Amanda, there are several ways to do this. The easiest way would be to rig up a tarp large enough to cover the treehouse with a hook or something to hold tarp in center. The other way which is more permanent yet harder to do is build a permanent wood roof, start in the center with a block of wood shaped to match the existing triangle and build out to walls, this method requires a compound angle saw. I would probably just tarp it. I hope this helps.