I'd agree with the consensus on this one, pack your kit in waterproof sacks in the backpack.. Good enough for the Royal Marines, good enough for anyone! I use small heavy gauge plastic rubble sacks to pack kit in, inside a 6X4' survival bag inside the pack. I've never had a leak yet in 20 years... I think I remember a company called Macpac making what they *claimed* were waterproof packs but I've never tried one.