On daf 15b we are trying to establish a case where we have more kasher schacht han pasul schach. Rava gives the following answer:
"(d) Rava establishes the Mishnah - by Sechach that is placed not parallel to the rods, but adjacent to them. In that case, there will automatically be more Sechach than rods. Why is that? Because each piece of Sechach stretches from the top of one rod to the top of the next one."
How is this more kasher schach? If you have kasher schach on top of pasul schach, this is still pasul schach. Since here, according to Rava, there is an equal amount of space as there is rods so there can't be more kasher schach than pasul schach?
David
TOSFOS (DH Rashi Garis) asks your question and argues with Rashi, giving an entirely different explanation for the Gemara. The CHIDUSHEI HA'RAN answers that according to Rashi, such a case of Sechach Kasher on top of Sechach Pasul is indeed valid. Since the Kosher Sechach is resting directly on the Pasul Sechach, and is greater in quantity than the Pasul S'chach, it is Mevatel the Sechach which is Pasul. Only when the Kosher Sechach and the Pasul Sechach each stand alone and are distinct from each other is the Sukah invalid.
Be well,
M. KORNFELD
(Note: See Insights to Sukah 15:3)