As much as two years ago HGTV aired a program that showed 1/2 round rain diverters which were placed under a deck between the joists. I would guess they were plastic or other man-made material. The concept was great because it kept the area below dry and thus made a liveable sitting area even in the rain.
I have searched and searched and can find no info on these units. Can anyone shed any light on this product -- the product name, the show on which I saw it, a manufacturer name, anything?!