A device mounted on a roof is provided with, in one embodiment, a first structural component including a first water guide board on the roof and inclined upward, the first water guide board having a groove at a downward bent open end; a vent under the first water guide board; a second structural component including a downward inclined second water guide board extending toward the first water guide board; a splash elimination wall under the groove and abutted on the vent; and a water blocking board secured to a bottom of the first structural component and abutted on the vent.