The Sea to Summit Ultra-Sil Pack Covers are very light and pack into a tiny stuff sack so they don't take up any space in your pack when they're not being used. They also come in a choice of sizes to fit most bags.
Sea to Summit says:
Ending the debate about whether to carry a pack cover or not, the ultralight Ultra-Sil® Pack Cover offers a tiny yet compelling solution. Weighing only 113g for a 70L pack, why wouldn’t you carry one?
Even though you may have your gear stored in waterproof bags inside your pack a pack cover will stop the fabric of the rucksack soaking up water and becoming much heavier. Lighter pack = happier trek.
