A roof that has no plaster [the boards are not wide but were a roof] R’ Yehuda says that Bais Shammai says even if you move the roof boards you must take out some of the boards and put in kosher sechach. Bais Hillel say either moving the boards or putting in new sechach will make it kosher.

R’ Meir says you must take out the roof boards and put in kosher sechach.

Succa 1:8

If you put on the roof of the succa metal spits or bed boards going one way and kosher sechach going the other way, if both cover equally, the succa is kosher.

If you dig out a haystack to make a succa, it is posul as you did not make the sechach with an action.