stakes and palissades are imune to rain
25° and 45° wood and corewood parts wont get any rain damage
Darkwood seems totally imune to rain also

all stone parts are imune to any sort of water

I build a roofing top piece and place it on top of my gates to prevent them from rotting. All roofing parts are immune to rain. I was unaware that the angled wood parts seem to be immune (probably a bug), but everything else made of wood should be susceptible.

Nothing rots to below 50% health, so generally it doesn't matter aside from gates which need HP to keep enemies out.

Iron gates and gratings seem to be immune to rain also.

Stakes and palisade wall don't rot, but gates do.
Most of other wooden structures do rot.

Is it me or does Darkwood not seem to rot readily?

darkwood is tarred,,, literally waterproof