Clive,
I modified a carry out to be a roof mount along with an additional ground stand option. I had used three screws per mounting point and my choice were self drilling hex head screws. I made that choice because the tips are sized so that the tread gets a good bite out of the material being screwed into. The drilling tips are sized properly for the thread. My choice of sealant adhesive is 3M 5200 which you can get at Lowes. I put some under the metal mounts, some on the screw threads, and then when the screw mounting was finished I lapped over the "tabs" and screw heads. My only caution is that the roofs are thin and you need to tighten the fastener carefully the last several turns by hand so as to not strip the holes out. The sealant/adhesive does a lot of the mount securing. Photo of dish mounted with screws covered with sealant.
Later Ed