You don't say how long you'll need it this way or where you live. I would build a knee wall down the center high enough to have at least a 3-12 pitch and use 2X's (width dependent on length needed) for raters , sheet and use regular roofing. You are basically building a roof with pitch so you get runoff. Other vise you would have to use a membrane roofing system which is pretty expensive, and if you have kids they will probably be on it and could cause damage.
Be sure you live your life, because you are a long time dead.-Scottish Proverb