The Chicago Rabbinical Counsel, pursuant to the psak of Rabbi Gedaliah Schwartz and Rabbi Shmuel Fuerst, has made it known that a certain commercially available fencing product is kosher to be used as Schach. The original psak can be seen here. However, they include a link to a caveat: it is only kosher when it is supported by some other schach other than this fencing- on bamboo, for example- and also that it lies perpendicular to the other schach. The warning can be seen here. If you remember your High School math, the Cartesian terms would be that this Schach is only kosher on the X axis. If you place it on the Y axis, it will be passul.
Let's call it "X Axis Schach."
Here is a picture of the schach in use in a sukkah.
In the picture you can see a black line. This is one of the wires that are woven through the reeds to hold them together. A moment's thought should make it clear why this schach is only kosher perpendicular to the support bamboo. If a moment's thought does not do it, I note the explanation below.
Unfortunately, from my examination of this product, I believe it to be mostly open, חמתו מרובה מצלתו. It might be that since its use is limited to cases where it is perpendicular to other kosher schach, it can be assumed that between itself and the support schach there will be a total of צלתו מרובה מחמתו. This, however, is a risky assumption, but well within the purview of reliance on one's poseik. For skeptics, the only way to make a final determination would be to use a light meter and a flashlight in a darkened room. Rabbi Reuven Drucker, the Marah D'Asra of the Agudah of Highland Park New Jersey and Menahel of Yeshiva Rabbi Samson Raphael Hirsch in Washington Heights, is an expert in such determinations.
I was just informed that Rabbi Fishbane, head of the CRC, says that they did measure it and they found it to be tzilsa merubah. Rabbi Fishbane is universally respected in the Kashrus World, has well established his credentials as a man to be relied upon, and I cede to his opinion.
Oddly, in the printed CRC information sheet, this schach is discussed, and the paragraph ends by saying that as in all cases, one should ensure that the schach is Tzilso Merubah. Evidently, someone at the office was not convinced that it is mostly reed.
If the schach is placed parallel (Y Axis) to the support schach, then removing the wire would result in all the reeds falling down between the supports. If it is placed perpendicularly (X Axis,) then even if the wire were to be removed, the reeds would stay in place. Thus, when the fencing is parallel, the wire is the only reason the schach stays where it belongs, and under the rule of Maamid Bedavar Hamekabel Tumah the reeds are no more kasher than the wire itself. It is like having a metal lattice holding up the schach. (Many poskim are not concerned about maamid..)
ON EIRUV TAVSHILIN
On another topic: Some people, not chalilah in reference to anyone who will be eating in my house this Yomtov, are obsessive about the Eiruv Tavshilin. They are afraid someone (me) will forget to make the eiruv. Others might themselves be worried that they will get busy and forget erev yomtov. For those unfortunate compulsives, I have some advice. Make the eiruv two or three days before yomtov. As the Tur points out, his father, the Rosh, says that according to Rav Ashi, such an eiruv is kasher, and we pasken like Rav Ashi (as the basra.) True, the Mechaber (OC 527, see, for example, here, in Seif 4 and 25 ) holds that lechatchila one should be choshesh for the shitta of Ravva (and the Maharshal that says that the Rosh's analysis is incorrect) and only rely on an erev yomtov Eiruv, but bedieved it is certainly kasher. So for anyone taking this to heart, remember: if you make the eiruv a few days before Yomtov, or erev yomtov for the last days as well, it's kosher. But don't make a bracha, neither now nor later on erev Yomtov. And make sure the Eiruv food remains edible and available until Shabbos begins.