The Perils of Rain (Double) Roofs

BY BILL GUSTIN

On arrival, firefighters find a one-story ranch-style home with heavy smoke pushing from the soffit vents under the eaves overhanging the home’s exterior walls. The battalion chief in command of the incident suspects a fire that has either originated in or extended to the attic. Based on his observation, he orders firefighters, protected by a charged hoseline, to pull ceiling immediately on entering the front door. After forcing the front door, firefighters are surprised to find the house completely clear of smoke but are ready to fight fire over their heads in the attic when they pull the ceiling. When firefighters open the ceiling, they are again surprised and now perplexed to find only light smoke in the attic and no indication of fire. Moments later, the chief reports flames breaking through the roof. What is going on here?

At another fire in the same subdivision, firefighters find a house almost identical to the one in the previous scenario with dark smoke pushing from around the hurricane shutters covering the windows. This is a strong indication that the house is not occupied. It is common for winter residents of Florida to install shutters on their homes when they go back North during the hot summer months. The battalion chief, considering the pressure of the smoke and the delay and difficulty in removing the shutters to vent the windows, orders that the roof be ventilated. The chief is confident that the roof can be opened with relative safety because he is familiar with the construction of homes in this area. They were built in the 1960s with roofs of 1- × 4-inch tongue-and-groove sheathing supported by 2- × 6-inch rafters. This older “stick-built” roof is relatively substantial compared to the lightweight truss-supported roofs of newer homes and can withstand several minutes of fire involvement before showing signs of collapse and ultimately collapsing. Firefighters cutting the roof with a chain saw are, as in the previous scenario, surprised and perplexed when they find plywood sheathing instead of tongue-and-groove boards, 2- × 4-inch trusses instead of rafters, and asphalt shingles below their ventilation hole.

What do both of these fires have in common? The homes in both scenarios have had their original roofs covered by a second roof, called a rain roof or a double roof. When a roof ages and begins to chronically leak, the building owner may find that it is less expensive to construct a second roof over the original one than to repair the original roof. A rain roof built on top of an original roof also causes less disruption of the operation of a building than repairing or replacing the original roof. An office or a multiple dwelling can continue to be occupied while the rain roof or second roof structure is constructed over the original roof. This might not be possible if the original roof requires extensive replacement of rotten sheathing. Consider a leaky flat roof that would require replacement of rotten wood; new insulation board; and several layers of asphalt, felt paper, and a covering surface to make it watertight. It may be less expensive and disruptive to cover the original flat roof constructed with substantial dimensional lumber with a much lighter, less substantial pitched roof consisting of ½-inch-thick oriented strand board (OSB) supported by 2- × 4-inch peaked or parallel chord roof trusses.

Twice in my career I have fought two major fires involving the rain roof of a large shopping center—once in 1984 and again in 1988. The original shopping center roof was constructed of gypsum decking on steel bar joists. The flat rain roof consisted of 4- × 4-inch vertical members, 2- × 6-inch beams, 2- × 4-inch purlins, a ¾-inch plywood deck, and fiberboard insulation. The insulation on top of the plywood deck was covered with several layers of asphalt and asphalt-impregnated felt paper and topped with a ballast of gravel. Both fires required extensive cutting of the roof deck to get ahead of the rapidly spreading fire.

Rain roofs are also constructed over original roofs for aesthetic purposes. Say an old office building is in need of a facelift. Building a lightweight hip or gable roof over the original flat roof may be an expedient way to improve the building’s appearance (photos 1-3). In the Miami area, it is not uncommon to find single-family homes with lightweight roofs of a greater pitch built over the original roof. The new roofs are often covered with hundreds of pounds of clay tile to enhance the home’s appearance, but this will impose a significant dead load (photos 4-5). Building a second roof on top of a home’s original roof is also done to increase the size of a home. Consider an old home that is 25 feet wide with a flat roof. The home can gain an additional 15 feet in width by placing 2- × 4-inch wood trusses spanning 40 feet on the original roof and supporting the other end of the trusses with columns or a bearing wall. Rain roofs are also commonly constructed over mobile homes for aesthetics and to dampen the sound of rain hitting the original flat tin roof.

(1-3) This rain roof was constructed on top of the original flat roof for aesthetic purposes. A facelift for this old office building creates a dangerous concealed space for firefighters. (Photos by Enrique Rodriguez.)
(4) A Miami-Dade fire company conducting area familiarization found a new roof being built over the original roof of this single-family home and continued to monitor the progress of the renovation.
(5) The finished project. Note the decorative stone. Its height and pitch at the top are identical to the height and pitch of the original roof. (Photos by Tony Garcia.)

