I always recommend mounting tv antennas as high as possible.
Installing antenna on 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.
Tin roof mount where to place the mount.
Thus buildings with a metal roof or many metal components will face difficulty watching tv.
You can also buy non penetrating mounts that use weights rather than screws to hold them in place.
But what if you have a metal roof.
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.
So you may want to put the mount on the ridge capping at the top of the roof.
Be sure that the antenna can see over the roof.
A metal roof discourages the transfer of signal by reflecting it away from you.
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.