Due to their low weight and unique design, many metal roofs can be installed over old asphalt shingles.
First, though, a few things need to be checked:
In many areas, building codes allow re-roof only over one layer of old shingles. If you have only one layer, this should not be a problem.
If there are any existing roof leaks, those need to be addressed and rotten wood needs to be replaced.
If there are any signs of any existing weight problem, that must be addressed. These could include things like sagging decking or buckling trusses.
Before installing metal, a layer of underlayment must be installed to protect the granules of the old shingles from rubbing against the back of the metal. Underlayment is often 30-pound felt but there are also many newer synthetic underlayments. With some metal roofs, if you install 30 - pound felt, a slip sheet must be installed over it, to keep the felt from sticking to the back of the metal roofing (which could cause it to tear when the metal expands).
There are many types of metal roofs available. Most steel roofs weigh from 80 - 145 pounds per square (100 square feet) and most aluminum roofs weigh 40 - 70 pounds. Asphalt shingles weigh from 235 - 425, by the way.
A good resource of metal roofing info is the Metal Roofing Alliance website at www.metalroofing.com If you go to that site, you can investigate the variety of products available, link directly to manufacturers, or find experienced installers in your area.
