Rainwater is likely to cause a lot of corrosion to a standard leak sensor. And they aren't really designed to be wet "often".
If I were doing it, I'd use a sprinkler/irrigation system type rain sensor. Note that all the ones I've used in the past suck after a few years (the sun bakes them, they get clogged and start sticking, etc) but if you have relatively easy access to it they're not horrible. Wire it up to a z-wave contact/door sensor with support for dry contacts to interface it to the rest of your HA.