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I have been reading Canadian Wood-Frame House Construction to get an idea how houses are built and function, and am really confused about one aspect.

External floors are built just like external walls, just on their side. Internal boards->Vapour Barrier->Studs/joists with insulation. Which makes a lot of logical sense in walls and ceilings. But since water drains down, I have no idea how this is supposed to work for the flooring. I am using an example of a wood framed external floor, but the same principal extends to concrete slabs that have a vapor barrier built in. Assuming I spilt some water, or a pipe broke, a few gallons of water got on the floor. How does this standard design allow this excess water to be shed by the house?

The entire house is encased in a plastic vapor barrier, that is sealed and caulked at all the seams, it is one giant plastic bag. This house now has several gallons of water under the floor boards. What happens next? Or is a vapour barrier not a liquid water barrier?