Low slope(7%) roof- use corrugated?

Tony Davenport
You usually recommend a mechanically seamed/sealed standing seam system for low slope roofs. I need some of your wisdom on metal alternatives for a 7%(steeper than 1 in 12)slope shed dormer roof that is 40' wide and the rafters are 9' long.The roof is in good condition but the stick-down EDPM will need replacement in the next couple of years. The roof is painted white to reduce heat gain.It is not visible from anywhere on the property so appearance isn't important. Unfortunately the rafters are stuffed full of insulation between the drywall ceiling and roof deck -what a mistake!).The insulation accumulates heat all day and emits heat for most of the night into the bedroom beneath. My idea to remedy this is to use single 9' lengths of high profile(3")22 ga.white Kynar coated corrugated steel.The lap joint between adjacent sheets would be 6" wide. It would have a continuous butyl sealing strip inside the overlap covering at least one complete corrugation. The fasteners would be sealed and capped. Both ends of each corrugated would have continuous vented closure.The ridge would be vented.The white Kynar will maximize TSR .The light underside color will act as a radiant barrier.The large space under the high profile 3" corrugated will vent off heat accumulating under the corrugated panels.The thick gauge means it will be walkable & less likely to buckle in the hot summers. What is the most durable sealer to put between the overlapping corrugations? It must remain flexible because these panels will move a lot in the heat.I presume the fastener holes would need to be oval to allow for expansion? My hope is that you can tell me what else should be done to make this roof both watertight and heat reflective for a projected life of 20+ years.Or,is there a better alternative?
Todd Miller
Isaiah Industries, Inc.
Always work with the roofing manufacturer to determine these details. Overall your plan sounds okay though. A butyl tape or caulk sealant would be most durable.
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