Metal roof shell over concrete slab

PJ VS
8/4/2011
Hello, I think I have a metal roof question that is of interest to anyone building strong in an area visited by hurricanes (or tornados). I am a few weeks off from pouring a five inch 2,300 sqft reinforced concrete gable roof slab (with a 1 vertical:3.25 horiz pitch). The positives of a concrete roof are several (resists hurricane wind uplift, termite proof, fire resistant...). However, I live in a hot & generally wet area with clay soils. Thus. despite our extensive foundations, I still need to consider the strong possibility of settlement cracks in said roof. I have researched several admixtures, chemical paints, and rubber coatings. In the end I believe a white metal Aluzinc roof "shell" is a good solution for waterproofing a concrete roof. From the estimates I've received, a metal roof shell would cost less than a chemical or rubber one. I dont wish to put a whole lot of expansive bolts or screws into my new roof slab in order to apply this metal shell. I was hoping to come up with something clever. Do you have any ideas???? I came up with the concept of casting 2x4 galvanized steel Z (or C) purlins in my slab. They would stick out about 1" to 1.5". A radiant bubble insulation foil would also be layed above the purlins (silmilar to this: http://www.ecofoil.com/All-Products/Double-Bubble-Insulation/Double-Bubble-Foil-Insulation-Foil-Both-Sides-4-x-125-500-sq-ft-). My builder has come up the concept of using cast-in 1"x2" retangular galvanized steel 'hollow' tubes (picture a square 'pipe,' sometimes used for metal pergolas and the such), placed every 4'. He thinks the Aluzinc roof should be flush to the concrete roof (& perhaps the same radiant bubble product directly inbetween). The idea is that the screws would penetrate into the surface-flush tube and cavity within - ie. also not into the concrete. He says the tubes would be easier to cast into the slab since they present a shallower profile. Would either of these ideas work? What about condensation issues? The steel z/c-purlins would have to run horizontal along the roof (right?) so any condensation may pond along the purlin (would some ventilation stop this?). Similarly, the square tubes are hollow and (even if they are capped) may also cause condensation and corrosion within the tube. Any bright ideas or heedings? I have researched this concept to a dead-end. In short I think a creative solution would benefit the metal roof industry. Gracias, PJ
Todd Miller
Isaiah Industries, Inc.
8/6/2011
I will open this up to see what ideas exist out there for you. All best.
Eric Novotny
An informed customer is the Best Customer!
8/6/2011
Couple of interesting concepts for a roof in this case. The pre-cast option with the integrated pipes is interesting. You would not need to worry about condensation inside the pipe as long as you seal it up against any air movement inside the pipe. Condensation at that point would be limited to the surface. The horizontal square tubes would create a damn effect of any condensation off the back of the roof though. Radiant barriers need airspace to work properly between the hot surface and what you are trying to protect. Here is what my idea might look like. Cast the concrete with the square tubes in it, apply an underlayment across the roof, install vertical purlins across the horizontals, put your roof across those. Now you have protected the roof from condensation, have a underdeck venting in place, and can also install your radiant barrier on the surface of the concrete roof between the pre-cast square tubes.

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