Flashings or lack of, are more than half of what causes most of the leaks that I have been blessed enough to have to repair. I don't think I've ever had to repair one of my own flashings because I flash the crap out my jobs. Nearly every edge situation where there's a transition in materials or major pitch change requires a flashing. Even the drip edge around the edge of your roof is a type of flashing. It makes the transition from the roof pitch to the vertical fascia. There must be a flashing around that deck to deflect the water away from the edges and off of the house. At least, that's where I'd start to look for my problems. Remember, tar will not replace a good flashing made of a metal that will not react with the materials that contact it.
Tom in KY, been flashing folks for years.
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