I had my hardlid off the back off my ute for about a month, during which it was exposed to rain then intense sun, rain, intense sun etc. (damn Cairns weather!) and I started to notice that the plastic was starting to lift in one of the corners, sort of like a large bubble or warp . I threw the hard cover straight back on. It was a really hot day when I noticed it, and maybe the plastic had expanded because it was so hot, but unless you park undercover most of the time I would try and protect your tray with a soft cover at least.