Every re-roof requires a close inspection of the substrate. Incidental roof leaks, particularly ones that go unnoticed, can delaminate plywood sheathing or cause OSB to swell. The worst cases result in patches of rot. Leaks around deteriorated flashing are the most common. Ice damming in cold climates often destroys the sheathing at the mouths of valleys and sometimes in spots along the entire eaves, as well. These localized soft spots are often difficult to inspect from the attic, particularly if the damage is at or beyond the exterior plate line or over a cathedral space. Walking the roof helps, but frequently the problem doesn't present itself until tear-off.

Most shingle manufacturers agree that nails must be placed below the adhesive strip. If nailed above it, the nails may miss the underlying shingle, risking that this course will slide down the roof. Heavier laminated shingles are more prone to tearing loose, unless there's at least º to Ω inch of the underlying shingle above the nail.
The Journal of Light Construction Most shingle manufacturers agree that nails must be placed below the adhesive strip. If nailed above it, the nails may miss the underlying shingle, risking that this course will slide down the roof. Heavier laminated shingles are more prone to tearing loose, unless there's at least º to Ω inch of the underlying shingle above the nail.

Craig Bannister of Bannister Roofing/Siding based in Burlington, Vt., handles this the way many roofers do: He inserts a clause in his estimate, which clients sign and return with a deposit, specifying a $50 charge for every sheet of plywood that needs to be replaced. Customers occasionally balk at this, expecting a few thousand dollars added to the job if the entire roof needs to be re-sheathed. But most of the time Bannister convinces them that it's usually only needed to cover incidental problems. “I wouldn't be in business long if I doubled all my estimates,” he says.

A Hard Line on Roof-Overs Finding hidden problems assumes that the old roof has been torn off to begin with. When they see the labor and dumping fee for the tear-off, clients are quick to ask about roofing over the top of the existing shingles. Many code jurisdictions allow for re-roofing over as many as two existing roof layers.

“I used to do this, but not any more,” says Mike Guertin, a remodeling contractor based in East Greenwich, R.I., and author of Roofing With Asphalt Shingles (The Taunton Press). Guertin's hard-line approach is unusual, but easily justified. For starters, the International Residential Code now requires a self-adhesive waterproofing membrane applied directly to the sheathing for the first 3 feet of the roof, which then requires cobbling together a ramp to bridge the bump between the membrane and the existing shingles. A more important reason is the added weight that a second or even a third layer of 300-pound-per-square shingles can put on a roof. Many older homes weren't framed to handle that much dead load. And new shingles just don't last as long over existing shingles due to the increased heat build-up in the mass of asphalt. Heat is asphalt's number one enemy, and the new shingles tend to dry out and curl prematurely.

The Plank Problem Typically, sheathing problems are isolated to small areas, but old plank sheathing presents more of a challenge. The issue, Guertin explains, is the gaps between boards. On older homes, planks were often spaced for wood or slate roofs with wider exposures than the standard 5-inch tab on asphalt shingles. As a result, at least one, but often many, of the shingle nailing lines end up falling smack between boards, or so close to the edge that the nails lack holding power. And there isn't much leeway to reposition this nailing line (see illustration, left). Often the only alternative, Guertin says, is to re-sheathe the roof.

Fortunately, the plank problem is one that can be easily assessed from the attic. Guertin insists on crawling up there before taking on any roofing job — a step that few roofers take. “You can't see all the problems from underneath,” Guertin says, “but it's surprising what you can find.”

The most important problems often aren't with the sheathing or even the roofing, but with the presence of attic moisture. In extreme cases, the bottom of the sheathing will be black with mildew. Corrosion of fasteners sticking through the sheathing may indicate excessive condensation. In cold weather, moisture materializes as frost on the bottom of sheathing or on the ends of roofing nails, depending on conditions. The attic inspection is really the only way to appraise how well roof ventilation is working and to determine if the homeowner has an indoor moisture problem, both of which are vitally important to the durability of the structure and the comfort of its occupants.