Re: Using N'sarim [roofing boards or planks] for s'chach - 4 t'fachim or even those less....
I was learning with my havruta the following: SA Harav Hilchot Sukkah 629:29-33
As my havruta has asked me "how can we make a g'zeira to a g'zeira?". * IE the Talmud was gozeir against n'sarim gt:: 4 t'fachim - and posqim said nowadays even if lt: 4.
Original Talmudic g'zeira: see s'ief 29 fn 195-199 [viz. No nsarim allowed gt: 4 t'fachim]
Then see s'if 32 "v'achshav bizman hazeh... " fn 209 and his lashon "g'zeira shema yeisheiv" [viz. Any nesser big enough NOW for a tiqra [ceiling] is also under the G'zeira]
See Rambam MT Hilchot Sukkah 5:7 and Hagahot Maimoniyyot 6 in the name of Sefer Mitzvot Qatan
[Also Tur 629 quoting Sma"q and Darchei Moshe 629:9 quoting Hagahot Maimoniyot.]
Then See Shulchan Aruch R Caro O"H 629:18 where he terms it "minhag" [as per Tur and sources above] but seems more machmir in terms of scope [namely any nesser period - regardless of size. Whilst the posqim above seemed concerned only about a nesser that is contemporary use for roofing. Perhaps SA was writing shorthand]
Then see Mishanh Brurah 629:49 "Yeish mai-harishonim .."That it's "assur midina"[Shaar haTziyyun 72 - same sources as fn 209 above]
In summary The G'zeira of SA Harav may simple be that of a "minhag-G'zeira" - analogous to Qitniyyos. Rambam's MT himself uses this term.
The MB apparently morphs the Minhag to be Machmir as actual "din". The dynamics here are unclearBottom Line:Post-Talmudic G'zeirot/Policies are Still being promulgated by posqim. Some are more transparent as g'zeirot, others seem to be "masquerading" as something else.
* to answer my havrusa's question I will say that there are TWO versions of G'zeirot [there are many more, but we"ll keep it simple]
Gz'eira#1 is TALMUDIC; Gz'eira#2 is Post-Talmudic "hanhagah. "There is a hanhagga from Rishonim to protect the g'zeira of the Talmud, or to simple redefine its parameters [as stated above] Based upon Rambam, this is a top-down "hanhaggah" legislated [not pasqened] by rabbis and its long-term authority is determined by acceptance.