How do you put a new metal roof over an old metal roof on a mobile home?

This will depend on the existing roof material. Also, whether you can do it without confronting disapproving neighbors or code enforcement. And, you will want to make some observation of the roof structure below the surface. With basic carpentery skills, a flatish existing roof material, and freedom from disapproving by standers, I would think it not too difficult. A person should think the details through, perhaps coming back to them at another time when reaching a sticking point. The main supporting members of most roof surfaces are either rafters or purlins. Rafters run up and down a roof, (eave to peak). Purlins run across a roof. Once a roof is already sufficiently framed, either will work. In a reroof I might prefer rafter style so any slight leakage would be able to run down and out more quickly. I would probably want to use 2x4 rafters on their sides, rather than standing, each rafter being 24" apart. Although, it would be wise to get an idea of the primary framing your existing roof already has. If it is 16" OC, (on center), you would probably want to put your new wood 16"OC, over the old. The trickier parts to think through would be the top, bottom, and face of your new, secondary roof framing. There are decisions to make at each one of these. Example: Do I want to leave a ridge vent at the top? What kind of eave shape do I want? Vented at the eave, or not? Do I want to extend past the old roof? And so on... Another consideration could be whether you want to put in some reflective insulation or seal the old roof for extra protection. Also, do you want to put more OSB plywood on your new, secondary roof frame, or just leave it bare as long as there is adequate support? For example, if you put 2x4 rafters on their sides, 24"OC-and added cross supports every 2 to 4 feet, then you could probably fasten standard 26" metal roofing well enough to avoid rippling noises. Just make sure you put the cross supports at points that the metal sheets overlap from top to bottom. The 26" roofing sheets are designed to allow side-to-side overlap and remain on a 24"OC rythm pattern. How ever you do it, remember to address the ridge, eave, and face considerations. The face would be where, on some roofs, you can see the profile of the roof shape, or pitch. It's these three places that are most likely to leak in heavy storms and precipitation.