Gemara

R. Meir says that a vessel gets the law of its Ma'amid (what makes it usable);

(Beraisa - R. Shimon ben Gamliel): If a glass vessel was punctured and sealed with a drop of molten lead, R. Meir is Metamei, and Chachamim are Metaher.

Sukah 21a (Mishnah): If one supports his Sukah with legs of the bed, it is Kosher;

R. Yehudah says, if it could not stand by itself, it is Pasul.

Question: What is R. Yehudah's reason?

Answer #1 (R. Zeira or R. Aba bar Mamal): It is not Keva (fixed).

Answer #2 (the other of R. Zeira and R. Aba bar Mamal): He is Ma'amid with something that is Mekabel Tum'ah.

(Abaye): This is only if he supported the Sukah on the bed. If he put Sechach on the bed, it is Kosher.

Rishonim

Rosh (Sukah 2:1): According to the opinion that R. Yehudah disqualifies because he is Ma'amid with something that is Mekabel Tum'ah, if one put Sechach on a wall of rocks, why is it Kosher? He is Ma'amid with something that [is Pasul for Sechach, for it is not Mekabel Tum'ah]!

Rosh (ibid.) and Ran (DH ul'Inyan): The Rif brought Abaye's teaching, who explains R. Yehudah's opinion. This connotes that the Rif rules like R. Yehudah.

Rosh (ibid.): The primary opinion is that R. Yehudah disqualifies because it is not Keva. The Yerushalmi says so. Why are Rabanan Machshir when the roof is less than 10 above the bed? This is a repulsive dwelling! It seems that R. Yehudah explains the first Tana. There is no argument.

Milchamos Hash-m (Sukah 10a): If one supports his Sukah on Mechubar (something attached), this is like using attached Schach. On top of a tree is Kosher, for he is not Ma'amid with Mechubar. Rather, Mechubar supports the Ma'amid. This is Kosher, for it merely serves like the ground.

Ran (Sukah 10 DH Rebbi): One opinion disqualifies because he is Ma'amid with something that is Mekabel Tum'ah. Even though we say that everything is Kosher for walls, this is when he does not rest the Sechach on them. Mid'Rabanan, one may not be Ma'amid with something we may not use for Sechach, lest one say that just like it is Kosher to be Ma'amid, it is Kosher for Sechach. A Mishnah (22b) is Machshir a Sukah in a tree, and R. Yehudah does not argue, even though a tree is Pasul for Sechach. That is because a tree is not appropriate for Sechach. It is not common to use it for Sechach. Therefore, R. Yehudah does not decree. This is difficult. A tree is appropriate for Sechach, like the Mishnah (11a) of one who draped a vine over a Sukah! The Ramban answered that on 22b, he did not rest the Sechach on the tree. Rather, the floor of the Sukah is in a tree. He inserted poles in the floor of the Sukah and put the Sechach on them. There, even though something attached supports the Ma'amid, we are not concerned, for it is merely serving like the ground. The Mishnah connotes like this. It forbids going up in the Sukah on Yom Tov. This shows that one must go up, i.e. the floor is in the tree, but not that the floor is on the ground and the Sechach is on the tree. This is what Abaye taught. It is Pasul if the Sechach is [directly] on the bed, but if it is on poles in the bed. This is not Ma'amid with something that is Mekabel Tum'ah, for the Sechach is not on the bed, even though the pegs and poles rest on the bed.

Ran (DH ul'Inyan): It seems that R. Yehudah does not disqualify based on his opinion that Sukah must be Diras Keva, since the Gemara did not say so. Also, Abaye holds that Sukah must be Arai, so he must be concerned for Ma'amid with something that is Mekabel Tum'ah! We hold like this. Even though we put Sechach on walls of materials Pasul for Sechach (since they do not grow from the ground), for it is not common to use them for Sechach. Also, all know that these things are called a Dirah (dwelling), and not a Sukah, so we need not decree. The custom is to put sticks on the walls (and put the Schach on the sticks). This is a mere Midas Chasidus, lest it seem that he puts the Schach on the walls. This is according to the Rif. The Ba'al ha'Ma'or rules like Chachamim.

Terumas ha'Deshen (91): One may use iron nails to connect poles of the Sukah, or tie them with worn out clothing. The Rosh holds that R. Yehudah disqualifies because it is not Keva. The Rif agrees, for he did not bring the argument, even though it is very relevant, e.g. to one who put the Schach on poles inserted in the ground. He holds that it is not Keva when there is not 10 Tefachim above the bed, so he taught only about Schach on a bed. We disqualify a sheet that holds up the Schach, and without it the leaves would fall. There, the sheet is not only Ma'amid. It also gives shade, therefore it is considered that most of the shade is due to it.

Poskim

Shulchan Aruch (OC 629:7): It is not clear whether or not one may put a ladder on the roof in order to put the Sechach on it.

Taz (9): The Safek is whether or not the ladder is Mekabel Tum'ah, or if we disqualify all ladders due to those that (have holes to hold the rungs, and) are Mekabel Tum'ah.

Question (Taz 10): We should say that the ladder is Batel in the majority of Kosher Schach! We do not disqualify a Sukah because he was Ma'amid with something that is Mekabel Tum'ah (Sa'if 8, 630:13)!

