Over the years, many Amateur Radio operators have successfully used a metal roof as the ground plane or radial system for their HF vertical antenna. This is possible with a metal roof on a home, barn, garage, carport, RV, porch and metal sheds or awnings, and even on commercial buildings. The actual height of the metal roof above ground is not especially important for an HF quarter-wave vertical antenna.
In order to take advantage of a metal roof as a ground plane, you cannot elevate a quarter-wave vertical high above a metal roof. The key is that the antenna feed point, which is usually at the mounting bracket, should be the same physical level as the metal roof to offer the best ground plane effect. Additionally, the metal roof sections must be bonded together to create one large electrical surface.