Must the commonly sold bamboo Sechach mats contain a reliable Rabbinical supervision?
Yes. This is due to the fact that in some countries the mats are actually used for roofing or other vessel purpose in which the ruling in Shulchan Aruch dictates that it may then not be used as Sechach. Thus supervision is required to verify that the mats are not being made for an invalidating purpose. As well, supervision is required to verify that the sticks of the mat are woven using material valid for Sechach, such as the more expensive cotton string, in contrast to the cheaper synthetic strings which are invalid for Sechach.
 Rabbi Don Levy of OK laboratories brought on the OK web site.
 Although cotton is invalid for Sechach, since this is only a Rabbinical invalidation due to having lost their original form, they are allowed to be used to support the Sechach as we do not make a decree upon a decree. [Rav S”Z Aurbach; Rav Wozner -Piskeiy Teshuvos 629/6] Nevertheless it is best to get materials which themselves are valid. [See Piskeiy Teshuvos there!]