It seems from the din of making succos under trees that the problem is that the sechach needs to do the shading and not other trees. Can one therefore use a clear shlock or cover for the succah? The sechach will therefore do all the shading still?
Answer:
It is true that in discussing sechach pasul, the Gemara and the Poskim refer to issues of shade and sunlight (if the kosher sechach provides sufficient shade without requiring the non-kosher sechach, the succah is valid). Based on this, the Shevet Halevi (4:57) cites from somebody who suggested that there is no problem of covering the sechach with a nylon see-through sheet.
However, the Rosh (cited by the Beis Yosef 631) writes that sechach protects from the sun and the rain, and Rav Wosner proves from here that even a plastic sheet that means to protect from rainfall is considered a layer of sechach pasul.
His conclusion is therefore that a plastic sheet is certainly sechach pasul, and the status of the succah will depend on the two opinions mentioned in the Shulchan Aruch (629) concerning a kosher succah with a protective sheet. On account of the safek one should therefore sit in the succah, without reciting a berachah.
See also Har Tzvi, Succah 22, who also discusses the issue.
