We have 80-100 ft. trees that are high above our Sukkah. Is our Sukkah valid?
The Sukkah needs to be directly under the sky without anything intervening between it. Trees may not hang over the Schach of the Sukkah regardless of its height.
The בכורי יעקב, however, writes that if the trees were only on the side of the Sukkah even though their height towers and shades off the whole area it is still kosher since they are not hanging directly over the Sukkah.
If the tree is only hanging over a very small part of the edge of the Sukkah, at times one may still eat in the Sukkah. The following conditions need to be met:
1) שאם ינטל הצל עדיין תהא צלתה מרובה מחמתה – If one were to imaginably take away the Schach underneath the part the tree hangs over it, the Sukkah would still be more shady than sunny.
2) The tree cannot cover an area of 4 Amos or more on the side of the Sukkah, otherwise, the whole Sukkah would be invalid even if the Sukkah was gigantic.
3) The tree cannot cover 4 hand-breadths or more over the middle of the Sukkah, or in a very small Sukkah (a Sukkah which is the exact required length and width), even 3 handbreadths.
Ideally, the Sukkah should not have anything hanging over it at all, regardless.
Shulchan Aruch Admur Hazaken 626:2-4.
Biur Halacha 626: 1.ד”ה תחת האילן. Bikurei Yaskov 626: 2. Moed Lchol Chai 21:30
Bikurei Yaakov 626: 1. Kaf Hachayim 626:6.