Maamid with Schach
Maamid
1. The Rishonim argue whether schach may be placed on something that is unfit for schach. In other words, may it be set down or supported on something that can become tamei, e.g., metal, in such a way that if that thing weren’t there, the schach would fall? This is called the pesul of "maamid" (ראשונים סוכה כ''א :). Some say it is assur out of concern someone will think that just like that material may be used to support schach, it may also be used as the schach itself (הר''ן והריטב''א ).
2. Others hold that there is no issur whatsoever to place schach on something that cannot be used for schach itself (רא''ש, תרומת הדשן צ''א ).
3. Practical halachah. As far as the halachah, the poskim hold that one should lechatchilah not place schach on something pasul. Bedieved, or if there is nothing else to put the schach on, one may be meikel (מג''א תרכ''ט סק''ט, ח "א כלל קמ''ו ס''ל, שו''ע הרב תרכ''ט סי''ג, מ''ב שם סקכ''ב ).
4. Examples of bedieved. If one already built his sukkah and placed his schach on something that can become tamei, he does not need to dismantle it (שו''ע הרב סי''ג ). It could be this is only if fixing it is difficult or a big hassle (מו''ר בעל שבט הקהתי ). Similarly, if one ate on the first night of Sukkos in a sukkah whose schach was placed on something that can become tamei, he does not need to eat again in another sukkah (חזו''א, ארחות רבינו ח''ב עמ' רי''ח ). Also, if one went to his in-laws and their schach was placed on something unfit to be used as schach, he may eat there (הגר''נ קרליץ, חוט שני סוכות עמ' ר''ט ) and say a brachah on it (הלכות חג בחג פ''ז ס''א הע' 2).
5. Awning on aluminum tracks. If one has a balcony covered by an awning that is on aluminum tracks and he removed the awning to make a sukkah, he should not place the schach directly on the tracks since they cannot be used as schach. Instead he should place wooden beams perpendicularly over the tracks and put the schach on the beams (בית דוד פ''ו הערה 3).
6. Iron beams. If someone has an area for his sukkah that has iron beams, e.g., a balcony that has an iron beam every meter, or if someone wants to support a schach mat with iron wires attached to the walls on either side, lechatchilah he should not put the schach on the iron itself. Instead, he should place wooden beams perpendicularly on the iron beams or wires and put the schach on the wooden beams.
On a Stone Wall
7. Even though stone may not be used as schach, it is not a problem of maamid to lay schach down on a stone wall. This is because it is unusual to make a stone roof – thus, there is no concern people will think stone may be used for schach (מ''ב סי' תרכ''ט סקכ''ב ). Even according to the Chazon Ish, who is machmir on "maamid demaamid" (below, 11), one would be able to place schach on a stone wall that has iron reinforcement rods inside the wall since the iron is not visible and is only there to strengthen the wall (חזו''א סי' קמ''ג סק''ב ד''ה ולסמוך ).
On Something Pasul Derabanan
8. Most poskim hold that one may lechatchilah place schach on something that is only pasul derabanan for schach. Since the whole issur of maamid is a gezeirah and a lechatchilah, Chazal did not extend the gezeirah to something that is only pasul derabanan (ביה''ל תר''ל ד''ה כל, שו''ת מנחת שלמה תנינא סי' נ''ה, הגר ''נ קרליץ חוט שני סוכות עמ' רי "א ).
9. Placing schach on walls. In light of this, the Pri Megadim (א''א סק''א ) is unsure whether one may place schach directly on top of the sukkah walls if they are wooden panels wider than four tefachim: since miderabanan they cannot be used as schach (שו''ע סי' תרכ''ט סי''ח ), putting schach on top of them is maamid on something that is pasul miderabanan as schach.
10. However, since most poskim are meikel (above, 8), doing this is completely fine (בכורי יעקב סקי''ד ).
Maamid Demaamid
11. Chazon Ish's opinion. Some poskim hold that just like we are machmir lechatchilah for maamid, one must also be machmir lechatchilah for maamid demaamid, i.e., the schach's supports should also be placed on something that is kosher for schach deoraisa. This is based on Rashi's opinion that the gezeirah of maamid is that anything that is held up by something pasul is itself considered pasul (חזו''א קמ''ג ). According to this, "maamid demaamid demaamid" etc. etc. is also a potential problem, and all the materials must be fit to be used for schach (שו''ת מנחת שלמה תנינא סי' נ''ה ).
12. Accordingly, lechatchilah, one may not use nails to secure the wooden beams supporting the schach if the whole weight of the beams is supported by the nails since nails create a situation of maamid demaamid. Similarly, one may not place schach on walls if the walls are connected with nails, screws, or plastic zip ties and the walls would not stand without them. This is also considered maamid demaamid.
13. However, one may attach a wooden beam to a stone wall with wooden nails that can hold up the beams themselves and then add iron nails as reinforcement. Then, he can put the wooden beams that will support the schach on top of this beam.
14. Most poskim allow it. However, most poskim allow maamid demaamid lechatchilah. Accordingly, one may secure the schach’s supports with nails, other metal pieces, or things that cannot be used as schach (שו''ע סי' תרכ''ט, מג''א סק''ט, ביאור הגר''א שם, שו''ע הרב, חיי אדם, בכורי יעקב, מ''ב סקכ''ו ). This is the predominant minhag, especially considering that the pesul of the first maamid is only lechatchilah, not a pesul bedieved; there is certainly no need to be machmir for maamid demaamid (הגרי''ז הלוי, מועדים וזמנים ח''ח על ח''א סי' פ''ב, גאב''ד טשעבין הובא בארחות רבינו ח''ב עמ' רי''ז ) unless one wants to go above and beyond and be machmir to satisfy all opinions.
15. Holding down the schach. One must make sure not to use something that cannot be used for schach to hold down his schach so that it does not get blown away by a normal-strength wind; this is also because of maamid [see Issue 60 (paragraphs 41 and on) in the Koveitz Chukai Chaim (first volume) for more details about this].
