The Gemoro on 17b (second loshon) says that Rav holds Sechach Posul (even in the middle of a sukah) is posel at 4 Amos. The Gem brings an earlier Mishna "Nosan Aleha Neser" re a plank 4 tefochim wide, which is Kosher - this is fine according to Rav. However, that Mishna concludes that one should not sit under the 4 tefach plank!
Our Magid Shiur pointed out that it would appear that even according to Rav (in this second loshon), Schach Posul of less than 4 Amos does not invalidate a Sukka, but one should still not sit underneath it!
Do you have any comment on the above?
Kesiva VaChasima Tova
Meir Eliezer Bergman
Shalom and thanks for your question.
The Avnei Nezer states that even according to Rav one may not sleep under such a neser.
G'mar Chasimah Tovah.