For example, before I put my bamboo-strip sechach mat on top of my sukkah frame, which is made of metal tubes, I put wooden boards on top of the frame to support the sechach. If the metal uprights poking up above the top of the frame at the corners happen to reach above the level of the boards, they may end up poking up a corner of the sechach. Would such a sukkah be totally non-compliant with the sechach-eligible-support rule, totally compliant, or non-compliant just near (how near?) the poking metal?
In a discussion of the whole issue of maamid, the Chazon Ish writes:
דסכך הנסמך על מקבל טומאה יש לומר דנעשה סכך פסול, וחברו הנסמך עליו הוא כדין נסמך על דבר שפסול לסכך בו
So it sounds like he's saying that no part of the sechach may rest on something that is mekabel tum'ah (with the exception of a stone wall, as he says further), and that if it does, it becomes invalid (according to the opinions that disqualify such sechach - as Gershon noted, the accepted halachah is that it's okay after the fact).
Our minhag to assume the safek of the Shulchan Aruch is an issue of maamid is based on the second answer of the Magen Avraham (629:9- although rejected by most acharonim including the taz and the Aruch haShulchan). According to the Machtazis haShekel, the text of the Magen Avraham's first answer is relevant to our halacha:
"Therefore it appears to me that he restricted using a ladder because it in itself is invalid schach [see siman 632]."
The Machtzis haShekel infers from the bracketed reference the following: If the reason would have been because of maamid, even a small amount would be invalid, but according to my answer that the ladder itself is schach pasul, it is only invalid if there is a shiur (4 tefachim) as delineated in siman 632.
So, according to the machatzis haShekel, one should not sit under that area even if it's a small peice of schach, and, according to those who hold fully of the gezeirah, even one bamboo pole could split the suckkah if it extends across.
I'm not sure that the Machtzis haShekel is muchrach, as you can say that if you held of the maamid answer you don't need siman 632 because b'diavad it's kosher based on the mechaber in 631:8. However, I have not to date received my supra-commentary license.
Nevertheless, if the Sechach was placed directly upon a metal frame, the Sukka may be used for the Misva during Sukkot.