I've been talking a lot to my parents about mounting a mag mount antenna on the roof of our house by drilling in a metal plate. meanwhile, they're skeptical about the lightning threat, but I'm more wondering about how well this would work and if it actually would be grounded?
Would it work? Yes. But there's a lot of details that would be tacked on to that.
The sheet metal would need to be at least 1/4 wave length radiating out from the base, so for 2 meter VHF, 19" in all directions, or 38" across.
Then there's the issue with what grade and how long the coaxial cable would be. Magnetic mounts are usually intended for mobile use, and will have a short length of RG-58 coaxial cable. If you can reach your radio with 10-15 feet of cable, it's probably going to be OK. Much longer than that, and feed line losses become an issue. You would want higher grade coax for anything longer.
"Drilling in a metal plate" raises some questions. Are you talking about screwing down a piece of sheet metal onto the roof and placing the magnetic mount in the middle? That would be fine, however a couple of things to think about:
1. Make sure the screws do not pierce the roofing material all the way. A leaky roof will reduce your chances of getting to keep an antenna on the roof.
2. Sheet metal that has enough steel in it to hold a magnet runs the risk of rusting. that rusting will lead to rust streaks coming off the roof. Might be fine if it's not visible from the street.
Lightning is always a threat, doesn't matter if there is an antenna on your roof or not. But sure, putting an antenna on the roof has some risks. It doesn't even require a direct lightning strike, a nearby strike can induce a lot of energy into the antenna and cable. That can damage your radio, start a fire, etc. Using a proper lightning suppressor is required by the National Electric Code. So is properly grounding the antenna. That's not impossible to do, but it's not cheap, either.
Yeah, some hobbyists skip the grounding and ignore the code and usually their houses don't burn down.
It would not be grounded. Magnetic mounts don't necessarily ground through the mount. It relies on capacitive coupling to work. That's fine for RF, but it's not the DC ground that's required by code. The sheet metal may not be grounded that way.
Depending on the radio and how it's powered, it -may- find a path to ground through the coaxial, to the radio, to the power supply and into the home electrical ground. A lightning strike finding a path through the coax, through the radio, through the power supply and into your home is going to cause a lot of damage.
I know, not what you wanted to hear, but doing this sort of stuff correctly is something you can do, it just takes some money and some work. Mounting an antenna on a house is a big step, and as always it depends on the person that owns the house. Even when you own the house, the spouse can often over rule you.
A good option might be to set up a temporary antenna mast that you can put up and take down as you need it. There's a lot of people that do that when camping, and it can work pretty well. Only caution in a residential environment is making sure nothing ever comes close to power lines. That'll do worse things than damage your radio or burn your house down.
