[Cob] roof suggestions wanted

[hms.mommy at juno.com]

Hi everyone,
Like many of you, I'm hurrying to finish my project before the hard
freezes.  It's a play castle, sides straight up, castellations along the
roof--like a rook in a chess game.  The roof is going to be flat.  But, I
am one who has little construction experience (yet!) so I'm looking for
suggestions on how to do the roof.  I was thinking of using a piece of
plywood, and I envision nestling it in at the bottom of the castellations
and providing drainage pipes for runoff.  But what I don't know is what
kind of material to use on top of the plywood to make it waterproof. 
Ideally, one of the castellations would be more widely separated than the
others, and I will provide footholds along the wall so the children can
climb up to the roof.  So whatever goes on the plywood to make it
waterproof should be something that can handle a certain amount of foot
traffic.  Anyone have an idea what I can use? 
Carrie