deck waterproofing and ventilation

Hi all:
I'm renovating a heritage house in Victoria BC and repairing an exterior stair and deck/landing over an insulated space. The inspector says I must treat the deck like a roof, with waterproofing and ventilation.
The landing is 4' x 4' covered by a roof overhang. Head room below is less than 7'.
How about membrane under the deck flooring, then just vent the insulated space under the deck with holes in the first stair riser, under the deck lip, through to the space above the insulation, then out holes under the door threshold? Sorta rules out washing the deck with a hose, but then you can't wash your soffits with a hose and not get water up the vents either, right?
Thanks
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