During last month JMT trip, I was using just a plastic sheet (drop cloth from HomeDepot) which was providing some moisture barrier but not good enough for rain soaked ground and sharp sierra pebbles.
Several thru hikers were using Tyvek sheets for ground cloth, due to its light weight, complete moisture/ water barrier and toughness.
Q. where do you get a piece of Tyvek (about 10ft by 8 ft which I plan to cut to my BA tent size). Home depot sells this by rolls costing around $100/- for builders.
