I'm guessing that you are referring to a corrugated steel roof, with only one layer of steel and no extra roofing material. This kind of roofing cannot be drilled into because that cuts a hole to let rain into the building.
Is this "Galvanized Iron" (GI) wire? I'm assuming that you make the wiring tight so the box does not move.
There are two issues with using the GI wire:
- Mechanical - Eventually, the wire can loosen a little and the box will not be stable anymore.
- Grounding - Electrically, after the backbox is held in place with the GI wire for a while, tarnish will prevent a proper electrical contact for ground.
I haven't seen anywhere that this type of mounting is "Acceptable". Beam clamps may be a better alternate to using GI wire.
I don't know if these will actually help you, but if there are beams holding up the roof, these clamps may help you. Check it out.
Google "electrical beam clamp". There are dozens of possibilities, and many different variations for the clamps. The claps are rated in how many pounds (or kilograms) of weight they will carry.
If you use two clamps for each electrical backbox, mechanically - `the backbox will be mounted quite solidly, and electrically - the backbox will be properly grounded.