If a person puts a bundle of reeds to dry in the sun on top of the succa, it would be posul for sichach as it was not put up for shade. Chazal said therefore even if you did put it up for shade since you might forget and put on top not for shade and use it. If you opened the bundle it is kosher.

Everything is kosher to be used for walls of the succa.

Succa 1:6

Chazal said that since a real roof is definitely posul for sechach, we don’t even allow a board that is 4 tefachim wide since it looks like a real roof. If it is 3 tefachim wide R’ Yehuda says it is kosher and R’ Meir says it is posul.

If he put a 4 tefachim board within 4 amos of the 4^th wall and there are 7 tefachim of kosher sechach on the other side of the succa, at least those 7 tefachim of the succa are kosher as a ‘dofen akuma’ applies. This literally means a bent wall, since the posul sechach in this case is considered like a bent wall.