For a slide-out awning like the Solera 6' Long Slider (Slide-Topper) in White w/ White Hardware # LCV000163274, you only need enough room to mount a rail if one isn't already present from the manufacturer. Most of the time the manufacturer already has the rail installed that you will use for a slide-topper like this. It only needs a slight drop for water run off purposes.
I recommend using the Butyl tape # CAM25013 to ensure you have a good seal between your rail and camper. view full answer...