re-roofing job

I have a cedar shingle roof, and it is time to replace.
I am thinking that I don't want cedar anymore.
I am considering asphalt, and metal.
I plan to live in the house at least ten years.
Can I put the metal roof directly onto the purlins that the cedar roof sits on?
Does there need to be an underlay of some kind? There is an unheated attic between the roof and the second floor of the house.
If I go with asphalt, do I need to place a plywood underlay?
Thank you in advance,
Ron
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Go with composition (fiberglas) shingles, not asphalt. Yes, pull the cedar up, and what its nailed to. Lay minimum thickness 1/2" CDX plywood spaced properly and felt paper atop that.
Don't think the separation distance vertically is the same for nailers for metal vs. cedar shingles. Sure someone knows here
Ice damming can be a problem on any type of roof if the local climatology presents such cold temperatures and precipitation. Administering to that condition vs. living in the far south U.S. presents 2 entirely different solutions. A "gray" zone of the 2 is your choice of which to administer.
Cannot ascertain putting any type of roofing directly on purlin. http://www.answers.com/topic/purlin
--
Dave

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