Rainwater Systems & Galvanic Action

Our house has verandahs on some sections of the external walls. Traditionally, water from the main roof is collected in a gutter, and then fed on to the verandah roof below via "spreaders". This is the arrangment that we want to adopt (the other option is to put separate downpipes for the roof and verandahs, but this is tedious and messy looking).

For months we have been trying to decide what colour fionish the verandahs would be. We were going to use a "colorbond" product that has a zinc-alume coating with expoxy coloured coating. In the end we have decided that we want an un-painted look, so are verandahs will be plain galvanised corrugates steel (it has to be galvanised with zinc, not zinc-aluminium as the latter is so bright and never dulls-off to a patina).

In addition, the we decided that the ridge-capping has to be plain galvanised steel (with roll-top profile).

All this has implications for the deign of the rainwater system. Water that has been on a zinc-alume roof, cannot run-off onto a zinc ("galavnised") roof, as electrolytic action results in corrosion of the zinc-roof. (However, water that has been on a zinc ("galvanised") roof can run onto a zinc-alume roof without corrosion problems. There is a known sequence of the order in which various metals can be used, called the galvanic series or some such, if you need to know more.

So, we have decided that our entire roof rainwater system will be zinc galvanised. This is good, as it gives us the traditional appearance/patina we want. But its bad, as the newer zinc-alume products are much more long-lasting. Oh well....it will just mean that I have to make sure the gutter drains well, and doesn't get clogged up with leaves.....