The subflooring underneath isn't wet. So I thought maybe it's coming from the roof. There's a water heater vent which goes through an interior wall, near that spot. I thought maybe rain is leaking down the side of the vent and spreading out onto a board at the bottom of the wall, and channelled along tongue and groove channels in the flooring.
But when I got up on the roof with a tar-gun, there weren't obvious leaks around the side of the vent. It would maybe leak a little (which I sealed off), but it didn't look too bad.
But, is the water heater vent supposed to have a cap on it? There's a strange 4-sided crinkly thing around the sides of the top, but no cap. I don't understand how, even if it were lacking a cap, water could get into the floorboards. I'd think that rain would go down into the water heater, if it were falling into the vent - not end up in my floorboards.
The vent for my gas house heater *is* capped, though.
What should the water heater vent look like?