Essential Details for a Successful Reroofing Project

Reroofing an existing home is one of the most impactful exterior renovations a homeowner can undertake. A new roof not only improves curb appeal and property value but also provides critical protection against the elements for the next 20 to 30 years. However, the success of a reroofing project depends as much on the details as on the quality of the shingles themselves. From ice dam prevention to valley construction and flashing details, getting the small things right is what separates a roof that lasts from one that leaks.

Assessing the Existing Roof Before Starting

Before laying a single shingle, a thorough inspection of the existing roof structure is essential. For a roof that has been in service for 25 years or more—as is the case with many reroofing projects—the underlying decking, flashing, and ventilation systems may need attention. Walking the roof to check for soft spots, visible sagging, or deteriorated sheathing can prevent costly callbacks after the new shingles are installed.

One critical preliminary assessment involves checking for evidence of ice dams. If the homeowner has experienced water infiltration around windows or interior walls during winter thaws, ice dams may have been the cause. As roofing contractor Richard Wright of Natick, Massachusetts, points out, adding a second layer of shingles over a roof with a history of ice dams will only compound the problem by adding more insulation on top, making the attic colder and the snowmelt pattern worse.

Ice Dam Prevention

Ice dams form when heat escaping from the attic melts snow on the upper portions of the roof. The meltwater runs down until it reaches the cold eaves, where it refreezes, forming a ridge of ice. As the dam grows, it traps water behind it, forcing it under the shingles and into the house.

Preventative measures include:

• Improving attic insulation and ventilation to keep the roof deck cold
• Installing ice-and-water shield membrane at the eaves, extending at least 24 inches past the interior wall line
• Ensuring continuous soffit-to-ridge ventilation to maintain cold roof temperatures
• Sealing all attic penetrations (plumbing stacks, wiring, chimneys) to stop warm air leakage

Valley Construction Methods

Roof valleys are among the most vulnerable points on any roof. Water from two roof planes converges here, creating a channel that must handle significant runoff during heavy rain. Three common valley methods exist:

Wright recommends the woven valley as the most practical and visually appealing option for asphalt shingles. When installing over existing roll roofing, the woven valley can create a sunken appearance. To avoid this, cut new shingles lengthwise to the exact width of the valley and install them vertically in the valley before weaving. This raises the valley flush with the rest of the roof plane.

Regardless of valley method, always keep nails as far from the valley centerline as possible—at least 6 inches back—to prevent them from being exposed to the concentrated water flow.

Chimney Flashing: Working with Lead Counterflashing

Chimney flashing is often the most delicate part of a reroof. Older homes frequently have lead counterflashing embedded into the chimney mortar joints. This lead sheet is bent downward over the step flashing to shed water. During a reroof, the instinct is to pull the lead away from the chimney—but this can crack the old, brittle mortar.

Instead, carefully curl the lead upward from its outer edge, not from the mortar joint. Install new lead step flashing for each course of shingles abutting the chimney, tucking it under the counterflashing. Once the step flashing is in place, push the lead counterflashing back down and gently mold it back into shape against the chimney using a block of wood and a hammer.

Surprisingly, detailed drawings of this flashing procedure are printed on most bundles of asphalt shingles. Taking the time to review these instructions on the job site can prevent costly mistakes.

Drip Edge and Layering Considerations

When reroofing over an existing layer of shingles, the drip edge at the rake and eaves deserves attention. Leaving the old drip edge in place means the new shingles will sit above it, creating a visible step in the roof edge. The alternative is to cut off the face of the old drip edge with tin snips and install new drip edge directly over the existing shingles.

A related consideration is the total number of shingle layers. Most building codes allow no more than two layers of asphalt shingles. If the existing roof already has two layers, the old shingles must be stripped entirely before installing new ones. Exceeding two layers adds excessive weight to the roof structure and prevents proper nail penetration into the decking.

Roof Ventilation: A Critical but Often Overlooked Detail

Proper attic ventilation is essential for roof longevity. The International Residential Code (IRC) requires at least 1 square foot of net free ventilating area for every 150 square feet of attic floor area (1:150 ratio) when a vapor retarder is not installed on the warm side of the ceiling. For balanced ventilation, half of this area should be intake (soffit vents) and half exhaust (ridge vents or gable vents).

When reroofing, this is the ideal time to upgrade ventilation. Adding a ridge vent, ensuring soffit vents are not blocked by insulation, and installing baffles at the eaves can dramatically improve roof performance.

Reroofing Cost and Lifespan Data

For more on roofing systems, see our article on asphalt shingle selection and installation and sustainable building materials for long-term performance.