In that case, you are probably correct. The leak is in the seam.

If the flat roof is EPDM, looks like bicycle inner tube material, you
can glue another piece 6" wider on each side of the seam. Use If the
material looks like it is a tar sheet, you can use torch-down sheet 6"
wider on each side also.

I think you said the material is roll roofing. Were these simply
nailed down? Maybe it just needs a little tar and mesh (tar coated
baddage looking thing.

Looks like I need to see the problem to really help you. PerhapsIf you
were in the DC Metro area, you could get some material from Washington
Roofing or Bradco Supplies.

Good luck with it.

Bryan