If a roof has more than one layer of roofing, the roof should be torn off. In most cases this is a code requirement. Codes rarely permit more than two layers of roofing. To determine this, all you need to do is contact your local building inspections department.
Below, are many other reasons you may need to consider completely tear-off the existing roof system(s).
Ice dams – ice dams can be a big problem. In areas where there the average January temperature can be below 30°F, and no ice and water protection membrane is present on a roof, then a full tear off should be considered. Twenty year old buildings with no ice and water protection and no prior problems, can suddenly experience thousands of dollars in damage when a freak cold front hits.
Incompatible shingles – if a heavy weight architectural style shingle is used to cover a light weight strip shingle such as 3-tabs, then the roof will look good. However, if a light weight shingle is used to cover a heavy weight shingle, the light weight shingles have a tendency to show all the bumps and ridges (called telegraphing) and won’t look good.
Existing roof is in poor condition – if the existing roof is in really poor shape, such as tabs being severely curled or if the rows are crooked, then complete tear off and replacment should be considered.
Shorter Life Span – There is no known documented research, but most roofing professionals agree that with an overlay, the average lifespan of the shingles will be shortened by about 10%-20%.