Tosfos, at the very end of 14b, writes that the Halachah follows Shmuel. The opinion of Shmuel on 14a is that everyone agrees that one may not cover the roof of the Sukah with planks which are 4 Tefachim wide. It is a big Chidush to say that the Halachah follows Shmuel because the general rule (see Nidah 24b) is that the Halachah follows Rav in questions of what is permitted and what is forbidden (as opposed to monetary matters, where the Halachah follows Shmuel). The question of planks on the Sukah roof must be an exception to this rule.

I do not think that this is connected to the issue of Dofen Akumah, because the planks are in the middle of the roof, not on the side.

Best regards,

Dovid Bloom