Actually, the tape is more waterproof than the snap caps were. It has been over 3 years that I first applied the tape. I have been through some horrendous downpours without a single leak. Even spent a week or so with several inches of snow on the roof. If it were going to leak, it would have done so by now.
I'm not talking about "duct tape". This tape a thin sheet of aluminum with a very sticky adhesive on one side. The roll I have (enough to last a lifetime) is from 3M.
Before I discovered the tape, I was replacing rivets and snap caps. It was a hassle and I still had
leaks from some of the rivets where the fore and aft cabinets are fastened to the roof. I intended the aluminum tape to be just a stop gap until I had time to get some help and do it right. But I haven't had any problems at all since applying the tape. Still have a couple of bags of rivets and snap caps around here somewhere. Even if I need to replace a rivet, I will use the tape to cover it instead of the snap caps.