I agree with jonwienke - use a mast bolted to the house and anchored in concrete at the ground. Here's why...
Using a tripod on the roof with any antenna that has anything more than a very small wind load is a recipe for disaster. The constant antenna flexing caused by wind will eventually lead to a roof leak unless properly maintained annually. As jon said, the stress on those 3 small mount points is much, much greater than a long mast bolted to the house. And, it shouldn't be bolted just at the top. I'd recommend attaching at the top and at 2 even points in between the top and ground.
Don't skimp on concrete for the base. For a mast that size, I'd put no less than 400 lbs of concrete. You'll be surprised at how small a hole that actually fills. 400 lbs should make a hole about 12" in diameter and 3' deep. I also suggest putting a "sleeve" in the concrete, rather than the mast itself. You will put a piece of mast with an inner diameter just slightly bigger than the outer diameter of the mast in the hole. That short piece of mast should extend about a foot from the ground, and go all the way to the bottom of the hole. This will enable you to remove the mast if necessary for maintenance, replacement, etc. without tearing up the mounting concrete.
Your ground wire needs to be as short as possible and, as jon said, go directly to the electrical main ground, as well as to it's own ground. The ground wire must be at least 4 gauge or bigger. Don't skimp on the ground. I would suggest talking to a local electrician to find out the minimum grounding requirements to meet electrical code. Any competent electrician should be able to tell you the wire gauge requirements, as well as the ground rod requirements (ground rod material, length, depth, wire gauge, number of rods, connection to the house main ground, etc.). If your ground is missing or doesn't meet code requirements you'll pay for it with the insurance company if you ever do need to file a claim for any electrical issue. It's very clearly stated in every insurance policy that any electrical problem caused by any installation that does not meet code requirements will not be honored. Insurance companies are NOT flexible about this. Don't find out AFTER a lightning strike that your installation isn't up to code. Even if you get very few lightning strikes annually, it takes only one and it doesn't need to be a direct strike. Lightning energy travels very far very fast. A strike a half a mile away can still cause damage to you. Don't take chances here.
Finally, make sure your mast is of a material suitable for an antenna mast. It's easy to Google to find out suitable materials and mast wall dimensions suitable for your type of installation. If your mast wall is too thin, it can bend and break under wind load.
You mentioned that you don't want your antenna on the roof for visibility reasons. I'm sure you already know this, but I want to mention it anyway. If you put your antenna below the roof line, it WILL change the radiation pattern and your house WILL block signals from the direction of the house to the mast. Perhaps you can live with the reduced signal level and with missing signals from that direction. I would do some research re: location of signals from which you want to receive and send. Make sure you know what you'll be blocking at your selected installation location. It would be bad to go through the effort of properly mounting your antenna only to find out that you cannot receive wanted signals.
If you have any way of doing a temporary installation, even if the height is lower than the anticipated final height, that would be an exercise worth doing. For example, if you know someone that has a portable tripod for field use, and can borrow that for a test. Even mounting the antenna on your mast and just leaning it against the house for a test (on a calm day, of course) would be good. My point is, it won't take much time to test from several locations and the results might give you some clarity about the performance of one locations vs another. Yes, the ideal location is right by your main electrical entrance. However, if your house blocks your most important signals at that location, then you might want to choose another location. Mounting the antenna above the roofline would make this unnecessary. However, if you're determined to keep the antenna visibility down and keep the antenna below the roofline, then it's worth the effort to do some testing.
Good luck and 73 - David, AG4F
Using a tripod on the roof with any antenna that has anything more than a very small wind load is a recipe for disaster. The constant antenna flexing caused by wind will eventually lead to a roof leak unless properly maintained annually. As jon said, the stress on those 3 small mount points is much, much greater than a long mast bolted to the house. And, it shouldn't be bolted just at the top. I'd recommend attaching at the top and at 2 even points in between the top and ground.
Don't skimp on concrete for the base. For a mast that size, I'd put no less than 400 lbs of concrete. You'll be surprised at how small a hole that actually fills. 400 lbs should make a hole about 12" in diameter and 3' deep. I also suggest putting a "sleeve" in the concrete, rather than the mast itself. You will put a piece of mast with an inner diameter just slightly bigger than the outer diameter of the mast in the hole. That short piece of mast should extend about a foot from the ground, and go all the way to the bottom of the hole. This will enable you to remove the mast if necessary for maintenance, replacement, etc. without tearing up the mounting concrete.
Your ground wire needs to be as short as possible and, as jon said, go directly to the electrical main ground, as well as to it's own ground. The ground wire must be at least 4 gauge or bigger. Don't skimp on the ground. I would suggest talking to a local electrician to find out the minimum grounding requirements to meet electrical code. Any competent electrician should be able to tell you the wire gauge requirements, as well as the ground rod requirements (ground rod material, length, depth, wire gauge, number of rods, connection to the house main ground, etc.). If your ground is missing or doesn't meet code requirements you'll pay for it with the insurance company if you ever do need to file a claim for any electrical issue. It's very clearly stated in every insurance policy that any electrical problem caused by any installation that does not meet code requirements will not be honored. Insurance companies are NOT flexible about this. Don't find out AFTER a lightning strike that your installation isn't up to code. Even if you get very few lightning strikes annually, it takes only one and it doesn't need to be a direct strike. Lightning energy travels very far very fast. A strike a half a mile away can still cause damage to you. Don't take chances here.
Finally, make sure your mast is of a material suitable for an antenna mast. It's easy to Google to find out suitable materials and mast wall dimensions suitable for your type of installation. If your mast wall is too thin, it can bend and break under wind load.
You mentioned that you don't want your antenna on the roof for visibility reasons. I'm sure you already know this, but I want to mention it anyway. If you put your antenna below the roof line, it WILL change the radiation pattern and your house WILL block signals from the direction of the house to the mast. Perhaps you can live with the reduced signal level and with missing signals from that direction. I would do some research re: location of signals from which you want to receive and send. Make sure you know what you'll be blocking at your selected installation location. It would be bad to go through the effort of properly mounting your antenna only to find out that you cannot receive wanted signals.
If you have any way of doing a temporary installation, even if the height is lower than the anticipated final height, that would be an exercise worth doing. For example, if you know someone that has a portable tripod for field use, and can borrow that for a test. Even mounting the antenna on your mast and just leaning it against the house for a test (on a calm day, of course) would be good. My point is, it won't take much time to test from several locations and the results might give you some clarity about the performance of one locations vs another. Yes, the ideal location is right by your main electrical entrance. However, if your house blocks your most important signals at that location, then you might want to choose another location. Mounting the antenna above the roofline would make this unnecessary. However, if you're determined to keep the antenna visibility down and keep the antenna below the roofline, then it's worth the effort to do some testing.
Good luck and 73 - David, AG4F