I had a new flat roof done in 2001. It is leaking where the carport roof abuts to
The main house. I always though they should have used a flasher.

It has a 10 year warranty to 2010.

There definitely should be flashing where the two structures meet. Since you have a warranty you should call that installer, if he is still in business.

Flashing yes, flasher no. Flashers get in trouble with the law for indecent exposure.