The tarp must be well taut, with a slope of at least 45°, and the bales must be placed on pallets (preferably in two rows) and not in contact with the ground. The hay must have been properly dried in the field (residual humidity less than 20%). The tarp is not waterproof, but hydrophobic (i.e., it does not retain water but expels it through gravity or evaporation).

It works like this: when it rains, some of the water slides off the surface and ends up on the ground, while the rest penetrates the cover, which retains it and then carries it to the lowest point. In case of heavy and persistent rain, some of the water passes through the cover, wetting the straw (the percentage varies depending on the intensity of the rain). However, since the cover is breathable, it allows the hay to dry. For this to happen, it is important to store the hay in a sunny and ventilated location (not close to trees, walls, embankments, buildings, or other piles). In case of heavy and persistent rainfall above average, there is a risk that the resulting intense humidity will delay and prevent drying.
It is the user's responsibility to periodically check that the cover is installed correctly and that the covered material is stored properly.