I have researched root barriers for under pea shingle paths and also for a green roof. From previous experience the woven mat type still let seeds through, and if a dandelion seed can get through, then I don't rate the success on keeping plant roots out. I don't know how successful a fibre-type mat is. Any experience and can anybody recommend a brand? As for the green roof, I have a bonded rubber waterproof layer, on which I propose to put a polypropelene carpet upside down. I then want a root barrier, but nothing heavy. I have a roll of lightweight frost protection barrier which is trememdously impervious: having tried to force a screw through, it winds around the thread, so even a really thin layer seems more impervious than the thick woven root barrier. What do you think about 3 or 4 layers of the frost barrier? On top will be a proprietary tray system to hold water and allow it to run off, then lightweight planting medium.