Bach (DH Kasav ha'Ran): The Ran holds that the Isur to be Ma'amid the Schach with sometimes Pasul for Schach is only mid'Rabanan. It is a decree lest people use it for Sechach. Rashi connotes that it is Pasul because the Ma'amid is the Ikar of the Sechach, so it is as if he uses for Schach something that is Mekabel Tum'ah. We hold in general that the law depends on the Ma'amid (Shabbos 15b). It seems that the Rosh agrees. This is why he could not answer why one may put on top of a stone wall. He holds that it is not a decree. Therefore, it seems that letter of the law one may not put Schach on something that does not grow from the ground. If he did, it is Pasul. Sukah 2a says 'if one made iron walls and put Schach on them, will you say that it is Pasul?!' It is Kosher if the Schach is on sticks. It is called "Schach on them", because it relies on the iron walls, just like "he put the Schach on an Achsadra" (a room open on one side).

Magen Avraham (9): According to the Ran, we can explain why we permit Schach on metal spits (631:8). We require [a majority of] Kosher Schach on them, so there is no concern lest he use only spits. The Bach holds that letter of the law, it is Pasul if the Ma'amid is Mekabel Tum'ah. How can he explain Schach on spits? However, the Beis Yosef (Sa'if 8 and Sof Siman 630) permits Ma'amid with something (e.g. a bed) that is Mekabel Tum'ah. He relies on the Terumas ha'Deshen. Here he disqualifies the ladder for Schach. Also the Rosh and Maharil (87) permit Ma'amid with something that is Mekabel Tum'ah. Also, we can say that b'Di'eved it is permitted. It is forbidden only l'Chatchilah. Sa'if 8 permits tying the poles with garments, since the Schach itself does not rest on what is Mekabel Tum'ah.

Mishnah Berurah (22): Some say that the Safek is whether one may be Ma'amid with something that is Mekabel Tum'ah. L'Chatchilah, one should be stringent. B'Di'eved, or if he has nothing else, one may be Ma'amid on something that is Mekabel Tum'ah (630:13).

Kaf ha'Chayim (50): Many say that l'Chatchilah one may not be Ma'amid with something that is Mekabel Tum'ah, but b'Di'eved it is Kosher.

Rema: Therefore, one should not put Sechach on it. One may not even put it on the Sechach to hold it. The same applies to anything that is Mekabel Tum'ah, such as a bench or chair, which are Mekabel Tum'as Medras.

Magen Avraham (11): The Maharil says that we are concerned lest they became Tamei. This connotes that if they are Vadai Tehorim, e.g. they are new, one may use them. This is difficult.

Gra (DH v'Hu): (Damesek Eliezer - the Rema is difficult, for he learns from Maharal.) The Maharil (87) says that the Halachah follows Chachamim, and even those who rule like R. Yehudah, the primary reason is because it is not Keva, like the Rosh wrote!

Mishnah Berurah (24): Also what holds the Schach in place is a Ma'amid.

Kaf ha'Chayim (55): It is not clear whether one may be Ma'amid with something that is Pasul for Schach mid'Rabanan, e.g. a board four Tefachim wide. Perhaps this is like a Gezeirah for a Gezeirah. Therefore, one who fears Shamayim should not put Schach on walls made of such boards. However, the Ritva (Sukah 11b) permits to be Ma'amid with something that is Pasul for Schach mid'Rabanan.

Shulchan Aruch (631:8): If one roofed his Sukah with spits, and put more Kosher Sechach on top of them, it is Kosher.

Question: The Rema (above) forbids using a Kli that is Mekabel Tum'ah should not be used to hold up the Sechach or to be Ma'amid (hold it in place)!

Answer #1 (Mishbetzos Zahav 6): We decreed lest one use such Kelim for Sechach. All know that metal spits are Pasul for Sechach!

Answer #2 (Kaf ha'Chayim 32): He puts reeds on the spits, and the Sechach on the reeds, so the spits are merely Ma'amid the Ma'amid.

Minchas Shlomo (ha'Gaon R. S. Z. Auerbach, Ztz"l, 2:55): The Chazon Ish (OC 143) says that there is no Heter of Ma'amid d'Ma'amid. He learns from Milchamos Hash-m, who permits only because the tree serves like the ground. In all other cases, what is Ma'amid the Ma'amid, or even a third level Ma'amid, is like one (a regular) Ma'amid. Therefore, one may not use nails or worn out clothes (or string or anything that is Mekabel Tum'ah) to tie the poles [on which the Schach rests]. Almost all Rishonim hold that it is a mere decree mid'Rabanan not to be Ma'amid with something that is Mekabel Tum'ah, lest people use it for Schach. There is no such concern for Ma'amid d'Ma'amid. However, if one puts Schach on spits, just a piece of wood separates the spit from the Schach, this is not Ma'amid d'Ma'amid. The Schach stands on the spits, with a Chatzitzah.

Note: Kaf ha'Chayim (32, above) calls this Ma'amid d'Ma'amid, and so does Minchas Yitzchak (4:95), and it seems that the Ran calls this Midas Chasidus, i.e. it helps.

Minchas Shlomo: However, if the ends of poles rest on the spits (and the Schach is not directly over the spits - PF), and all the more so if one fixes the poles with nails, this is Ma'amid d'Ma'amid, and Chachamim did not decree about this. Every Sukah is on the ground (or on something on the ground). This shows that Ma'amid d'Ma'amid is permitted.

B'Tzel ha'Chachmah (5:44): We forbid only when the Schach could not stand at all without the Ma'amid that is Mekabel Tum'ah, e.g. Schach on a bed. This includes what could not stand in a standard wind on dry land. If it could stand in a normal wind, just he fears lest a fierce wind scatter the Schach, he may use something that is Mekabel Tum'ah to tie down the Schach. The Sukah is Kosher before and after such a storm, even though without what is Mekabel Tum'ah, the Schach would have blown away. However, it seems that during the storm the Sukah is Pasul.