Tearing off the old shingles can add anywhere from a few hours to a day to a roofing job. It can also raise the cost of the job from 25% to 50%. And then there's the problem of disposal. On Nantucket Island, Mass., for instance, shingles cost $250 a ton to discard. "Say you're doing a 30-square roof. You could be spending $750 or $800 in dump fees," contractor Jim Lydon points out.

For all those reasons, Lydon tells clients it's OK to roof over existing shingles if (1) it's a relatively new building and "basically up to code"; (2) the old shingles are "nice and smooth," providing a relatively even surface for reroofing; and (3) the beams and boards of the roof structure are in good enough shape to hold a nail.

Wood damage

Hank Jaworowski, owner of Contemporary Home Remodeling Corp. in Smithtown, N.Y., says the condition of the sheathing is the deciding factor in determining whether to roof over or reroof.

Generally, the rule of thumb for houses in Jaworowski's area is to roof over one layer of shingles and tear off if there are two layers. That will vary, depending on whether the sheathing is plywood or tongue-and-groove. TG can carry the weight of three shingle layers; plywood generally can't.

If the roof has one layer on it, Jaworowski asks the homeowners about leakage. An attic inspection for water spots or an inspection from the ground -- subtle ripples 3 to 4 inches wide indicate plywood delaminating -- is a good way to spot damage to the roof structure. Walking the roof will reveal spongy areas, also indicating decayed sheathing. When Jaworowski determines there's wood damage, he invites homeowners to go up on the roof with him and see for themselves. Three out of four take his word for it.

"Eighty percent of the time, if they have one layer of shingles and they leave it up to us, we roof over," Jaworowski says. Under those circumstances, he prefers to use architectural shingles, which "hide a lot of [aesthetic] imperfections."

No shortcuts

Jimmy Waller, vice president/development for Waller Construction in Lakeland, Fla., says 98% of the time, Goff-Waller Roofing, the company's roofing division, tears off old shingles before installing new ones. Roofing over, which Waller describes as essentially a "cosmetic" procedure, may shave 20% off the cost of the job -- the company's average asphalt-shingle roof job is $5,000 -- but that shortcut, he claims, will cost the homeowner 40% to 50% extra the next time a new roof's needed. (That's what Goff-Waller charges when crews have to remove multiple layers of compacted shingles.)

Waller says removing existing shingles provides the only foolproof means of inspecting the flashings and decking and providing a solid, secure grip for fasteners. Waller says that in Florida's climate, at least, roofing over creates a lasagnalike layer that, over time, will cause the house to bake and the roof to sweat.

"We feel removing the shingles down to the deck is the most prudent thing to do," Waller says. "The only time we roof over is when [the homeowner] requests it, and they have to sign a change order specifically saying we were directed not to remove the existing layer."