If one makes his succah like a conical hut, or he leaned it against the wall, R’ Eliezer invalidates it since it has no roof; but the Sages declare it valid.  A large reed mat —- if made to lie upon, it is susceptible to contamination and we may not use it for s’chach; if for shade, we may use it for s’chach and it is not susceptible to contamination.  R’ Eliezer says: Whether [the mat is] small or large —- if he made it to lie upon, it is susceptible to contamination and we may not use it as s’chach; if for s’chach, we may use it for s’chach and it is not susceptible to contamination.