STOPAQ AMCORR makes a family of products that have been very successful in solving many roof leaks. Sloped and flat roofs alike. The product that I would recommend is called EZ WRAP. It comes in 2" or 4" wide rolls that are 33 feet in length. The product does not need any adhesive and can be applied in temperatures ranging 15 - 176 degrees F. It works wonders as a sealant around windows, as a sealant over flashing, or in place of flashing. It is UV stable, non-hazardous and is a permanent repair.
If you have any questions, feel free to contact me at firstname.lastname@example.org