Which of the following is a concern when installing a new layer of shingles on top of an old layer?

Installing a new layer of shingles on top of an old layer can lead to significant concerns, particularly regarding water damage. When multiple layers of shingles are added, it may inhibit proper drainage of water and can trap moisture between the layers. This moisture buildup can lead to a higher risk of leaks and damage to the roof deck and underlying structures. If water is retained, it can cause deterioration of the wood, mold growth, and ultimately compromise the integrity of the roofing system.

Ventilation issues also arise when a new layer of shingles is added, as the additional material can restrict airflow, leading to insufficient ventilation in the attic space. This can further exacerbate moisture issues and increase energy costs due to heat buildup. The weight of the new shingles is indeed a concern, but unless the roof structure is significantly compromised or overloaded, it may not be the primary issue compared to moisture-related problems. While cosmetic appearance is an important factor for homeowners, it does not pose a functional risk like potential water damage or ventilation issues would. Thus, the gravity of water damage makes it a critical concern in the context of roofing practices.