The minimum pitch for an asphalt roof for Asphalt shingles is 2/12 or more as long as you use at least one layer of ice and water shield and one layer of synthethic felt under the architectural shingl ... read more
I would recommend not less than 40 degrees. If the shingles are a dark color in direct sunlight they will seal in one day from the solar radiance from the sun even if temps are low.