[Cob] Roofing question & thoughts

[drub]

In the scheming stage.  Looking forward to real cobbing next summer.

Am thinking of the roof design.  Have seen pictures of several cob 
houses with fairly flat roofs.  Gotta believe a membrane was used.  Some 
have been planted.

    * What kind of membrane was used?  Conventional roofing membrane? 
      Some sort of swimming pool product?
    * Is a membrane a good, long term choice?  As materials become more
      expensive and less available, will this sort of membrane be
      serviceable in future years of depletion?
    * Would a conventional asphalt shingle be a better, more sustainable
      choice?  It would likely be more available.  True?
    * What other materials should be considered?  Tiles?


Thanks for your thoughts,
David