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.
Installing antenna on metal roof.
Whatever you do do not put holes in your roof to attach your antenna.
So you may want to put the mount on the ridge capping at the top of the roof.
In order to mount an antenna on your roof without damaging it you should attach the antenna to the gable the fascia or the chimney.
I always recommend mounting tv antennas as high as possible.
They can also be put lower on the rows of screws that hold the metal sheeting to the roof batons.
Be sure that the antenna can see over the roof.
You can also buy non penetrating mounts that use weights rather than screws to hold them in place.
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.
Thus buildings with a metal roof or many metal components will face difficulty watching tv.
But what if you have a metal roof.