If you're talking about the vapor barrier on the underside of the slideout I had the same problem with my Monaco MH. This is specific to Monaco products, but may offer clues to other brands. Monaco recommends operating slides when the coach is still air'd up. I once operated my slide after dumping air and the twist caused by not being square caused that vapor barrier material to bunch up on the roller and tear. Once that happens any subsequent operations of the slide out will continue to tear that material. My dealer performed the repair by installing what looked like hard rubber runners where the floor of the slideout met the metal rollers. Sounds very similar to what has been offered here using sheet metal. Keep in mind that slideouts are extremely heavy and sheet metal may not be able to handle the weight. You will also need to seal whatever you install so moisture does not get in and rot the wood subfloor.