Square foot is the major factor of cost. There are 3-tab shingles, architectural shingle and metal roofs. Most factor of cost depends on the type of roof that the customer chooses.
We advise against this for a few reasons. If your putting a metal roof on, laying on top of the shingles will terminate the 40 year warranty on the metal purchased. New shingles may not lay flat over existing ones resulting in “bumps” in some areas. Shingles are incredibly heavy, excessive weight on the roof structure can result in a sagging appearance.
The primary factor for knowing its time, is the age of your roof.
Typically, it acts as a release between the two material. The type of roofing product used will determine the weight of felt needed for the job.