When installing a metal roof, it is generally better to start with a clean surface by removing the existing shingles. While it is possible to install a metal roof over the shingles, it is not the best practice for several reasons:
Weight: Adding a metal roof on top of shingles can add significant weight to the roof structure. This could potentially exceed the load capacity the roof was designed for, leading to structural issues and even collapse.
Moisture Trapped: By installing metal over shingles, you create a sandwich-like structure that may trap moisture between the layers. This trapped moisture can cause rot, mold, and other problems over time.
Uneven Surface: Shingles may have irregularities or bumps that could transfer through to the metal roof, leading to an uneven appearance and potential water pooling areas.
Roof Warranty: Many metal roofing manufacturers require installation over a clean surface to uphold the roof warranty.
It is important to follow the manufacturer's guidelines and local building codes when installing a metal roof. In most cases, they will recommend starting with a clean surface.
Regarding the ice and water shield, it is not necessary to install it over the shingles if you are opting to remove them. The ice and water shield is typically installed directly on the roof deck before installing the metal roofing panels. It provides an additional layer of protection against water infiltration in areas prone to ice dams and heavy rain.