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Thread: Flat roof ventilation

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    Default Flat roof ventilation

    Does anyone have any advice on cutting or removing insulation on top of a metal flat roof?

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    Quote Originally Posted by mnfire46 View Post
    Does anyone have any advice on cutting or removing insulation on top of a metal flat roof?
    Personally, never seen insulation on top of the roof.
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    I assume you are referring to a corrugated metal roof deck with insulation and roofing material on top of the metal. Sometimes found on commercial/industrial structures. Such a roof is likely supported by lightweight metal trusses. Due to the early collapse potential, it is unsafe to operate on a roof supported by lightweight metal trusses that have been directly exposed to fire. Collapse can occur within 5-10 minutes of exposure. This time frame can be reached prior to FD arrival. So as soon as we step onto such a roof, it may be ready to fail. Vinnie Dunn says in "Collapse of Burning Buildings" that if a fire in one of these buildings is serious enough to require roof venting, then the roof on that building should be considered unsafe to operate on. There is also a possibility that metal truss or I-beam will elongate when heated and push out the supporting wall, also causing collapse.

    The following is paraphrased from Francis Brannigan's "Building Construction for the Fire Service":
    The insulation is attached to the metal deck with adhesive. When a fire causes heat contact with underside of metal deck the adhesive vaporizes and expands. This can occur at temperatures as low as 800 degrees. The vapor can't get through the roofing material but it can be forced downward through cracks at seams of metal deck. The vapor then ignites and causes flame rollover at ceiling level. The flame is often blue in color, but may not be seen due to very heavy black smoke. This then causes more heat contact with underside of deck, repeating the process. The burning vapors can become a much bigger fire problem than the original contents fire ever would. To stop the process stream should be aimed at underside of decking in order to cool it and stop the vapor production.

    Another tactic suggested in "Fire Engineering" by BC John Mittendorf, LAFD, is to cut away the insulation and roofing material. This allows the vapors to escape out above the roof, thereby interrupting the process. It would have to be done in an area beyond the fire boundaries so as to be safe to operate. It is a slow operation but can be done with any wood cutting blade.

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    Quote Originally Posted by mnfire46 View Post
    Does anyone have any advice on cutting or removing insulation on top of a metal flat roof?
    While we've not seen this over metal roofs, many places have added over wood decked roofs with 2" foam board insulation and then the rubberized membrane. Last year we were able to tear apart an old elementary school roof with this condition and played with numerous tools/scenarios. We found that a carpet knife or large handled razor knife used to cut the membrane first helped, then cutting through the board insulation and decking. In this case the insulation board was glued down, making it difficult to find the structural members underneath without using the triangle inspection cut. Nearly all the new "disposable" buildings seem to be built with membrane material over board insulation over lightweight trusses, making for a recipe for disaster as CaptnJak alluded.

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    Cutting metal deck roofs supported by bar joists is controversial. I know the FDNY doesn't do it, but a lot of other department do. Your best bet to get through the insulation is to step cut the insulation first, pull it back, and then make the roof cut. Here is a video https://www.youtube.com/watch?v=BMWXuXfecQY and here are some pics from Columbia, South Carolina 7 Truck https://www.facebook.com/photo.php?f...e=3&permPage=1.

    An interesting note is that in John Norman's Fire Officer Handbook of Tactics (he's from FDNY) he says that roofs built on steel bar joists CAN be cut when a few precautions are taken. 1. cut 60' back from where the roof appears to be weak or is sagging and 2. have a hoseline positioned below cooling the bar joists with a hose stream.

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    What do we accomplish by cutting a hole 60 feet from the fire area? We are not really providing the desired vertical ventilation directly over the fire. We may be pulling fire (think flow path). We may be venting smoke. I'm not sure I see a tactical advantage in cutting the roof 60 feet (or more) from the fire. Steel bar joists are known to sag vs. wood joists that are known to snap. But is it guaranteed that the sagging will occur? And if it does are we guaranteed to see it? If a roof does sag it may be difficult to get out of the area due to slippery hot liquefied tar. I assume if we are cutting a roof, no previous vertical ventilation has taken place. Can't we then have high heat accumulations building up remote from fire area due to mushrooming. Can't this high heat affect all the joists? We are told that failure of one truss can lead to failure of other trusses. Brannigan and Dunn both put firefighter safety above roof cutting on these types of roofs. It should be noted that neither one of these experts have ever been known to let safety concerns cripple firefighting efforts. They just respect and honor risk vs. reward principle. As we all should.

    I should add that if we accidentally cut the top chord of a truss, it may cause collapse of that truss (and subsequently others possibly) regardless of location in relation to fire area.
    Last edited by captnjak; 01-27-2014 at 10:52 AM.
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    Quote Originally Posted by MemphisE34a View Post
    Personally, never seen insulation on top of the roof.
    They are all over the place up here on commercial flat roofs. Usually a metal corrogated roof deck, covered by 2 to 4 inches of of poystyrene or styrofoam sheet insulation screwed down to the roof deck. This is covered with a rubber membrane that may of may not be tarred over and cover with 1 to 2 inch rock.
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    My experiences have been with rubber membrane covering a 2 inch thick polystyrene over a metal deck, rubber over a 4 inch Styrofoam over metal deck and a real doozie - 6 inches of spray-on expanding foam over metal decking. All had metal joists.

    The rubber membrane, I find using an axe in short chops works best. If covered with rock, turn the axe and use it like a shovel or to push the rock out of the way first. Once through the membrane, use the axe to cut through the foam. Have found a roof hook or the adz end of a Halligan works good to get the foam up.

    On the layered spray foam one, that poised a challenge. Tried the axe-cut method but the foam was basically adhered to the decking. What worked best was to take the K-saw, cut your outer perimeter for the hole you intended to make. Then move over 3-4 inches, cut around a second time. Using the roof hook and the adz end of a Halligan (like a garden hoe), we pulled up the cut section and created a channel down to the decking. Then we cut the decking with no problems. Took a bit longer as we had to cut the same hole 3 times, but the labor effort was actually less intensive.

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