These are the steps I'd take If I were to mount a tripod to my roof.
1. Figure out the placement of the tripod, and bend the tripod's feet so they make flat contact with the shingles.
2. If possible, I'd use an ultrasonic stud finder to locate the rafters and to place at least one (and preferably two) of the tripod's feet directly over a rafter.
3. I'd fully coat the contact surface of each tripod foot with a good layer of blackjack.
4. When all three feet were coated, I'd press the tripod firmly in place.
5. I'd then circle the top of each foot's bolt hole with a complete ring of blackjack (smaller diameter than the fender washers).
6. I'd put the fender washer on the lag screw, and drive the lag screw home through each foot of the tripod.
7. I'd be extra careful to not overtighten the lag screws, as they can easily strip out the OSB roofing material.
But that's just what I'd do. You might want to ask an experienced roofer or professional antenna installer what he or she would do.
Actually, I have this decision coming up for myself. I recently re-sided and removed the wooden chimney chase I had my anemometer mounted to, and now I have to decide whether to mount the anemometer directly on a tripod on the roof, on a bracket on the side of the eaves, or on a free-standing 4x4 post in the garden. I like the idea of not having an ersatz lightning rod on my house, but the garden post places the anemometer only 15 feet in the air, and that's downhill from the house itself. Decisions, decisions.