When asphalt shingles are installed over existing shingles, where does leakage commonly occur?

Leakage commonly occurs at penetrations when asphalt shingles are installed over existing shingles due to several factors related to the roof structure and the nature of roofing materials. Penetrations refer to any areas where objects, such as chimneys, vents, and skylights, extend through the roof surface. These areas require careful sealing to prevent water from entering the roof assembly.

When new shingles are applied over old shingles, the underlying shingles may create an uneven surface. This can lead to inadequate sealing around the penetrations, where flashing may not properly align or be adequately secured, allowing water to infiltrate. Additionally, if the existing shingles are damaged or deteriorated, this further increases the risk of water intrusion at these critical points.

While leakage can also occur at eaves, ridges, and valleys, penetrations tend to be the most vulnerable areas, especially when new layers of shingles are added without addressing the sealing and flashing needed around these important features. It is essential for inspectors to pay close attention to these areas when evaluating roofs with multiple layers of shingles.