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I used a magnet. The bows are steel and the roof skin is aluminum. There are diagrams of the specific bows (and everything else) for each particular bus on the Prevost parts assist web site – which I found useful for planning my approach to putting my roof airs on. As has been noted, proceed slowly due to wires, etc in ceiling. For one roof air I was able to run a bow through the middle of the unit to make the location work. Fishing the power wires and condensate lines – which I tagged into the condensate lines of the cruisairs – is fun. Supplemental roof support should be added and there are various ways to do that – some simpler than others. Drop me a note if you have questions or need more info. Mike