Simple treehouse plan
Notice how the roof overlaps on all sides to shed the rain. To add a degree of waterproofing to your treehouse, lay down some layers of felt (at least three) across the roof. Alternate direction of the felt with each layer and do not nail any layer down just yet. When you have all the layers in position, set down strips of 2"x1" so they are directly above the roof supports. Fix the strips down firmly with 2" screws.
Once the felt is secured, paint over the whole of the roof with a thick tarry paint to seal any gaps around the screws and to provide a tough finish. You can alternatively fit a pitched roof instead of the flat one shown here.
Pitched roofs are much more resistant to rot than flat because rain will rapidly drain away and will be shed more reliably away from the walls This keeps the house drier and better protected. The diagram below shows one way to add a pitched roof. This will need two roof panels and two extra side panels to fill in the spaces. Once assembled, cover with felt as for the flat roof.