A couple of months ago some friends held a performance in the courtyard of a renaissance house. A good rule of thumb in this area, of course, is that no matter what the weather man says it will probably rain, so tarps were suspended over the area.
As anybody who has built with tarps knows, they will collect rainwater and eventually burst unless somebody applies some real engineering to the project. In this case, the problem was solved by attaching hoses to the lowest-hanging parts of the tarp, thus diverting the water. If I can only figure out how this was done, it might be practical in the construction of temporary sheds and the like.
Below is a tarp-and-rain-related accident waiting to happen. Note the water-filled bulge above the bed:
This is the now abandoned home of my former neighbor Mo. The picture is taken some days after he was sent to the hospital, and I was asked to collect some of his stuff. He was what is normally called a "homeless guy", except that he did have a home, this shack made of birch poles and tarp and scrounged odds and ends, situated quite close to the local train station but out of sight of everyone. I hesitate in calling Mo a friend, as he had some real serious issues and could be sort of scary when high or drunk.
I imagine what he would normally do about that rainwater catchment above his bed was simply push off the collected water after each rain. He didn't quite have his act together to make a more maintenance-free solution, though there was nothing wrong with his skills or potential.
You'd think just building a roof with a really steep angle would solve all problems, but it would need to be so steep that it for all practical purposes would be a wall. And you'd be back to square one. A method I've used (example below) is making sure ridges below the tarp create grooves that lets the water flow downwards.