Are you certain that you need to be up at roof level to achieve reception at your house in Phoenix? Typical elevation angles are between 30 and 50 degrees, so unless something is blocking you to the southeast, I would think that a pole cemented into the ground would work in most locations.
With that concept in mind, you could go to a steel yard (or even a scrapyard) and get a full 20 or 21 foot length of steel pipe (3 or 3-1/2 inch outside diameter),
and cement at least four feet into the ground with a pillar mount. Before cementing, get a 1-foot length of the OD pipe that you will be using on your motor drive mount, and weld to a transition to the larger pipe. A free standing pole could be 10-12 feet off the ground; relatively accessable with a tall stepladder, but above all local objects.
If you MUST go to the roof to clear obstructions, use the same technique, and cement a full length of pipe into the ground about the same depth. Using gradually smaller diameter lengths, weld several transitions down to the size of pipe that you really need, leaving about one foot sticking out of the top.
Pipe is generally in 1/2 inch increments, so 3 or 4 2-foot pieces of each smaller diameter should fit into each succeeding pipe like a glove. Weld them into place.
Now for the tip of the week: Let's assume you need a 2-inch OD mast for the motor. Get a 6-inch length of 2-inch inside diameter pipe, and confirm that it slides over the smallest diameter pipe easily. Drill a pair of 1/2 or 5/8 inch holes into it...or have a welder torch holes of this diameter, and weld a machine nut
for that size bolt (1/2 or 5/8 inch) over each hole. Appropriate machine bolts
will be installed after you have finished the next step, and painted the metal.
Near the peak of your roof, measure horizontally how far it would be to span between either side. Weld a piece of flat bar of that length to your above
"clamp" with the two machine bolts, centered on either side. Drill at least
3/8 inch holes near the ends of the flat bar. These will anchor your "tower"
to the side of the peak. The trick is to dig a hole below. centered on the
middle of the pipe, which is leveled for vertical in that hole (4 feet deep)
and cemented into place.
A crazy contraption, but something I used to do out of necessity in Alaska, where the highest elevations at south barely touched 20 degrees, and rooftop mounts were usually the only way to get a few satellites.