16 days ago - someonesneaky - Direct link
Deterioration:
If an item is outside, not under a roof, is in standing or running water, is being rained on, it will decay
That means its durability / hit points will gradually go down until the item vanishes
That's how corpses, apparel, weapons, et cetera left outside eventually disappear.
Most things have a decay rate, but some things do not - all minified buildings, metals such as steel and silver, all stone bricks. Items stored on shelves will not deteriorate, which lets them be safely stored outside, no matter the weather, terrain, or roofing status.

Rotting:
If a perishable item such as food is left unfrozen, it will slowly rot. You can see the timer when selecting it, it'll tell you that it'll spoil in X days. It doesn't matter where or how you store it, anything that can rot will rot if you don't freeze it. That's the only way to keep products fresh.
In general, perishable items are raw food (meat, vegetables, etc), meals, most plant matter, and such. But it also includes herbal medicine, Ambrosia, wort, but does not include kibble, chocolate, or hemogen.
Some perishables have a very long shelf life, like hay, pemmican, and herbal meds. You can store them for a long time, preferably in shelves.

So even though the hay isn't deteriorating, it's slowly rotting away, because it's not refrigerated. You'll need to either store less in the barn, or build your animals a dedicated freezer - animal flaps work perfectly well for holding in temperature, especially if it's a smallish room with an airlock.