How to validate a Sukkah that had its Sechach placed before its walls:[1]
It suffices to slightly lift the Sechach and then replace it. [It is not necessary to lift up the Sechach three Tefachim, or even one Tefach, and any amount suffices.[2] One must lift up each individual piece of Sechach in order to validate it.[3] Some Poskim[4] rule that one is required to fully lift up each individual piece of Sechach. Meaning that one cannot lift one end of the beam and then go around and lift up the other end of the beam, and rather the entire beam from both ends must be lifted off the Sukkah at least slightly. However, one is not required to lift all the beams of the Sechach simultaneously, and some Poskim[5] even rule that one is required to lift them up one at a time. Practically, one may lift many pieces of Sechach at a time, if he chooses, although some are stringent especially by a Biblical invalidation.[6]]
[1] See Admur 629:28; 631:12; 636:3; M”A 636:3; M”B 636:8; based on Michaber 631:9; See Piskeiy Teshuvos 626:4
[2] Implication of Admur 629:28; Poskim in Piskeiy Teshuvos ibid footnote 11
Other opinions: Some Poskim rule that one is required to lift it three Tefachim as otherwise it is considered as if it is still resting there due to Lavud. [Az Nidnbaru 13:38; 14:41]
[3] Implication of Admur 631:12; Michaber 626:2
[4] Orchos Rabbeinu 2:214; Piskeiy Teshuvos ibid
[5] Simple implication of Admur 631:12 and Michaber 626:2 “Lift up each one alone and replace it and then lift up the one next to it and replace it”; Koveitz Mibeis Levi 3 in name of Rav Wozner
[6] See Piskeiy Teshuvos ibid footnote 14