Roof replacements do not always involve stripping the existing shingles. In fact, sometimes reroofing on top of older materials has financial and functional advantages. That said, deciding to keep or discard your shingles requires the help of a professional. You should call a contractor to assess the age, condition and workmanship of the existing roof before going ahead with your project.
Contractors can only overlap asphalt shingles. Wood or slate shingles should never double up nor mix with other materials. Although some municipalities prohibit layering shingles, most building codes do permit up to two layers. A third poses safety concerns; the weight would exceed the support of underlying structures.
When done professionally, old shingles can provide a cushion for new ones. In fact, they can act as a second barrier against weathering. More importantly, the choice to reroof over existing singles is one of cost and convenience. By not ripping up the existing roof, you save on labour and waste disposal.
Pulling up existing shingles is sometimes the only choice. Contractors cannot build atop older materials without prepping and repairing the existing roof. Unfortunately, with the shingles still intact, the contractor cannot replace or assess the underlayment. Thus, reroofing is infeasible for older homes, as the underlayment nears its replacement date.
Depending on the condition of the existing shingles, reroofing can also void the warranty on new shingles. To avoid this from happening, consult a professional. A contractor can advise you on your decision and help avoid such problems from arising. Many even come with service guarantees and special manufacturer warranties of their own.