Side note: Plain metal roofs are extremely common here in Texas for the same reasons noted they are common in Louisiana, but I noticed that someone above mentioned that the old roof is sometimes left underneath for insulation purposes.

Our neighbor's house burned just before New Year's and one of the things that made it very difficult to put the fire out was that the attached garage had a metal roof that trapped flames underneath, where they could not be reached by water. Also, the firefighters were concerned that there might still be wooden shingles under the existing outer roof, which they told us would engender a much higher fire hazard/difficulty in putting out the current fire because there would be more material to burn. It turned out that the old shingles had been removed when the house was re-roofed, but we made sure to check in turn that our old shingles had been removed when our house was re-roofed after Hurricane Rita. (We don't have a metal roof.)

We still don't know what caused the fire at our neighbor's, but we've decided that, just in case, the wiring will be the next thing we get checked out.