(a) The Gemara discusses situations where Sechach was placed upon the Sukah
in such a way that the Sukah is Pasul (for example, the Sechach was attached
to the ground (Mechubar), or it was not placed upon the Sukah for the sake
of providing shade (l'Shem Tzel -- e.g. if someone dug out a Sukah in the
middle of a hay stack, or if the roof was placed there as the roof of a
house, Rashi 12a, DH Chada). In such cases, the Gemara explains, in order to
make the Sukah valid it is not enough to simply remove the Pesul by cutting
the Sechach from the ground (in a case of Mechubar) or by having intention
that from now on the Sechach is for the sake of shade (when it was placed
there for a purpose other than for providing shade). Rather, one must "re-
lay" the Sechach afterwards by lifting up each of the pieces of the Sechach
and placing it back l'Shem Tzel. This requirement for active placement of
proper Sechach, "Ta'aseh v'Lo Min ha'Asuy," is cited as the Halachah in the
SHULCHAN ARUCH (OC 626:2).
(b) What about when a Sukah is Pasul not because of something inherent in
the Sukah but because of circumstances external to the Sukah? For example,
what is the Halachah if a Sukah was built over 20 Amos tall, or underneath a
tree or inside of a house, and then its Pesul is corrected (by raising the
floor of the 20 Amah Sukah such that its Sechach is less than 20 Amos from
its floor, or by removing the tree or roof which had covered the Sukah)?
Does one need to lift up the Sechach after correcting the Pesul in order for
the Sukah to be valid?
The HAGAHOS ASHIRI (1:23) cites RABEINU BARUCH of Regensburg who asserts
that the rule of "Ta'aseh v'Lo Min ha'Asuy" applies, and one must lift up
the Sechach in such cases to validate the Sukah. However, RABEINU YITZCHAK
HA'LAVAN (cited by the Hagahos Ashiri, ibid.), as well as the KOLBO (cited
by the Darchei Moshe, beginning of OC 626), rule that it is not necessary to
do anything to the Sechach in these cases. The Sukah becomes Kosher as soon
as the height is lessened or when the tree is removed. They reason that the
only time that Sechach has to be actively lifted up and replaced is when the
Pesul is in the Sechach itself (such as Sechach that was Mechubar, or that
was not placed there l'Shem Tzel). If the Pesul is not in the Sechach -- the
Sechach was placed properly -- but the Pesul is in the circumstances
surrounding it, it is enough to do an action that removes the external
invalidating factor. Since the Pesul is not in the Sechach, the "Ta'aseh"
(the significant action required when making a Sukah) also does not have to
be in the Sechach.
The lenient opinion is cited as the Halachah (OC 626:2,3 and Mishnah Berurah
there).
(c) The Acharonim discuss a third case. The previous discussion involved a
case such as building an entire Sukah inside of a house. What about when
Sechach alone is put into place under a removable or hinged roof? In such a
case, after the roof is lifted off, will the Sechach have to be completely
re-laid in order to make it a valid Sukah? The MAHARIL (cited by the BACH,
end of OC 626) writes that this is the same as the previous case, and no
action is necessary. However, the Maharil quotes a certain "Ga'on Echad" who
says that one must lift up each piece of Sechach and re-lay it in this case.
It is not clear why he differentiated between this case and the previous
one. The Bach suggests that this Ga'on was Machmir in this case because the
Sechach could not really be described as "inherently proper Sechach" before
the roof was removed, because it was not part of a complete Sukah. Inserting
branches under a roof does not a Sukah make. This case cannot be compared to
a complete Sukah underneath a tree, or inside of a house because the Pesul
is, to a certain measure, inherent in the Sechach itself.
The MAGEN AVRAHAM offers another suggestion. Lifting off a roof which was
made to be removable, he writes, is not considered a significant action. As
we noted above (b), even if the Pesul is not in the Sechach itself one must
perform a significant action of some sort to make the Sukah valid. Removing
a removable roof does not qualify as such an action.
HALACHAH: In the case of the Sechach under the removable roof, the MISHNAH
BERURAH (626:18) appears to be Machmir and require that the Sechach be
lifted up in this case. However, this is only so if the Sechach was put into
place *while the roof was still in position*. If the roof was removed
*before* the Sechach was put in place (and thus the Sukah was once valid),
even if one later lowered the roof back onto the Sechach (to protect the
Sukah from the rain and the like), everyone agrees that it suffices to
merely lift off the roof again and the Sukah will be valid (Mishnah Berurah
626:19).