TT #405 · 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: "The festival of Sukkos you shall make" — A roof that has no plaster: Beis Hillel say one must loosen it or remove one board from between the others. What is the reason? Because it is written "you shall make" and not from that which is already made.
Derash on the verse (Sukkah) — an existing roof must be modified to become sechach.
To convert an ordinary roof into valid sechach one must either loosen the boards or remove one of them, thereby performing a new act of making.