Do you need both a pack liner and a pack cover for backpacking? Probably not. It's excess weight and both more or less accomplish the same thing. (Plus, a poncho and pack liner combo offers the true winning option—stay tuned for more on why!)
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Ripstop nylon (which is also very durable!) and polyester are great materials for heavy downpours because they are tightly woven together. Water-repellent surfaces on the cover made of polyester keep you dry.