I have seen some amazing renovations to mobile homes in the Miami area. Consider a 1950s vintage “single-wide” (12-foot) trailer. The owner of the mobile home will erect 2- × 4-inch wood trusses, spanning 24 feet over the top of the 12-foot-wide trailer, effectively doubling the width of his residence. He covers his new roof with ½-inch OSB sheathing and clay tile or asphalt shingles. The trusses will be supported by columns in the front, which commonly become a carport and at the rear by a bearing wall, which becomes an additional bedroom or storage enclosure (photos 6-7). The resident then covers the addition and the trailer’s original metal skin with plywood or OSB, wire lathe, and a thick coating of stucco. Sometimes ornamental cement stonework is added for a nice touch; the result is an attractive single-family home that makes the owner proud of his handiwork and can fool firefighters into thinking that they are fighting a fire in a conventionally constructed wood-frame residential building (photos 8, 9).

(6-7) The fire building is a single-wide mobile home, similar to the one to the left. Lightweight trusses supported a rain roof, as seen at the rear in photo 7. (Photos by Tony Garcia.)
(8-9) Amazing, illegal, but not uncommon in Miami-Dade County, this single-wide mobile home has been completely enclosed with stucco-finished wood-frame walls and a lightweight truss roof. (Photos by Abraham Iglesias.)

When an addition is constructed, its roof must be joined to the roof of the original structure. This is commonly facilitated by a “roof overlap.” An overlap is accomplished by either extending the roof of the original structure a few feet over the roof of the addition or extending the addition’s roof over the original roof (photo 10). Roof overlaps can make a structure vulnerable for a rekindle because fire can smolder undetected in the space between the two roofs and break out after fire companies have left the fire scene. My department has learned the hard way that firefighters must check for extension into a roof overlap whenever there is fire of any significance involving a structure with an addition.

(10) The added roof overlaps the roof of the original house. Holes in the original roof facilitate the installation of the addition’s roof trusses (note the metal hurricane straps strengthening the truss’s wind resistance). The holes in the original roof provide a path for the fire to spread into the space between the two roofs. (Photo by Enrique Rodriguez.)

Rain roofs and roof overlaps create a dangerous concealed space that can become a reservoir for fire and flammable fire gases that can fuel a backdraft or a flashover. The space between two roofs is similar to the area behind the knee walls of a finished half-story or attic level in the sense that it can conceal fire or contain hot, unburned fire gases that can burst into flame and seriously burn firefighters. A fire originating in or extending to the space between a rain roof and an original roof can feed on a generous supply of light-dimension, high-surface-to-mass roof trusses and plywood or OSB sheathing and flammable vapors produced when asphalt-impregnated roofing materials are heated.

Additionally, some homeowners will cut an opening in the original roof for a scuttle so that the space between the two roofs can be used for storage. A fire between a rain roof and an original roof cannot be suppressed by conventional attic firefighting tactics—that is, pulling ceiling below and directing streams into the combustible attic space. This would be very difficult or impossible because firefighters would have to penetrate the original roof from the underside of the original roof structure. Firefighters would have no choice but to direct streams in the space between roofs through vents or holes cut in the roof or gable ends.

A piercing nozzle can be very effective in getting water into the space between two roofs. Extreme heat and confinement of the space are perfect conditions for the indirect attack with water fog streams. This is performed by driving and flowing a piercing nozzle into the space or directing a fog stream though a small hole cut into the roof. Ideally, the heat and confinement of the space will cause the water to convert into steam and smother the fire.

A Bresnan distributor nozzle is also a highly effective weapon for fires involving rain roofs. Inserting and flowing a distributer nozzle through a small hole cut in the rain roof can apply water over a wide area (photos 11-13). When deploying a piercing nozzle or a distributor, control the flow with a valve connected into the hoseline at least 50 feet from the nozzle. This allows a firefighter on the ground to control the flow. If a piercing nozzle or a distributor must be inserted in more than one hole, shut off the flow at the valve on the ground, disconnect the hoseline from the valve, and allow the hose to drain. This will allow the nozzle to be deployed at another location without firefighters on the roof having to pull up more charged hoseline and drag it across the roof. It is critical to remember that most rain roofs are extremely lightweight and are very likely to collapse after just a few minutes of fire attacking their supports. Collapse of a rain roof can be hastened by imposing an additional dead load of heavy roof tiles and a live load of firefighters—who could fall into the fire burning in the space between roofs—and their equipment. Firefighters operating on a rain roof will be much safer if they are on an aerial ladder or work from the basket of an elevated platform. Similarly, directing streams through a gable vent or openings cut in the gable ends will be much safer than operating on a roof. Fire officers who send personnel on or under a rain roof have to wonder if the building department ordered engineering calculations to ensure that the structure and its original roof can bear the additional dead load imposed by the rain roof. Further, was the building department even aware that the rain roof was constructed? Operations on or under a rain roof must be conducted in accordance with a risk vs. benefit analysis, considering the risk of firefighters being injured by the collapse of a lightweight truss-supported assembly against the benefit of faster extinguishment of building materials that will probably have to be replaced.

