As a continuation of the series, we will next explore the ways in which site development can play a role in designing for resiliency.  The recent wildfires in LA have been a stark reminder that we must consider both the structure itself and the surrounding site when proposing design solutions that offer not only an aesthetic vision, but also ones that are thoughtful and responsive to the vulnerabilities of the surrounding area.  As stewards of the built environment, the design community has a responsibility to put forward solutions that consider the design of the surrounding site just as important as the building itself.  It is here where many of the resiliency strategies can help limit the spread of wildfires and mitigate the overall impact.

• Direct Flame
  • The direct contact that a flame makes can and will ignite materials that are of a combustible nature. Keeping flames low, providing fire breaks and building with non-combustible materials can limit the impact of direct flame contact.

• Radiant Heat
  • The radiant heat generated by a wildfire can ignite materials from a distance as far away as 30 feet.  Larger fuels often burn hotter, therefore posing a great risk to the structure.  Limiting large fuel sources to distances beyond 30 feet from any structure and utilizing non-combustible materials and protected openings like multi-pane tempered windows can help limit the risk of radiant ignition.

• Wind Driven Embers
  • 90% of homes ignite by direct contact with embers. Embers can travel anywhere from hundreds of feet and up to miles in distance with the right wind conditions.  These embers are often the first component of a fire to reach a site and not only ignite fuel sources around the home like vegetation, fencing and decks, but they can also pose a risk to the structure itself.  Utilizing hardscape materials, fire resistant landscaping and non-combustible decking and furniture can reduce a sites risk to ignition by embers. Home-hardening techniques like Class A roofing, using non-combustible materials for wall assemblies and sealing openings into the home are also effective strategies.

Site Characteristics

With an understanding of how fires travel in various ways to and through a site, the site itself must be examined to determine if any vulnerabilities exist that require design interventions to mitigate. There are a variety of decisions that go into the development of a site, but the first step is understanding the inherent characteristics that designers must ultimately respond to.

• Topography – The natural contours of a site can dramatically impact the way in which a fire can spread, with steep inclines in grade accelerating a fires pace. Dense vegetation and debris runoff are prevalent on steep slopes, providing additional fuel for fires. The decision to work with or modify the existing contours is an important decision in site development, as is the location and orientation of the structure in response to this.

• Wind Patterns – Much like topography, wind direction plays a significant role in the path a fire takes through a site. Understanding the direction the prevailing wind is coming from and how it is modified by both the terrain and surrounding structures will inform building orientation and location on a site. The design of additional site features like vegetation groupings, retaining walls (fire breaks) and locating prime fuel sources downwind of the structure can help reduce the impact of traveling embers.

• Site Adjacencies – Every site is part of a larger collective, a community.  As important as it is to understand the key characteristics of the immediate site, it is equally as important to understand the surrounding area and landscape.  Examining the surrounding area to understand if steep terrain, overgrowth, dense vegetation, utility corridors or close adjacent structures exist can and should inform design decisions on a project site.

Defensible Zones

Once the main site characteristics are determined and the structure is located, the immediate surrounding area is then developed in a three-zone design strategy.  These zones (0-5 feet, 5-30 feet and 30 feet to 100 feet) are designed to become increasingly more resilient the closer to the structure you get.

• Zone 0 (0 feet to 5 feet) Ember Resistant / Non-Combustible
  • Zone Intent: The goal in this zone is to create a non-combustible immediate barrier around the structure, which includes attached decks, to eliminate direct flame contact. The area closest to the structure should have fire-resistant materials, very minimal vegetation, and no combustible items like firewood.
  • Design Features:
    • Create separation between vegetation (if any).
    • Use hardscapes for features like driveways, seating areas and walkways that are of non-combustible materials like gravel, stone and concrete.
    • Select furniture that is made of non-combustible materials and separate from other potential fuel sources.
    • Decking:
      • If a deck is proposed, construct of non-combustible materials like composite boards, concrete or stone.
      • Limit any gaps between the deck boards and install a skirt made of non-combustible material to prevent embers from gathering.
      • Install foil-faced bitumen tape on the top surface the deck joists.
      • Limit growth under the deck by installing a stone bed over a weed barrier
    • Fencing, arbors and gates that are within 5 feet of the structure or attach directly to the structure should be made of non-combustible materials, like metal.
  • Routine Maintenance (post-occupancy)
    • Remove all dead plants or grass and dried leaves.
    • Do not store wood in this area.
    • Eliminate flammable plants and vegetation.
    • Do not store wood in this area.
    • Routinely clean gutters and roofs of branches, leaves and needles.
    • Do not install combustible systems like generators or propane tanks in this area.
    • Limit the storage or recycling in this area.

