I've used Glen Martin roof towers for many years (I also have ground-mounted towers, but you can never have too many antennas...) and they're very good.
Inexpensive, lightweight, strong, easy to assemble and because they have four legs, once installed you can tilt them over in either of two directions, which can be a very handy feature.
If you cannot access the attic, you need to find out where the rafters are. A stud finder (electronic instrument) might work for this. Sometimes roofing is too thick, and all wood also, so stud finders are fooled and can't really find the rafters through the roofing. In that case, you drill pilot holes! Yes, I've done this, on pretty much every house I've ever owned (15 of them now) and it's non-destructive.
I use a long but small diameter drill bit, like 1/16" x 5" or so, and start drilling. You can tell if the bit goes right through the roofing, or if it hits a rafter. Then, by drilling 1" to the left and right of the "I hit it!" hole, you can determine the approximate center of the rafter. Mark that by putting a wire coat hanger bent to prevent it from slipping all the way through (or a screw, or some other convenient "marker") and move 16" to the left and right of that location to see if you find two more rafters, on 16" centers.
Sometimes, they're not. I've found rafters on 24" centers, or on "not really quite" 16" centers, and all sorts of stuff. But with patience you can find three in a row across the roof, on each side of the peak. Mark all "rafter centers" and then use permanent roofing patch to patch the other pilot holes. You can get colored patch material that dries in 12 hours and fully cures in 24 hours, is completely weatherproof, and matches the color of your roofing so it cannot be seen at all, even with a close inspection.
Once you have the rafter centers marked, use a pressure treated (and then painted to a color you like, probably to match the roofing color) 2 x 6" overlapping three of them in a row on each side of the roof peak, and appropriate lag screws (I use hex head lag screws, usually 7/16" or 1/2" x 7-8" long, with fender washers under the heads to form a permanent weatherproof seal against the wood) to mount these exterior supports.
Then, you lag the roof tower mounting brackets to those using similar materials, and use roofing patch on all bolt heads. I use that very sparingly so I can remove the bolts if I want to. It does not take "much" roofing cement (I use "Henry" brand, which is fibreglas filled and incredibly strong) to do a wonderful job. Probably 8 ounces is enough to patch everything.
When you find the locations for the mounting brackets, of course the tower must be attached to them! Use a carpenter's level to assure the tower is perfectly vertical, measured from multiple directions; mark the hole locations for the brackets, remove the tower with its brackets, and drill.
My 9' G-M roof tower at my current home was installed by me, from "start to finish," including "finding the rafters," in less than one hour. It really doesn't take long.
I've tilted it over a few times for service or antenna changes over the past 9 years, and it still looks new, shiny, and exactly as it looked when first installed. In torrential downpours, it never leaked a drop -- nor did any of the other installations.
These things are a good deal for someone who already has a 2-story home -- you get a "free 20 feet" or more of elevation right from the start!
One disclaimer about the roof towers: They don't provide any "extra" hardware, at all. As such, don't lose any...