[Cob] Roofing material?

[Amanda Peck]

I had thought that Ondura would not be good.  I still think I wouldn't use 
it, but I did notice that one of the links mentioned earlier had this 
list--is Ondura a bitumen based product??:

from http://www.rainharvesting.com.au/roof_surface.asp

The most suitable roofing materials for collecting potable water* are:
1. Colorbond® steel sheets or tiles
2. Zincalume® steel sheets or tiles
3. Glazed tiles well fired
4. Concrete/cement tiles
5. Clay tiles
6. ** Fibro - new type only
7. Composite tiles - bitumen based

That list does not include any brush-on or membrane products.  I've looked 
at this before, and there are some people who advertise in Fine Homebuilding 
who do mention potable water and roofs in the same sentence....

EPDM either the sheet or the brush-on can be potable water certified, 
although these guys only mention fish-safe.

http://www.epdmcoatings.com/

On the other hand, these guys do (although one part of this one is an 
"isocyanate prepolymer" and they mention quite a few precautions during 
application--I think I may be sensitive to '"isocyanates" not sure if 
they're loose here):

http://www.gaco.com/01prd_02uc_lm60.html

and I think these as well.

http://users.rcn.com/sealproinc/protective_coatings.htm

All high tech and maybe not what we want for our cob--or low-"chemical" 
house.  The types of metal roofing might be a better bet for us.
...............................
Thad replied to me:

Just have to ask.   What roofing material would be available to use as 
rainwater collection that would be safe to use for potable water?  
Considering the abuse roofing material takes from the elements.  I would 
think that most any asphalt material would leech into the water.  Galvanized 
metal would certainly leech into the water.  Fiberglass would not last and 
would eventually degrade and leech into the water.  Plastic jugs or buckets 
cut to use for shingles are not UV stable and quickly become brittle and 
fail.   Other than stainless steel or copper which are obviously too 
expensive or glass; which would be fine in a greenhouse application, what 
would work?