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The term "rain roof" has been used for the past 25 years in the construction industry and the fire service to describe a complete roof assembly constructed over an existing one. The reason for this being done is that the original roof is deteriorated and that the most economical fix is to install a truss roof.
The preliminary evaluation by the contractor/architect/engineer is to ensure the existing exterior bearing walls will support the additional weight of the truss roof, plywood and shingles. A knee wall (a short wall usually built approximately to a height of the human knee) is constructed to raise the new roof over the existing one. This wall can be higher, for example, if covering a bowstring truss. The added weight of the new roof and the old roof will accelerate the collapse of the assembly during fire conditions because the more you load an item, the quicker it will fail during the thermal attack.
Once the installation has been completed, it is difficult to know that a rain roof system exists. If truck company members commence roof operations, often the information they give to the incident commander will be flawed. Remember, even with fire under them, the ceiling, cockloft and old roof assembly, including the stone-chipped roof covering, still exist. This is how six firefighters from FDNY were ambushed in 1978 at a supermarket fire. They were working on the roof when a knee wall failed suddenly, sending them to their deaths.
Pre-planning is paramount. When it is discovered that a rain roof has been installed in your area, the address should be tagged. This tagging will allow your communications section to advise all responding units to use caution. Also, it should alert the responding incident commander to go defensive early if a significant fire exists and no viable victims are to be found (or at least the incident commander can forego roof operations).
Size-up can be accomplished successfully if you are aware of the following: first, the area above the top floor windows will be taller than normal. By normal, we are looking for between 18 and 24 inches from the line of the window top to the underside of the eaves. The added height above this will be the knee wall. Second, we look for dissimilar covering materials. If the exterior wall is masonry and the area above the top windows switches to panels, siding or off-color masonry, suspect a rain roof. Although these clues are basic, they can assist in identifying a rain roof.
Combating a rain roof fire is also different. You are going to have a deep-seated fire that may not show itself. If you have a large volume of fire on the top floor and little or no flame or smoke has penetrated the roof, suspect a rain roof and prepare for a long-duration attack.
One small note for your consideration: I have always marveled at incident commanders who immediately go to ladder pipe operations, even if the roof has not yet burned off. In my opinion, a progression of tools should be used to fight roof fires, beginning with unmanned monitors, manned large-caliber streams, and pre-piped deluges on the engines. All of these allow water to penetrate under the fire. The roof is designed to shed water, so it's not rocket science to realize that ladder pipe-delivered water will only waste resources as it cascades off the covering.
Water supply is another problem to be addressed. Too many fire departments suffer the "octopus syndrome" - that's when the first pumper resembles an octopus with everyone's line coming off from it. It stands to reason if all responding units lead off from the same main, someone's not going to get sufficient pressure. Advise all subsequent units beyond the first alarm to seek another main and assign a water supply officer.
Safety should always be at the forefront of these incidents. Immediately upon the recognition that a rain roof exists, begin considering defensive operations. Also, augment your safety officer with assistants and ensure that the incident command system organization required is resourced and identified. Assign senior chiefs to your most concerned areas.