Daf Yomi - Sukkah 18 - Mimonofshoch

The Gemora cites a dispute between Rav Acha and Ravina if we apply the principle of lavud in the middle of a sukkah. If there is an area of open space less than three tefachim running across the entire length of the sukkah, we would need to attach the two sides together to close the gap in order for the sukkah to be valid. There is a debate amongst the Rishonim if this argument pertaints solely to a small sukkah or even to a large sukkah. Tosfos maintains that lavud would be required even in relevance to a large sukkah.

Tosfos on the previous daf speculates as to what the halacha would be in the following case: One placed two tefachim of ineligible s'chach and another two tefachim of ineligible s'chach separated by less than three tefachim of airspace. Does the principle of lavud apply in this case to combine the two sections of invalid s'chach creating in total four tefachim of ineligible s'chach, thereby disqualifying this sukkah or perhaps we do not apply the concept of lavud to be stringent?

Reb Akiva Eiger asks that mimonofshoch, either way, the sukkah should be disqualified for we are compelled to utilize lavud in this case, for if not, the sukkah is invalidated not because of the ineligible s'chach, but rather due to the fact that there is an open space that needs to be closed. If we apply lavud to close the gap to avoid the concern of the open area, the sukkah will become disqualified because of the combination of the two sides of ineligible s'chach totaling four tefachim. If we do not utilize lavud, the sukkah should be disqualified due to the open space?

In summary, how can we use lavud to close the gap and discard lavud to combine the two sides?