In some areas of the country new shingles are placed over old shingles. It is quite all right to overlay a second roof on shingles that are not totally lifeless, although I’m not sure that I would practice this methodology in the South. I’ll mention more considerations later.
It is even acceptable in some areas to put a third layer of shingles on a roof. I would not go over a third layer. If a roof needs to be replaced a fourth time, it should be stripped and new tar paper put down before shingling. The number of layers may depend on your particular building codes, so you need to check.
In general, the old shingles should not be so dry that they break or crumble when you flex or apply pressure to the surface of a shingle. Newer shingles will become mushy in hot weather. When old, lifeless shingles remain brittle when they become hot. If new shingles are placed over lifeless ones, the surface granules on the old shingles and pieces of shingles will continue to flake off. This could cause leaks to develop.
One of the real keys in making the decision to overlay new shingles over old ones is whether the roof will take the load. In areas where overlaying is practiced, the roofs are built to take heavier loads, such as snow. Rafters are heavier, and they are usually 16 inches on center. Many of the roofs in the South are constructed using rafters or trusses that are 24 inches on center. Quite often trusses are made from 2x4s, and although they are quite strong, their load-carrying capabilities can be limited by the types of joints that were used in their construction.
The pitch of the roof is also a factor. Usually, the steeper or more pitch a roof has the longer the shingles will last. This can be seen in the “bell” roofs in our area. The steeper part of the roof will outlast the lower pitch of the “bell” portion of the roof.
Even the thickness of the sheathing is greater on most Northern homes. The sheathing on a roof helps to distribute the weight of exterior loads such as snow more evenly on the rafters. If sheathing is too thin, it will bow under the load.
Southern summers are brutal to shingles. We get high temperatures, pounding rain, mildew and many other drastic and sometimes rapid changes in weather that rob the shingles of life.
You can give a roof a fair inspection without ever setting a foot off the ground. This can be done using a good pair of binoculars. The roof’s surface condition can be scrutinized easily by looking for patches where the grit has worn away. The lower edge of the shingle tabs will also begin to curl when shingles are in need of replacement. This curling can be seen very easily with binoculars before it is apparent with the naked eye.
I would recommend that it is better to remove the old roof. Removing the old surface gives the roofer a chance to examine the sheathing to see if there are any weak or problem spots that need attention. An example of a problem that may not be evident is caused by a buildup of leaves or pine needles. Leaves or other types of trash which accumulate on a roof cause moisture to be held in that spot. This moisture can be drawn up under the shingles and cause the sheathing to rot, even though there may be no apparent leak.