Fire Safety in the Wildland Urban Interface
By Scott McGlashan, McGlashan Architecture
Grim images and terrible stories from burned residential neighborhoods are becoming more common in our warming West. More and more of us are realizing that we live in areas where it’s only a matter of time before a real wildfire attacks, so it’s time to get serious about how to adapt our houses to resist fire. Visiting burned areas, I’m always struck by the stark juxtaposition of completely destroyed houses and others that appear untouched.
How can a fire roar through a neighborhood, reducing some houses to ash but leaving others standing unharmed?
The answer is in understanding how these fires work. Our popular imagination of fire is an advancing wall of flame marching along and burning everything in its path, but that’s not how modern wildfires have been most destructive.
Our worst fires lately have been fast-moving, wind-driven fires. High winds can take a tiny fire and quickly spread its burning embers far and wide. Neighborhoods are most threatened when a hail of hot embers comes blasting through, moving faster than firefighters can react, and quickly starting many small fires. It’s these wind-driven embers that we need to concentrate on if we want to protect our homes from the sort of wildfire that could threaten the East Bay Hills.
So how do you protect against hot flying embers? They will likely blow against your house, but most houses are built of materials like stucco or painted siding that are actually pretty hard to light on fire with just a spark. But look at the ground immediately around your house, where embers might fall. Is there a bed of mulch? A stack of wood? A chicken coop? A dry shrub? A tattered welcome mat and a box of shoes? These are perfect environments for nurturing a tiny ember into a hot fire. And when it’s right against your house, this small fire can create enough heat to actually ignite the house itself.
To safeguard against these circumstances, we should try to remove anything that might fuel a fire too close to the walls of your house: keep flammables, including trees and shrubs, at least a few feet away. Ideally, you can create a “moat” of gravel, stone or other non-flammable hardscape around the perimeter of the house.
Gutters or roof valleys full of leaves provide another easy place for embers to land and start a fire that spreads to the house. Clean gutters and roofs regularly and install gutter guards which prevent flammable duff from building up.
Another general rule is to avoid plastics, like vinyl windows, PVC gutters, or composite decking. These materials can be easier to ignite, more toxic when burning, and harder to extinguish than traditional building materials.
Most wood-framed houses have crawl spaces and attics that are vented to the outside. These openings near the ground and up in the eaves are important for preventing moisture problems, and are often required by code, but they also provide a much-too-easy way for sparks to find the perfect conditions to start a fire. Most attics are ultra-dry, with exposed wood and maybe even some sawdust piles from when it was built. Imagine how easy it would be for a tiny ember to ignite in there. And then imagine how hard it would be to put out that fire, even if you were standing outside with a hose.
Simply closing off attic and crawlspace vents can create moisture problems, so you’ll need to keep the vents open to let air pass through, but make sure every vent is screened with a metal mesh with openings no larger than ⅛”. For even more protection, you can install fire-resistant venting products that are designed to keep embers out. If you are adding on or building a new house, ask your architect about implementing an “unvented assembly,” which is an approach to designing attics or crawl spaces without vents or unconditioned enclosed spaces. These unvented assemblies are hard to install as a retrofit, but for new construction, if designed right, they perform very well in preventing fire and slowing heat loss.
Decks and railings present a particular challenge here in Berkeley. We love our outdoor spaces, and we have a fine tradition of creative woodworking on decks, fences, railings, arbors, and gates. It’s part of what gives Berkeley its character and it’s not something we should give up. But exposed wood, with many surfaces and gaps and crannies, is almost ideal for fueling a spark into a fire. I try to keep these sorts of outdoor wood elements separated from the house by either a gap or an extra layer of fireproofing. It’s good to be clear about the line of separation between what will likely burn (an old redwood deck with cracks full of duff) and what must be protected (the home itself). When I see an old wooden deck hanging off the side of a Berkeley hillside home, covered with oak leaves, with a messy woodpile tucked underneath, and tree branches poking through the railing, I can’t help but wonder: Will this home survive the next firestorm?
State officials in California have identified areas most at risk of wildfire. They are called Wildland Urban Interface zones, and they include a large portion of the East Bay Hills. In these areas, the state requires new construction to implement some enhanced fire protection standards beyond what is required by the local building code. These W.U.I. standards are sensible, easy to research, cost effective and not too hard to implement. They’re worth asking your architect about, even if you’re not in a W.U.I. zone.
It’s all about embers. If you have nothing that can stoke an ember into a fire up against your house, and you prevent embers from sneaking in through the vents and gaps, then when the wind-driven fire comes, you’ll be much more likely to find in the aftermath that yours is one of the houses still standing.
Fire resistant vents:
Wildland Urban Interface (W.U.I. or “woowee”) handbook: