Dean wrote and asked me:
My cottage has one layer of shingles. But my detached garage has two layers. They are getting very old with mould growing on them and are curling in spots.
Is it advisable to install metal shingles over the existing layer(s) of asphalt singles?
The low weight of metal roofing encourages such an application. Small amounts of curling are usually not a problem and a layer of underlayment is installed over the old shingles to minimize the impact of such curling.
Any biological growth will die once it is isolated from moisture and sunlight.
However, the building code in your area may dictate no more than two layers of roofing. Often even in those cases, though, building inspectors will waive the two layer maximum rule once the understand the low weight of metal.