[Cob] galvinised roofing

[Amanda Peck]

Yes, you're right.

But the fish are living in this environment.

So leaching might not realistically apply to a water catchment situation.  
The "first flush" is considered trash--leaves, bird and insect poop, and 
anything loose and oxidised on the surface of the roof.  After that the 
water doesn't stay very long on the roof, or presumably in any other part of 
the system except the tank.

I wouldn't know what form the zinc we would get from the galvanized 
metal--probably oxide.  But most of it in food is water soluble, i.e., we 
can get rid of it easily.

Another example of the kind of thing I'm talking about.

You can still buy fancy 23% lead crystal glasses and presumably decanters.  
Really, there's no problem about wine or brandy poured into the glass and 
drunk over the course of an hour or three.  There might be if you stored the 
stuff for six months or so in the decanter. The lead does leach out.  And 
there is a rumor that the fall of Rome was at least partly caused by lead in 
the pots and pans.

I think I'd fuss more about a slightly porous surface that mold can find a 
home in.

And a metal roof with rainwater harvestings has some advantages for our 
lovely cob homes.  Inexpensive both because the material is and because we 
will presumably not need a well.  The roof will presumably have a high 
recycled content, and is recyclable itself.  More, I expect, than can be 
said for those high-tech brush-on jobs--one of which is advertised in Fine 
Homebuilding)

(that said, I have an internet acquaintance who has a living roof, and he 
keeps showing us pictures of the vegetables he just brought down.  Makes one 
long for the living roof!)
..........................
Aaron Allen wrote:
I do not have a certifiable source for this info, but I can add that in my 
work in fish hatcherys galvinised metal was a big no no in the tanks for the 
reason that zinc leached from the metal in contact with the water and was 
deadly to the fish.  I do also know that overdoses of zinc (more than 30 
milligrams per day) are harmful to the human body (we are not able to 
process/eliminate it).  so the idea is not rubbish, and caution should be 
exercised untill conclusive proof can be found.