<< We want to put a new metal roof over an existing asphalt shingle roof.
There is only one layer of shingles. Some people have said that we need wood
firring strips, but I'm don't see why. Can't the metal be screwed down to the
existing shingles? Do we need felt paper between the shingles and the metal?
If we do need firring strips, what would be their purpose? >>
You need to back off, take a deep breath and talk to the people that know what
works. If you need explanations they can supply them.
First off, a metal roof is a much pricier covering than shingles, so you don't
want to screw it up with a half a**
ed installation. Your roofing vendor will
have complete instructions and will be willing to answer your questions in
detail if this is going to be a DIY project.
Next, do a Google search and you will likely be pointed to the Metal Roofing
Alliance. From there you will find lots of informative leads on types, tools,
and techniques. For example, it is possible with one system to go over exiting
shingles with their proprietary metal stand-off clips.
Finally, all metal roofing systems are unique, so you must install precisely in
order to have valid warranty. Carry on with your research and good luck.