Personally, I feel that slides (especially large slides) need covers. A cover to better protect it from leaves, from tree stains and from rain. Without a cover, one's slide (especially at seasonal camp site), looks like: http://i178.photobucket.com/albums/w...e/100_0353.jpg
If a mobile (traveling) TT, I highly recommend a spring loaded soft cover slide. Typical spring loaded cover that goes in / out - when slide is moved in/out. Within my area, these "soft top" slide covers are very expensive. But, it parked in "dirty" environment or one cannot resolve a persistent slide covers are are worth it.
If TT is at a seasonal site (or permanent site), perhaps installing a less expensive "hard top" slide cover would be better. If wondering, I park my Jay 29FBS at a seasonal site (even during winter months). Currently, I'm in the midst of creating a hard top cover for over my TT's large slide. It will be connected to the side of the TT (near roof top) using hinges (to allow some movement) and its outer slide end will be held up using telescopic steel poles. Thus, no weight (from this hard top) on the edge of the slide cover. So far, I created its light weight steel frame, have its telescopic 3 x vertical steel posts, created its hinges and currently saving my limited dollars for roof plastic. Plan to complete and install it this spring. To me, a light weight "hard top" slide cover (in 3 x sections) that can be easily removed (via removing its horizontal pins from its hinges) should work great.
Regardless if soft top (for traveling trailers) or "custom made" hard top method, I fell a slide cover on large slides is mandatory. Without a slide cover, its top appears like: http://i178.photobucket.com/albums/w...e/100_0358.jpg
For more details on "soft top" slide cover, surf: http://www.modmyrv.com/2008/07/10/rv-slide-topper
Hope this helps in your research....