You can cover it with tar paper and roofing shingles, if you can find some free. Hopefully you won't have to buy the materials.
You can make a slant shed roof. Use some of the boards from a pallet to screw some pallets together, and that can be used for your roof. You wouldn't want to walk on that roof, but it will be strong enough to keep the rain out of the coop. One side just needs to be a little higher so that the rain will run off the roof.
This looks fab,
I am just thinking about builing one too and this looks just what i am looking for.
Please o please post more photos as it continues.
Also can i ask, what did you use on the floor??