What I`ve seen so far.....
One roofer is happy to use battens (!), slides the end of the hook up under the slate above, until the hole lines up with the batten and drills through the hole in the hook, through the slate & into the batten. Slather it with quality silicon, and whack a nice sized screw in there. Then cuts strips of flashband, and sticks them over the hook.
MY roofer isn`t doing it like that. He`s going for the rafters (I go in the loft, and give him distance from internal pine end, and spacing), drilling the same way (yes, it penetrates the felt, but its over the rafter. IF there is a weep, it`ll run down the rafter, and out at the soffit. He`s also using pieces of lead, `cos he considers flashband to be a temp fix; rather than a permanent waterproofing solution.
To a point, I don`t care much. The roof, and its continued weathertightness, is the roofers` responsibility. If I get a call about a roof leaking, HE is going to go and fix it, at his own expense!