The shingle manufacturers vary between a 2 1/2" per 12" and a 3/12 slope.
If shingling even at these miinimum slopes, it is recommended to install a product similar to Grace Ice and Water Shield under them in lieu of standard felt paper.
How flat of a surface are you talking about?
You determine that by the amount of rise, expressed in inches typically versus the amount of run, also in inches per foot. The run is the horizontal distance from the one end to the other, so if you calculated from the gutter eave edge go one foot and see how many inches up the roof slope transitioned.
Take a 12" long board, 2x, 1x, etc.,
place it on the roof with one end pointing to the gutter and the other end pointing to the upper end, than leaving the stick pointing toward the upper end firmly on the roof lift the end toward the gutter up until it's level,
measure from the roof to the bottom of the board and that's your pitch.
If you take the time to make sure it's as level as possible your read will be fairly accurate, usually with in 1/4".
Like Ed said, Certainteed 'I believe' has spec's for a 2 1/2/12 pitch but most stop at a 3/12 pitch being the lowest for allowing shingles.
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