Here is my 2 cents worth of experience with Trex deck: Due to the composite/plastic nature of the product, it can sag over large spans, but not necessarily due to impact weight. On a southern exposure with lots of sunlight, the heat will absorb into the darker colors and soften them, thus leading to sags. Also be warned that a dark color deck board in the direct sun will absorb a dangerous amount of heat and can cause burns BEFORE your feet can tell your brain to get the heck out of there. The third item involves both composite decking and vinyl siding: Low-E high reflectivity glass will create a 'death ray' as the sun moves across the sky - it will concentrate solar rays by reflection and soften, warp or melt plastic products. Low-E is great for windows and bad for everything in the way.
The last house I lived in was a rental in a subdivision. The houses were 20 feet apart. Neighbor house had low-E glass and the vinyl siding on our rental had warped/melted and was ruined by a single dining room window on the neighbor house. You could actually feel the heat while mowing between the two homes on a sunny day.
