With wildfires destroying over 3,500 structures across northern California in the last week, it’s understandable that my kids are feeling concerned about the safety of our own home. Aside from the smoke, we’re well out of harms way, but that hasn’t stopped some age-appropriate brainstorming about fire safety. Maybe force-fields would help? How about everyone using their garden hoses to spray the fires? Why can’t houses just be fire-proof?
Fire-proof, in the 3rd-grade understanding of the term, probably isn’t possible, but houses can be made to be very fire-resistant. Depending on the materials and design of a building, it may be able to withstand up to four hours of intense flames, and even then structural problems might come up before the whole thing actually burns. Basically, the key is to build in materials that can absorb and withstand heat while remaining chemically inert— ie., not actually combusting themselves. From that perspective, the wood frames that hold up so many American homes are sort of a terrible idea, as the wood will both burn and transmit heat to other parts of the structure. Moving away from the idea of a rustic log cabin, we should really all be living in homes made of concrete.
Preventing conflagration with concrete
Concrete frames and walls provide a number of advantages over wood. The limestone, clay and gypsum that go into concrete are very stable, and thus unlikely to react with oxygen and heat during a fire. Instead, a concrete slab can absorb a lot of heat, trapping some of it in internal pockets and pores. This can help isolate the heat from a fire, as well as insulate the building from unpleasant hot and cold temperatures in less dire circumstances. If you want to maximize the impact of your concrete walls, you probably want to install them as insulated concrete forms (ICFs), which are modular systems to further compartmentalize your concrete slabs, keeping the buildup of heat from a fire as isolated as possible.
If a building isn’t concrete, there are other options to up its fire-resistance. Bricks, having been created in kilns, hold up to heat quite well. In a fire, they can absorb heat without being damaged, with the point of failure usually being the mortar that holds a wall together. Gypsum board used in drywall can absorb a fair amount of heat without burning as well, with Type X gypsum boards being packed with calcium sulfate and water vapor inside. When exposed to fire, the water vapor can help suck up a lot of heat before the gypsum has to get cooked too much, all of which will hopefully provide time for the fire to be dealt with. On the outside of your building, common stucco usually has cement, sand and lime as ingredients, which again are inert enough to absorb heat without burning themselves.
Bad and best practices
Even with concrete or brick walls, many buildings still have weaknesses that can make them susceptible to fires. Vinyl siding and framing around windows melts pretty easily, exposing any wood framing underneath. Single pane windows that get broken allow both heat and oxygen to pass into or out of a burning building. If the source of flames is from an external wildfire, roofs are often a point of combustion. Loose shingles or semi-open tile work, can provide openings burning embers to get into a house’s attic. Overhangs are another place where fire-resistant materials are likely to be joined to more combustible wood, exposing the roof to danger even if the walls are otherwise unscathed.
So what should my kids’ theoretical fire-proof house look like then? Starting with the yard, no trees or brush should be too close to the house itself. Instead of a wooden deck, a stone or concrete patio would act as a firebreak, protecting the concrete walls. Tempered glass windows, or maybe glass bricks with an internal wire matrix to avoid cracking, would be further protected by roll-down metal fire doors that could deploy automatically in response to extreme heat. A steeply pitched roof would encourage burning embers to fall to the ground, rather than sitting and burning on the building. Internal walls would be brick or concrete, maybe with gypsum boards if you needed a softer material for some surfaces. It might start to feel a little bit like a fortress, as long as no lava (“Or asteroids!” “Or monsters!”) show up, it should be one of the cozier places to be after a wildfire.
Source: Why is concrete fire resistant? by Colleen Cancio, How Stuff Works