corrugated metal roofs, I never make any holes in them I have however attached to corrugated deck roofs. I will all depend on the design of the roof, and what the manufacture recommends, as with any roof system check first.
I'm imagining those steel buildings, where all you see is steel beams and puffed-out plastic wrapped fiberglass insulation. If that's what you have, then there is NOTHING between that fiberglass and the sky but that single layer of sheet metal.
Naturally, you do not want to punch any holes in that metal. I've hung lights by attaching beam clamps to the lighter steel framing pieces. This is also a good place to use SO cord for the whips; MC seems to impart a twist to the fixtures, leaving you with a pretty sloppy look.
I was really impressed with the T-5's I installed. In a bay measuring about 50x80', with the lights mounted 22 ft. up, a dozen 5-lamp fixtures lit the place brilliantly.
In another place, I found the same fixture to be more than a match for the 400w Metal Halide lights that had been there.
IF you are lucky enough to have two layers of sheet metal - that is, steel-insulation-steel - for the roof, then you can mount directly to the inside layer. Nothing will really hold besides toggle bolts.
I'm finally done with my nap...if redsy is talking about roofs that are supported by steel bar joists, then he could simply throw a piece of shallow strut between two joists up at the roof pan and with twirl nuts and eyebolts hang the fixtures from that .
we also used a piece of 3/4 EMT with minnies and S- hooks to support 8' strip lights
Just a hunch but I think the corrugated roofing you are talking about, might be pan decking. lightweight concrete is poured on top of the pan decking. When you look up and see the lighting supports it appears they are attached only to corrugated roofing when in reality they are anchored into the concrete above.