Um... not so sure here. 1.5 in 12 pitch is considered "flat" by code,
my local inspector tells me. If you mean regular rolls of shingles (~
3ft wide, nailing strip at the top, some tar near the bottom, etc),
then no, you can't use it. Or, well, of course you can do whatever you
like.

Roll roofing needs a pitch to work. It can be less than for shingles,
but certainly not flat.. 1.5 might as well be flat -- the slightest
little backup of water, snow, or ice, or a little wind, or a slight
bubble or crease or whatever, and water will back right up under the
rolls and into the garage. Milage will vary depending on climate (snow?
driving rain?), and if you garage is heated. You need a flat roof
system -- membrane, tar, whatever -- but not rolled roofing.