Full Metal Jacket

Houses are not consumed by Chaparral fires, but by house-fires started by Chaparral fires.

That may seem like a fine distinction, but it has real consequences when planning for a fire safe residence. There are three modes of attack by these fires on buildings:

• burning embers

• heat radiation

• flame contact.

The CSIRO (an Australian government funded research organization - and full disclosure - with whom I worked for a couple of years in the 1970's at their Canberra Black Mountain Computer Research Laboratory) have generated significant data in their analyses of "How Bush Fires Attack". They conclude that ember attack is the most prolonged and persistent mode of attack, commencing before the fire front and persisting for several hours afterwards. Radiant heat is at dangerous levels for perhaps 5-10 minutes before and after the flame front passes. Flame contact may impinge on the building for just a few minutes. Strong winds associated with fires exacerbate the effects of all three.

We know from anecdotal evidence from those who have seen the fires ravage Upper Ojai that the fire line moves quickly and literally sweeps over structures. In our recent fire-safe building design, we have responded to all areas of vulnerability to fire, but have focused particularly on ember attack.

The re-model across the street that Lorrie and I (operating professionally as dba) designed for Margot is complete except for the fire screens which are to be installed at all glazed openings. Our thinking on fire screens/shutters/doors has evolved since they became one of the drivers for the design of own house on Koenigstein (and is reflected in their evolving nomenclature!). As it turned out, our fire doors were not installed until six months after we moved in and during that time we designed and re-designed the door and track system.

Our house was built with the line of glazing pulled back four feet from the face of the house. The fire doors pull across these 16' wide recesses (three to the north and three to the south) to create a hard shell designed to be impervious to the wind driven embers which can precede the actual line of the fire by up to a mile and arrive with a ballistic force capable of breaking the two layers of tempered window glass now mandated by the California Building Code in high fire districts. When the interior begins burning a house typically explodes in flame and ignites the landscape around it. It is in such a manner that structures - whether the fire line reaches them or not - can actually generate satellite fires that run ahead of the main fire line.

We had initially envisioned all-ipe fire doors for the house. We were early adopters of this tropical hardwood that has the same fire rating as concrete although we had used it for many years, beginning in the mid 1990's, primarily for its natural beauty and structural properties - 3/4" of ipe spans 24" as effectively as a 1 1/2" piece of redwood. Its advantages derive from its amazing density, but the weight of the wood is problematic when being used in a track hung door. Our 16' x 8' doors using a 2x frame with 1x infill would have weighed 800 lb. each.

Concerns with the logistics of hanging these doors and the structural consequences of supporting over a ton off of each side of the building led us to consider lighter, but still fire-resistant alternatives. In addition to their function as a hard shell to deflect embers, the doors also potentially function as barriers to heat radiation. In this latter role it seemed as though a cool roof material could be effective since a material designed to reflect solar radiation would also reflect radiant heat generated by a chaparral fire.

In the event, we chose a 24ga. sheet metal soffit panel in a 'cool roof' finish identical to our standing seam roof. The panels were attached to ipe strips laid into a 1/4" x 3" x 3" steel angle frame. They hang from a continuous galvanized track attached to a 1 x 6 ipe ledger. They are a handsome addition to the building and sit against the stucco facade awaiting their call to action - which we sincerely hope will never come.

Margot's situation, being a re-model of a conventional ranch-style house with a variety of glazed door and window conditions, is very different and requires a more flexible system of fire shutters. In addition, she has the ear of the L.A. County fire chief who has come down on the side of the radiation heat shield function being critical in the design of the fire shutters. Too bad: we were ready to adopt the Aussie notion that the key function of the screen is to deflect burning embers and perforated sheet metal or a wire cloth membrane is sufficient to the task.

The CSIRO addresses the radiation issue and provides experimental data on its impact on the building envelope. They note that at peak levels, radiation can crack and distort windows, doors and cladding materials, allowing breaches of the building envelope and ember attack on flammable framing members. Flame contact can cause building ignition when exposed materials, dried and prepared by sustained wind, ember and radiation attack, are contacted directly by flames.

In Margot's house we have tried to address these issues by using stucco, ipe and flame and ember-proof attic vents as well as installing double glazed tempered glazing at all new windows and doors. Her front door, eaves, decks (including their support structure and the side cladding) are solid ipe. Under floor venting - a typical weak point for raised floor structures - is protected by the ipe deck that wraps the vulnerable portions of the building.

The re-making of Margot's home, which was initially built with flagrant disregard for fire-safety and featured flammable fiber board siding, exposed rafter ends and plywood eave soffits, was an exercise in abating the inherent weaknesses of the existing building fabric.The fire screens will be the final piece of this defensive strategy.

Their design is complete in terms of the sliding mechanisms. We have opted for operable screens; most of the time, they will be parked above below or to the side of the opening they are designed to cover when there is a threat of fire or the house is left unattended. In Australia there is a trend to install fixed mesh screens that attempt to tread the line between view maintenance and fire protection. We believe that the problem is pieces of flaming debris dense enough and moving quickly enough to break through the window glass. We are less concerned, since we are confident in the fire resistive materials we have installed, about wind driven sparks.

Choosing to live in the wildland/urban interface requires a commitment to co-existing with endemic natural cycles of flood and fire. Our goal is to design structures that can endure - in an ecology in which fire is an occasional presence - like the oaks (Quercus agrifolia) that envelop Margot's house and bear the scars from past burns. We continue to wrestle with the precise nature of the fire screen infill.

Today, with temperatures hovering around a breezy 85 degrees, is a reminder that the season which mightily favors that occasional presence is fast upon us.....

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