BTW, the star-K says it's a davar hamaamid issue even when using monofilament "fishing line" that is not meqabel tum'ah. The maamid must be kosher sekhakh, which vinyl isn't. (I had thought it was a meqabel tum'ah issue only.) See their discussion (and sources) at http://www.star-k.org/cons-seasonal-schach.htm Tir'u baTov! -Micha