HVAC engineer. If that filtration involves separate outside inlet and exhaust ports, install the same sort of manual closures described above.

To minimize risk of gas explosions from gas leaks and broken gas lines during wildfires, earthquakes, flooding, and other disasters, consider avoiding natural gas or propane entirely, relying instead on electricity—ideally with a solar system to generate that electricity on a net-zero-energy basis.

Getting more information

With wildfire season upon us in California and as rebuilding efforts are moving full-steam ahead in many California communities affected by recent wildfires, we need to be smart about where—and how—we rebuild. The strategies provided above provide a starting point for what should be considered in wildfire country. But don’t stop here.

There is a lot more information available, including Regional Wildfire Retrofit Guides from the Insurance Institute for Business and Home Safety (IBHS) and the publication Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations, from the University of California. Steve Quarles, an author of much of this information and one of the nation’s leading experts on building design in wildfire country, will be a featured speaker at our Building Resilience 2019 Conference in Cleveland on November 7-8.

This conference will be a great opportunity to learn from the experts about not only wildfire, but the full range of vulnerabilities our buildings face. I hope to see you there!

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Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. To keep up with his latest articles and musings, you can sign up for his Twitter feed. To receive e-mail notices of new blogs, sign up at the top of the page.