Especially in the first few years, our high-quality backpack material and the DWR impregnation still keep it sufficiently waterproof. However, sometimes - especially after a thorough wash - a re-impregnation may be necessary. You can easily test this by dripping a small amount of water on the upper material of your backpack: if the water does not bead up but penetrates, you should impregnate the backpack. In heavy rain, we also recommend a rain cover to prevent water from penetrating through the zips and seams.