(11-13) A Bresnan distributor nozzle can be very effective for fighting a fire concealed between a rain roof and a building’s original roof. Firefighters deploying a piercing or distributor nozzle will be much safer if they work from an aerial ladder or from the basket of an elevating platform apparatus. (Photos by Enrique Rodriguez.)

A rain roof can fool firefighters into thinking that they have a fire out when it is still burning undetected and inaccessible under a rain roof overhead. Consider this scenario:

Fire companies arrive at a working fire in a three-story apartment building. A fire, originating on the second floor, spreads by way of combustible balconies to the third floor and into the cockloft (photo 14). Firefighters attack the fire aggressively with hoselines stretched and operated on the second and third floors. They stop fire extension into the cockloft by pulling ceilings and directing streams overhead (photo 15). These are conscientious firefighters; they pull the third-floor ceiling until they observe clean, unburned wood. Further, they are well aware of the propensity for combustible cellulose insulation to smolder undetected and precipitate a rekindle hours or even days after firefighters leave the scene. As firefighters were reloading their apparatus hosebeds, someone noticed smoke, followed by flame, issuing from the fire building’s pitched roof (photos 16, 17). The firefighters were dismayed and discouraged. How could they have missed fire remaining in the cockloft when their overhaul operation was so meticulous? At some point, one of the firefighters exclaimed, “I thought this building had a flat roof!”

(14) Fire starts on the second floor and spreads by combustible balconies to the third floor. (Photos by Stephen Wilcox.)
(15) Firefighters stretch hoselines to the second and third floors to rapidly attack the fire. The third-floor ceiling is pulled to check extension to the cockloft, and the fire is believed to be out.
(16-17) Fire rears its ugly head from a pitched rain roof that had been constructed over the original flat roof. (Photos by Stephen Wilcox.)

It turns out that this building DID have a flat roof when it was built in the early 1970s and still had a flat roof, but it was covered by a rain roof that was constructed sometime in the 1990s. Firefighters performing overhaul pulled ceiling and extinguished fire in the cockloft below the original flat roof; however, they were unaware that there was another cockloft over their heads that they could not see and could not reach. Firefighters now realized that the only way to fight this fire was from above. The firefighters proceeded to cut holes in the pitched lightweight rain roof from the basket of an elevating platform, then they directed a high flow-high pressure master solid stream from the basket of their elevated platform into the openings. The powerful master stream striking the underside of the roof had so much hydraulic force that it tore the sheathing from the trusses and sent it flying, exposing the fire.

A fire involving a rain roof is a perfect example of why it is essential that companies operating inside a fire building must report conditions to an incident commander (IC) who is observing conditions from a vantage point outside a fire building. When conditions reported from inside a fire building are not consistent with what the IC observes outside, it should raise an immediate red flag: Something is not right! For example, say companies operating inside report that they have knocked down the fire and checked the attic for extension, but the IC observes fire or smoke pushing from the roof. Could it be that the companies operating inside are unaware that there is fire over their heads in a space between the original roof and a rain roof? It is difficult for an IC to adequately observe conditions outside a fire building when he is sitting in an air-conditioned command vehicle, wearing a headset, and is engrossed in a command board and checklists. That may be fine as far as the incident command system is concerned, but it interferes with the IC’s ability to observe conditions. Additionally, consider that often the IC is the most experienced officer on the fire scene. If a fire department requires the IC to remain in a vehicle, then it is essential that the IC designate another officer early in the incident to be his “eyes and ears,” observing conditions outside the fire building and reporting them to the IC.

Rain roofs are also a perfect example of why fire companies must get out in their response areas, including mutual-aid jurisdictions and districts, and familiarize themselves with buildings under construction and under renovation. Firefighters who observe a construction dumpster in front of a building or any indications of construction or renovations ongoing at a building should be inquisitive: Could changes in the construction of a building affect the safety of firefighters?

Recently, I saw a disturbing television news report of a city councilman following a fire company in his car and videotaping the firefighters, who, in his opinion, were “joy riding” and “wasting fuel.” This pompous charlatan is obviously ignorant of how important it is to learn the structures in their areas and to practice driving large and heavy fire apparatus. The politician does not understand that when it comes to firefighting and firefighter safety, preparation is key.

Thanks to Chief Tony Garcia and Paul Dansbach for their assistance with this article.

BILL GUSTIN is a 38-year veteran of the fire service and a captain with the Miami-Dade (FL) Fire Rescue Department. He began his fire service career in the Chicago area and conducts firefighting training programs in the United States, Canada, and the Caribbean. He is a lead instructor in his department’s officer training program, is a marine firefighting instructor, and has conducted forcible entry training for local and federal law enforcement agencies. He is an editorial advisory board member of FDIC and Fire Engineering.


More Fire Engineering Issue Articles


Fire Engineering Archives

No posts to display