I also posted some sketches for the structure of the playhouse here: https://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.
Step 3: Upper Roof Frame
Note that each of the 2x4s in this sketch (not the middle-beam) coincides with a 2x4 underneath the plywood. This also means that any seams in the plywood will have a 2x4 on top of it, since you made sure your seams were directly on top of the underneath 2x4, right?
I screwed these 2x4s in from the underneath, so one person on top can hold them in place while the person underneath with the screw-gun can drive the long screws up through the first 2x4 and up into the top 2x4. You may miss a few times..unscrew and try again..repeat.
I found that by having the plywood sandwiched between the two 2x4's, there was a lot of strength added to the frame.
Step 4: Final Edge Frame
Note that unless you are an expert miller, the long 2x6's will not sit flush with the edge of the plywood. There will be gaps and such. THAT IS OKAY! This is where water can escape rather than pooling up inside like a swimming-pool. I found that when I added the layer of gravel, it filled up part of those gaps and made a lovely drainage.
Once the frame was built, I was then free to tar everything up to seal the roof. I checked the sealing after it had dried with a garden hose and found a few spots that needed more sealant. Repeat as necessary.
After that, I added a layer of gravel, then old carpet to help hold some moisture, and then soil.