The SWR of an antenna mounted over a metal roof is only one aspect to consider. If the SWR changes, you can be sure that the roof is reflecting RF power. Now the roof is at least partly replacing the soil below it, as the ground surface. This effectively reduces the height of the antenna above its ground and increases your take off angle. Adjusting the SWR does not restore the clean pattern.
Even worse is when the antenna is at the edge of a metal roof and too close to it. In that case, the ground under the antenna does not appear level and that can introduce undesired beam tilt into the pattern, making it skew away from being close to the horizon. With the back of the beam over the roof, the signal tilts into the ground while the other direction is above the horizon. If the SWR changes with direction of the beam, you know the ground reflection is not even.
