Excellent, thanks so much. You're all pretty much hitting the nail on the head. The deck isn't covered; one wall is the master bedroom and where the deck door is. The other 3 walls are just 4-foot balcony type walls. Yep, the deck floor is a few inches lower than the interior floor so you have to step down onto the deck. It's sloped so the water does run out a corner gutter. Yep, there's a bedroom directly under the deck. What happens in a heavy rainfall (especially hurricanes like Irene and Sandy! I'm in Philadelphia, PA) is the water seems to be leaking in around the door, then running down inside the wall to the room underneath where the water drips out around the light switch. Scary, right? When this happens I immediately cut the power to that room of course. The light switch is directly beneath the deck door, same wall one floor down, so I think it's safe to say that's where the leak is happening but I can't be positive. For starters I guess I'll try squirting some silicone all around the deck door.