Materials that are "Mekabel Tum’a" – susceptible to the Halachic status of ritual impurity – may not be used as Sechach for the Sukka. The Shulhan Aruch rules that one should also not use such materials to support the Sechach ("Ma’amid"). The Rabbis were concerned that if people supported the Sechach with these materials, they might ultimately use these materials for Sechach itself, in which case the Sukka would be invalid for the Misva. Therefore, one should not support the Sechach with materials that are not suitable as Sechach.
For this reason, it is customary when building a Sukka with a metal frame to first place wooden slats on top of the frame, and then place the Sechach over the slats. Metal is not suitable as Sechach, and it should therefore not be used to support Sechach. Hence, one should first place wooden slats over the metal, and then the Sechach over the slats. In such an arrangement, the metal is "Ma’amid De’ma’amid" – it supports that which supports the Sechach, rather than supporting the Sechach directly. And according to the Shulhan Aruch, materials unfit as Sechach may be used as a "Ma’amid De’ma’amid." Although some later Halachic authorities rule that a Sukka may not be used if even the "Ma’amid De’ma’amid" is made from material that is disqualified as Sechach, nevertheless, Hacham Ovadia Yosef ruled that Halacha follows the Shulhan Aruch’s position in this regard. Thus, one may use a Sukka with a metal frame, provided that he places wooden slats over the metal and the Sechach over the wood.
The Mishna Berura discusses a case of a person who has on his property four concrete walls, and he wants to place Sechach over the walls to form a Sukka. Even though concrete is "Mekabel Tum’a" and thus invalid for use as Sechach, the Mishna Berura allows placing Sechach directly on the concrete walls. Since nobody would mistakenly use concrete as Sechach, the decree mentioned earlier does not apply in such a situation. And even if there are metal poles in the concrete to hold it in place, the Sechach may nevertheless be placed directly on the concrete, since the primary material is concrete and the wall can stand even without the metal poles.
May one use plastic fasteners to hold the Sechach in place on top of the Sukka?
The Hazon Ish (Rav Avraham Yeshaya Karelitz 1879-1954) allowed the use of plastic fasteners, even though they are unsuitable as Sechach and serve to hold the Sechach in place. He explained that assuming the Sechach is capable of staying in place and withstanding ordinary winds on its own, even without the fasteners, they are not considered as supporting the Sechach, even though they add extra reinforcement. So long as the Sechach is weighty enough to stay in place under normal circumstances without the fasteners, the fasteners may be used even though they are made with plastic. It goes without saying that fasteners made from cotton may be used for this purpose, as well.
Summary: If one uses a metal frame for his Sukka, he should first place wooden slats over the frame and then place the Sechach over the slats. If one uses four concrete walls for his Sukka, he may place the Sechach directly over the walls. One may use plastic fasteners to hold the Sechach in place, provided that the Sechach is heavy enough to stay in place under normal circumstances even without the fasteners.
