A Sukkah'ed Sukkah

(3) If one spread a sheet over the roofing as protection for those sitting in the sukka due to the sun, or if one spread a sheet beneath the roofing as protection due to the falling leaves, or if one spread a sheet as a canopy over the frame of a four-post [kinof] bed, the area in the sukka beneath the sheets is unfit. In the first two cases, because the sheet is susceptible to ritual impurity, it renders the otherwise fit roofing unfit. In the case of the canopy, one is not sitting under the roofing of the sukka; rather, he is sitting inside a tent. However, one may spread the sheet over the frame of a two-post [naklitei] bed, which has one post in the middle of each end of the bed. When spreading the sheet over the posts it forms an inclined rather than a flat roof, and a tent with an inclined roof is not considered a significant structure.