Hi,
I'm installing a Bosch on-demand water heater in a house that has metal roofing (in Hawaii) and need to pass the propane combustion product vent pipe through the roof, which will probably require cutting a hole through one of the corrugations of the roof. It's not the round continuous corrugations but the type that has level spots and then triangular corrugations about every foot. My question is: what is the best method to seal the hole around the pipe waterproof (it rains A LOT here). My best idea so far is just to just seal it with that sticky black flashing sealer (9'' wide) or maybe roof cement.

Thanks very much.

How big is this pipe?

Can you move it to a position that exits the roof in the flat area instead of on the raised?

If you can do that,you can use a typical roof boot(available through a heating and air conditioning company, or maybe even the local lumber yard)