From the Fall 2010: Building of the Earth Roof.

Attention to detail is extremely important in making sure that the roof will remain waterproof.
One small oversight and water will find its way into the house.

 The plywood is cleared of nails and swept of dust.  

Then the waterproof membrane is laid down....very carefully.
It is shingled from the low sections up, overlapping layers at least 3 inches.

 The curved section is laid down.

The entire roof is now covered with the waterproof membrane.

 All the overlapping seams are sealed.
Note that the waterproof membrane runs up the vertical wall on the right.

 As a cheap, final insurance, a seamless sheet of 6 mil poly is laid down on top.

On top of that, a drainage layer is laid.  

This has a filter cloth that keeps soil from clogging things up,

and allows water to travel quickly downhill to drains.

The photo above shows the waterproof membrane (shiny black), the seamless poly 

(White on the left and right....cut wide around the drainage hole.)

the drainage layer (closest to you), and a screen poking out of the drainage hole to keep leaves out.

We then hauled (or had others haul) over 400 buckets of compost and topsoil.

And planted the roof.