The Chazon Ish in Hilchos Sukkah 143:2 writes:
ולכן לדידן שאנו נוהגין להחמיר בדין המעמיד הסכך בדבר המקבל טומאה כדעת רמב"ן ור"ן, אין לקבוע הכלונסות במסמרים.
Therefore according to us, who are stringent in the law of supporting the schach with something which can become tamei, one should not stabilize the planks (which support the schach) with nails.
(This is the source of the famous no-nail "Chazon Ish sukkah")
Qusetion: The Mishnah in KEILIM 11:3 states:
העושה כלים... וּמִן הַמַּסְמְרוֹת, שֶׁיָּדוּעַ שֶׁנַּעֲשׂוּ מִכְּלִי, טְמֵאִין. מִן הַמַּסְמְרוֹת, בֵּית שַׁמַּאי מְטַמְּאִין, וּבֵית הִלֵּל מְטַהֲרִין:
One who makes vessels... from nails that were known to have been made from vessels, are impure. From ordinary nails: Beit Shammai says: they are impure; and Beit Hillel says that they are pure.
We see from this Mishnah that (according to Bais Hillel, as is the halachah) only nails which are definitely recycled from other vessels are mekabel tumah, but regular nails are not mekabel Tumah.
So why does the Chazon Ish not allow supporting the schach with nailed planks?