• Zone 1 (5 feet to 30 feet) Intermediate Area – Lean, Clean and Green
  • Zone Intent: The goal in this zone is to reduce the risk of fire spreading to the structure from close fuel sources, via radiant heat and additional embers.
  • Design Features:
    • Limit vegetation to fire-resistant, native and water retaining plants, and locate in groupings with ample separation. Keep plant selection to low growing plants.
      • Burning vegetation can ignite adjacent materials through radiant heat and create additional embers as they burn.
    • Trees and shrubs should be limited to small clusters in discontinuous groupings.
    • Create fuel breaks with non-combustible pathways like gravel, stone and concrete.
    • Furniture groupings, play structures and fuel sources like grilles and propane tanks shall be separated.
  • Routine Maintenance (post-occupancy)
    • Mow grass to 4” max in height.
    • Clear vegetation from ignition sources (grilles, propane tanks, etc).
    • Clear debris and dead vegetation.
    • Limit the storing of any combustible materials like firewood.
    • Trim trees regularly.

• Zone 2 (30 feet to 100 feet) Fuel Reduction Area
  • Zone Intent: The goal in this zone is to restrict the movement of fire and to limit its ability to progress towards the structure via traveling embers. This zone is primarily governed by vegetation and overgrowth management.
  • Design Features:
    • Vegetation Spacing:
      • Space between shrubs:
        • Flat or mild slope (less than 20%): Two times the height of the shrub.
        • Mild to moderate slope (20–40%): Four times the height of the shrub.
        • Moderate to steep slope (greater than 40%): Six times the height of the shrub.
      • Space between trees:
        • Flat or mild slope (less than 20%): 10 feet.
        • Mild to moderate slope (20–40%): 20 feet.
        • Moderate to steep slope (greater than 40%): 30 feet.
    • Locate secondary structures (ADUs, sheds) at least 30 feet from the primary structure.
    • Allow for extra vertical space between shrubs and trees.
    • Keep at least three times the height of any shrubs, between the shrubs and the lowest branch of trees.
  • Routine Maintenance (post-occupancy)
    • Clear areas around secondary structures and propane tanks.
    • Keep 10 feet of clearance to bare mineral soils and no flammable vegetation for an additional 10 feet around their exterior.
    • Keep 10 foot distance around wood piles, down to bare mineral soil, in all directions.
    • Remove all tree branches at least six feet from the ground.

Other Considerations

Beyond defensible space requirements, there are additional features and systems that can be installed to enhance the resiliency of a site and provide coverage continuity in the event of a weather-related event.

Water Features – Pools, spas and fountains can provide an owner with the ability to draw water from an on-site source quickly to proactively saturate the grounds ahead of a fire or reactively extinguish flames during an active blaze.  Planning ahead infrastructurally is key, as both a standalone power (generator) and pumping systems should be integrated into the design to properly utilize these features.

Landscaping Irrigation – The irrigation system used to support landscaping can also be activated in the event of a wildfire to saturate the surrounding area and limit the ability of embers and flames to ignite vegetation.  Note, to be used during the event of a fire, assume city wide utilities will not be in service and standalone water and power sources will need to be relied upon.

Underground infrastructure – Running utility infrastructure underground and where permitted, with flexible conduit can reduce the risk of line rupture and ignition.

Generator – Installing a generator can keep critical infrastructure running like power, along with life safety systems like fire alarm and fire suppression devices.

Monitoring System – Consider site and structure monitoring services for smoke, heat and ember related triggers.  Wildfire defense systems on the market utilize satellite data sources, cameras and heat sensors to track wildfires for an individual property 24 hours a day and can alert the owner to potential hazards. These monitoring services can also be tied to site irrigation or fire suppression systems for immediate activation.  More traditional heat and smoke detectors can be used in and around the structure to jointly notify both the owner and the monitoring agency.

Beyond Utility

The strategies discussed in this post have been explored at their foundational level with a focus on the utilitarian function to mitigate the effects of wildfires.  As design professionals, our role is to consider how to deploy each of these strategies in a manner that takes full advantage of their utilitarian function, while also creating compelling spaces that feel true to the fabric of the neighborhood and reflect the overall design strategy of the project. 

---

What’s Next?

In the next post, we’ll explore home hardening strategies like materiality, envelope composition, building systems and fire protection systems to better understand their role in resiliency and the impact they can potentially have in a wildfire event.

To learn more about how we can work together to incorporate these design strategies in your project, please reach out to us at info@h-h-architects.com.

We’re here to help you navigate the re-build process. For a clear roadmap that outlines the essential steps ahead please download our Project Flow Chart here.