My slide out awnings are made of Sunbrella and therefore have 'edge seams' which are naturally tighter than the center of the awning. This edge seem prevents good drainage. Usually, I retract the slides after rain. But that's a pain as rugs, etc. have to be moved to do so. Some try to put beach balls under the awning on top of the slide out, but they can be a pain to remove. One guy in the park took 3 foam 'swim noodles' and duct taped them together, two as a base, one above to form a 'triangle'. Then he slid this noodle triangle on top of the slide out under the awning. It seams to work. His 5th wheel is in place year round so storage of these noodles is not an issue when traveling. I have heard others have done likewise assembling pvc pipe into a support frame. Storage of these pvc pieces could be a headache. I may try the noodle solution.