Building a Wildfire-Ready Home or Business

With global climate change and ongoing drought in California, it’s a sad fact of life that we all need to be prepared in case we get the unwelcome message that a fire is heading our way.

As an architect helping rebuild after 2017’s Tubbs Fire, I got a first-hand look at how a wildfire can devastate a community and the people and businesses in it. While a large fire can destroy everything in its path, it’s clear that, with a little planning and careful design, some buildings are less likely to catch fire or to be completely destroyed.

Whether you’re building a new house or business space or renovating an existing one, there are some steps you, your architect and landscape designer can take to increase the chances that it will come through the next wildfire relatively unscathed.

Wildfire Defensible Space

A defensible space is an area surrounding the property that creates a buffer between your building and any combustible materials on the property surrounding it. It reduces the likelihood that your building will catch fire by reducing the fuel a fire needs to burn and by providing emergency personnel with the space they need to fight a fire that threatens your building.

California law requires property owners to maintain 100 feet of defensible space around buildings and structures on their land. While the specific requirements for defensible space will depend on your city and the particulars of your individual property, the County of Sonoma and fire safety organizations such as CalFire, FIRESafe MARIN and DisasterSafety.org have created helpful guidelines for maintaining several zones within the required 100 feet of defensible space, each with recommendations for keeping the area clear of potential fire hazards.

The definition of zones differs somewhat between organizations, but the general guidelines include similar recommendations:

Immediate Zone — 0 to 5 feet from buildings/structures

No combustible materials or vegetation
Replace combustible materials (like natural fiber doormats, hangings, signage or furniture) with metal, rubber or other non-combustible materials
Keep any vegetation in this area watered
Clear all fallen leaves and needles
Remove tree limbs that extend into this zone
Use inorganic, non-combustible mulches
Consider hardscaping around the base of structures in this zone
Place garbage and recycling bins outside this zone

Intermediate Zone — 5 to 30 feet from buildings/structures

Don’t use shredded bark mulch; use only compost or heavy bark mulch
Use only fire-resistant plants
Maintain space between shrubs at least 2 times the plant’s height (more on steep slopes)
Keep tree branches trimmed to at least 10 feet from other trees
No vegetation around fences, sheds or other outdoor structures
Remove branches within 10 feet of any chimney
Remove tree branches lower than 6 to 10 feet from the ground

Extended Zone — 30 to 100 feet from buildings/structures

No grass over 4 inches in height
Maintain shrubs and trees in spaced groupings
Maintain vertical space between trees and other vegetation

Access Zone — 0 to 10 feet from road and driveway edges

Remove vegetation within 14 feet overhead or 10 feet from sides of roads or driveways
Maintain at least 12 feet of unobstructed pavement for vehicles
Make address numbers clearly visible from the road

Use Fire-Resistant Building Materials

With much of the Bay Area falling within a Cal Fire-designated “High” fire hazard zone, it makes sense for home and business owners to use fire-resistant materials in their buildings.

Embers – small fragments of burning or glowing fire debris carried on the wind – are the most significant cause of building ignition during a wildfire. Using fire-resistant materials for vulnerable portions of your building can help protect your home or business from these airborne hazards.

Roofs

The type of roof you choose for your building will depend on a number of factors, but selecting a fire-resistant roof should be near the top of your list of considerations.

Roofing materials are fire-rated from Class A to Class C, with Class A providing the highest level of fire protection (unrated roofs are the most vulnerable and not recommended). In general, asphalt fiberglass composition shingles and concrete and flat/barrel-shaped tiles are the most fire resistant, while untreated wood shake is the least.

Cleaning your roof and gutters regularly to remove accumulated debris is crucial to maintaining its fire resistance.

Because a roof’s assembly can impact its performance in a wildfire, it should be installed by a licensed professional with experience with the type of roof you select.

Siding

Exterior siding is another vulnerable aspect of any building. During a wildfire, siding can ignite or allow fire to penetrate the building and into the building’s interior.

Siding materials can be combustible – solid or composite wood or vinyl, for example – or non-combustible, such as some stucco, fiber cement or metal siding. An intermediate category is ignition-resistant material, which includes some treated woods that have undergone a weathering process and passed a flame-spread test.

A building’s trim can also increase its vulnerability to fire. In addition to using an ignition-resistant or non-combustible material for trim, make sure there is no debris in any corners or gaps between the siding and the trim.

As with roofing, an installer’s experience and expertise can help ensure your siding maintains the highest possible fire-resistance.

Windows

Windows are a complex subject when it comes to fire safety because there are many variables, including window size, placement, framing and materials.

In general, large windows are more vulnerable to thermal stress during a fire, meaning they can break, allowing embers to enter the building. Annealed or tempered glass is less likely to break than plate, but reflective film and low-e coatings on windows may provide some protection from the heat of a fire if you can’t afford to replace older plate windows.

Decking

Because decks often abut a main structure, the material you select is crucial to maintaining your home or business’s fire resistance.

While metal and concrete decks are non-combustible, many popular materials used for decking, such as wood (treated or untreated) and plastic composite, are combustible. Adding metal flashing to the base of a building at the intersection of the siding and the decking can provide some extra fire protection, as can enclosing the deck’s perimeter to prevent the accumulation of debris and leaves underneath.

Equip your deck with fire-resistant furniture and décor, and regularly inspect it for rotting wood, which can be highly flammable. Clear any combustible debris, such as leaves or needles, from the deck’s surface and surroundings, and never store combustibles (like wood) under your deck.

Fencing

Like decking, any fencing that abuts or sits close to your building should be as fire resistant as possible. Certainly, any fence within the immediate zone should be made of non-combustible materials.

Fences that permit air flow – such as latticed fences – can prevent debris (or embers) from accumulating, which can help prevent ignition. Regular maintenance to prevent debris build-up and replace rotting wood is important.

Talk to Your Architect About Wildfires

Wildfires can affect any area, but some homes and businesses are more vulnerable due to their location, building type, or activities. Talk to your architect as you begin planning your project to explore what steps you can take to mitigate your risks.