There are two options available for reroofing installations. One would be to tear off the old roof before applying the new one (tear off). The second would be to lay new shingles over the existing roof (lay over). While the second choice is the less expensive of the two options, it is not necessarily always the best choice.
There are advantages to tearing off the old roof before installing a new one. For example:

• If there are any defects in the roof deck, they will be revealed when the roof is torn off. These defects should be repaired before applying the new roof.

• If condensation problems exist in the attic, they too will be revealed when the roof is torn off.
Properly designed attic ventilation can then be installed in order to help eliminate such problems.

• When the old roof is torn off, waterproofing shingle underlayment can be installed before applying the  new roof. This will help protect against leaks created by cyclical ice damage and wind-driven rain.

• Tearing off the old roof and starting with a clean deck before reroofing may result in a smoother finished roof system. Although there is added cost to these advantages, each lessens the likelihood that the validity of the manufacturer’s shingle warranty will be impaired. If the old roof is torn off, your contractor should be responsible for the cleanup and disposal of the old shingles, but make sure your contract states this clearly.

If you do plan to reroof over existing shingles, first check if your local building codes limit the number of roof layers that can be applied to a residence in your area. Most restrict maximum to 2 layers. Your contractor should know the pertinent code requirements.

You may be asking yourself, “What can I expect my roof to look like as this aging
process takes place?” One or more of the following conditions may occur over time:
Curling: As the asphalt hardens over time, the granules which were once securely
embedded begin to break away. Occasionally you may have seen the colored
granules in your gutters. Also, as this hardening advances, the asphalt layers begin
to shrink. Of course, all of this is occurring at a microscopic level and is not
something which will be noticeable on a daily basis. As the asphalt layer shrinks,
it is being countered by the shingle reinforcement, which resists shrinking. We
now have a situation in which the top and bottom coatings are shrinking and the
reinforcement is remaining stable. As a result, the edges of the shingle may begin
to curl over time. In addition, organic shingles may exhibit signs of curling which
might be considered excessive, however, this is not a manufacturing defect and
would be considered part of the normal weathering process of organic shingles.
Surface Cracking: Another manifestation of the normal aging process may
be the development of surface cracks. For example, as the flexibilizing oils of the
asphalt are depleted due to heat, the shingle becomes more brittle, to the point
where surface cracking may appear. The stresses created by thermal shock and
the movement of the roof deck also increase the likelihood of surface cracking.
Blisters: During the course of natural weathering, small bubble-like raised
areas known as blisters may appear on the surface of the shingles. The blisters
may be small and pea-sized or as large as a quarter. The blisters may be open,
exposing the asphalt, or closed. Blisters frequently result when minimum ventilation
requirements are not met.
Staining: Finally, over a period of time, shingles may develop dark brown or
black streaks that are sometimes mistaken for soot, dirt, moss or tree droppings.
In actuality, this discoloration may be caused by algae growth. Although most
roofing systems are susceptible to algae discoloration, it is most readily visible
on white or light-colored shingles.

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If you can get into your attic you should be able to see evidence of leaks. Look for staining and/or rotting wood at walls and penetrations.