If one spread a sheet over [the s’chach] because of the sun, or beneath it because of the falling leaves, or if one spread it over a four-poster bed, it is invalid. However, one may spread it over a two-poster bed.

If one raised onto it a grapevine, a gourd, or ivy, and covered it with s’chach, it is invalid. But if the s’chach exceeded them, or if he detached them, it is valid.  This is the rule [regarding the validity of s’chach]: Whatever is susceptible to contamination or does not grow from the ground, we may not use for s’chach; but whatever is not susceptible to contamination and grows from the ground, we may use for s’chach.