Definitely not recommended, but if you have a roof that is
gable without chimneys, skylight, dormers, walls, and flashing then you might
be able to cheat otherwise in fact it would be chalking alone that would be
keeping water out. On a roof chalking does not hold up to the continuous stress
and harsh weather elements like shingles do.
Two layer systems are also very dangerous due to trapping
water as well as trapped condensation resulting in mold concerns and hazards.