How to stop a flat roof from leaking?

Look into products and procedures for RVs and Manufactured homes. (youtube.com can be your friend).

Here in Idaho, the people who put flat roofs on their houses a few years ago have all changed them to gabled roofs so they have perfect run-off. Saves a lot on correcting the damage and leakage problems year after year.

I think your best action would be to find a reputable roofer. Flat roofs are tricky. Persistent leaks could cause mold and damage to your home.

Did your roof come with a warranty? Usually flat roofs are guaranteed 10-20 years. A professional will charge anywhere from $400 to $1500 for a 1500 sq ft flat roof to be sealed. A bit pricey really.

Have you tried Karnak 19 Ultra Rubberized Flashing? The stuff is amazing.

There's a product you can get called 'flex seal' it comes in a spray and a tape. But it is incredibly amazing stuff and that will for sure stop the leak! I would suggest using the spray for this job

Here in the rainy PNW, I've notice all the flat roof owners out fixing their roofs in the middle of the rainy season for just that reason..... flat roofs just don't work and are money pits.

I would never have a flat roof for just that reason..... I would replace the roof, and at least tilt it up on one end and extend the roof past the house to create a overhang all the way around the house with gutters to move the water away. Lots of videos on YouTube.... https://www.youtube.com/results?search_query=changing+a+flat+roof+to+a+sloped+roof

We also have a flat roof in snow country. I wanted to replace it with a slightly slanted roof but could not find a contractor who would agree to my vision. We had to have the roof completely redone. It had been tar and gravel, which is very smelly and unpleasant, which we had removed by a roofing company and replaced with a rubber sheeting well sealed at every edge. It takes constant work. As you know, large level areas always sag after time. My husband is on the roof after every melt and rain to sweep the water off to prevent extra strain on the roof base and the growth of fungus in standing water. He also constantly checks the seams for cracks. Iven so, we had a leak that stained our new ceiling tile in one corner.

I suggest you replace your flat roof with a slanted roof. Since then I have discovered that I could have had my vision if I had been willing to have a metal roof rather than shingles to match the rest of our roof. Perhaps that will work for you.