I also posted some sketches for the structure of the playhouse here: http://www.instructables.com/id/The-Structure-for-Cob-Playhouse/
This instructable is for the roof. I did not take any photos during construction, so I have tried to makes some sketches and provide some photos that will at least give you the basic idea.
*disclaimer: I am not a professional builder, artist, or construction worker. I apologize in advance for my crude sketches and even worse construction skills. This project was done using scrap wood, many peices were already cut, so I had to sort of jig-saw puzzle much of this together.
The plywood was cut to match the angle of the roof, so I had to do some eye-balling and measuring to match the 2x4's to that angle.
The #2 sketch shows how I put those 2x4's on. Note on the photo how the edge of the 2x4's are slightly raised over the edge of the 2x6. The plywood that will be plased on top needs that, or there will be a bump from the corner of the 2x6.
Step 2: The Plywood
Note how the seam of my plywood peice will be in the middle of a 2x4.
If you wanted, you could go ahead and apply a coat of that roofing sealer/tar stuff to all of the seams, and let it dry.