Tear Off Vs. Overlay
The question often comes up about whether an existing composition shingle roof should be torn off or if an overlay (or recover) is possible. Overlays save on time, labor, and disposal costs. If the existing roof has only one layer, lays flat, and there aren't any problems with the roof deck, removal is not necessary.
Regardless of whether you overlay or tear off, you should properly ventilate your attic. In almost all cases, continuous soffit and ridge vents will provide maximum cross ventilation.
If any of the following are true, a complete tear-off of the existing roof system(s) should be considered.
Too many layers - 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.
Bad decking - if spongy areas are noticed when walking on a roof, or if you see sags between the rafters or trusses, there's a chance that some of the roof deck may be deteriorating or be damaged. Deterioration can be attributed to dry rot or delamination of the plies in the plywood due to glue failure. Dry rot is wood rot caused by certain types of fungi and if it isn't taken care of, it can spread. If there is suspicion of bad decking, then a full tear off should seriously be considered
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%.