Ok, too many people get confused by a roach and a filter. A filter is normally fiberglass, and its what you get at the end of a cig. A roach, [The proper term for it, not just the end of the joint] is a bit of rolled up card. It doesnt filter anything, it keeps the end of the joint open, making the hits about 1000X better, and it saves any weed being wasted cos you can smoke it right down.

A filter from a cig WOULD filter out some THC, but you'd still get enough through to get high, but why bother? Why waste any THC?