TT #409 · Parashas Reeh · Seventh Aliyah · Passuk 16:13
The festival of Sukkos you shall make for yourself seven days, when you have gathered in from your threshing floor and from your winepress.
Torah Temimah points to Sukkah.
English: "When you have gathered in..." — one may cover [a sukkah] only with something that is not susceptible to impurity and that grows from the ground, for the verse says "when you have gathered in from your threshing floor and from your winepress"—the verse speaks of the waste of the threshing floor and the winepress.
Derash on the verse (Sukkah) — the definition of valid sechach.
Sechach must be material that grows from the ground and is not susceptible to tumah—such as the waste products of the threshing floor and the winepress.