The flat roof on my Bethany Pick-upper has some sag and water damage that i'd like to fix. I already have the plan to rebuild it with a slight curve to help shed water. My question is what would be a good cheap and durable material for the exterior of the roof? I was thinking FRP Wall Board like you can get at home Depot, it is water and mildew resistant, seems like it would hold up to branches scrapping along it. I would like to avoid expensive rubber style roofs. I see a lot of newer campers have a material like this wall board on it. http://www.homedepot.com/p/Unbranded-4-ft-x-8-ft-White-090-FRP-Wall-Board-MFTF12IXA480009600/100389836?MERCH=REC-_-PIPHorizontal1_rr-_-202090190-_-100389836-_-N