When building a Sukka, one should preferably ensure that the Sechach (covering of the Sukka) does not rest directly on metal, or on other materials susceptible to Tum'a (ritual impurity). The Sages enacted this law out of the concern that if people would use materials that are susceptible to Tum'a to support the Sechach, they might then use such materials for the Sechach itself, which would invalidate the Sukka.
This Halacha is very relevant nowadays, when many people construct Sukkot from metal frames. One who builds such a Sukka should not lay the Sechach directly upon the frame. Instead, it is customary to place several wooden beams along the top of the frame and then lay the Sechach upon those beams, such that the metal supports the Sechach only indirectly. This arrangement is referred to in Halacha as "Ma'amid De'ma'amid." Nevertheless, if the Sechach was placed directly upon a metal frame, the Sukka may be used for the Misva during Sukkot.
If a person's Sechach fell on Yom Tov, he may ask a gentile to replace the Sechach to enable him to fulfill the Misva of Sukka. Ideally, he should ask a gentile to ask another gentile to replace the Sechach. If this is not feasible, then he may ask the gentile to replace the Sechach himself, but preferably he should not state the request explicitly.
If one did not, for whatever reason, construct a Sukka before Sukkot, he may build a Sukka during Hol Ha'mo'ed Sukkot. He may also hire a skilled Jewish workman to build the Sukka on his behalf, and he may even pay the workman for his services. Since building the Sukka constitutes a Misva, it is permissible to hire a Jew to perform this service on Hol Ha'mo'ed. Once the Sukka is erected and the person uses it for the first time, he recites the Beracha of "She'heheyanu." Even though he had already eaten in somebody else's Sukka on the first days of Sukkot, he nevertheless recites "She'heheyanu" over the experience of eating in a new Sukka.
(Taken from Hacham Ovadia Yosef's rulings in Hazon Ovadia – Sukkot, p. 54 and onward)
Summary: The Sechach should preferably not be supported by metal directly; if one builds a Sukka with a metal frame, he should lay wooden beams across the frame and place the Sechach upon the metal. Nevertheless, one may use a Sukka if the Sechach was placed directly on metal. If one's Sechach fell on Yom Tov, he may ask a gentile to replace it for him. If one did not build a Sukka before Sukkot, he may build one on Hol Ha'mo'ed or hire somebody – even a Jew – to build it on Hol Ha'mo'ed, and when using it for the first time he recites "She'heheyanu."