Must the items which support the Sechach [The “Mamad”] be themselves kosher for Sechach?
From the letter of the law the Sechach may be placed over a wall made of any material, even a material which can contract impurity. However the Sages decreed that initially the Sechach should only be placed on something which is not able to contract impurity [Regarding other items which are invalid for Sechach-See footnote], or on something which one would never come to use as Sechach such as stone wall and the like.
Must the support of the support be of materials valid for Sechach? The Sages never decreed that the support of the supports be made of materials valid for Sechach and thus any material may be used even initially. One may thus make the walls of the Sukkah from materials invalid for Sechach and place over them material which is valid for Sechach. [See footnote for other opinions]
The reason no decree was made by the support of the support: As in any event everything is ultimately supported by the earth which is too invalid for Sechach. It is thus not relevant to decree against the support of the support.
Nailing in the supports of the Sechach: Based on the above that no decree was made against using invalid Sechach to support the support of the Sechach, it is thereby allowed to nail in the supports of the Sechach or tie them down, as the nail and rope are merely a support of the support. [However regarding nailing or tying the Sechach itself, see further.]
If the supports were mistakenly made of material which are invalid for Sechach does the Sukkah remain valid? The Sukkah remains valid, and it is permitted to even initially dwell in such a Sukkah in order to fulfill the Mitzvah.
May one place items which are invalid for Sechach, over the Sechach to support if from flying away? The decree of the Sages applies as well to items placed on the Sechach for purposes of weighing it down. [Thus initially one may not use nails, or rope which is invalid for Sechach, to nail or tie down the Sechach to the Sukkah.]
List of items which may not be used to support the Sechach:
- –Metal poles
- –Plastic rope
If there are no valid supports available may an invalid support be used?
The following opinions rule like Admur: Magen Avraham and Chayeh Adam. The Michaber and Rama do not make mention of this stringency.
The Mishneh Berurah brings that “There are some Achronim which have ruled that the direct support are to initially be made of materials Kosher for Sechach” thus implying that there is room to question this as an all ground common ruling.
 Admur mentions countless times that the prohibition is regarding using items which can contract impurity as supports, thus implying that other invalid items may be used as supports. However from the fact that Admur explained the reason why stone walls are valid to have the Sechach rest on them is because one will not come to think that a stone roof is valid, as opposed to saying simply since they do not contract impurity seems to imply that all invalid items may not be used as supports. Vetzaruch Iyun.
In other Poskim: Some rule that only items which can contract impurity are invalid while others rule [Minchas Yitzchak 4/45] that all invalid items are invalid to be used as supports. [See Piskeiy Teshuvos 629/7]
 The reason for this decree is due to worry that one may come to use the support material for Sechach, and thus if invalid material were allowed, one may come to use invalid Sechach. [ibid]
 The Mishneh Berurah rules like the Michaber and Admur that there is no need to be stringent to have the supports of the supports also be made of materials Kosher for Sechach. Furthermore, even regarding the direct support the Mishneh Berurah brings only that there are opinions which are stringent, as opposed to simply ruling like their opinion.
The Chazon Ish rules that the support of the support are to likewise be of material which are valid for Sechach. Accordingly the walls of the Sukkah must be made of materials which are valid to be used as Sechach. However it is implied from his wording that if the walls are made of stone they are nevertheless valid.
 Mishneh Berurah 629/22