Roof systems go way back. The asphalt composition shingle is a modern version of a shingle system invented as far back as the Egyptian dynasties, but no one can be sure of its origin.

Wood shingles and shakes were probably the first shingles used in America by the colonists who brought the shingle concept from Europe. Slate remains common throughout Europe and the USA. Thatch is sill used in Britain and elsewhere in Europe. Clay barrel tiles and hand-formed roofs made from malleable metal go back to ancient Greece and Rome and remain in common use. Each of these roofing alternatives have cost, appearance, availability and performance characteristics that affect their desirability for a homeowner. Composition asphalt shingles are a modern addition to the roofing material inventory, and widely used in the USA. Overseas, composition shingles are not so widely used.

The principle behind shingling is ancient and proven: to keep water moving down sloping roofs until it runs off and away from the house. It doesn’t matter precisely what material is used so long as the slope is adequate. Materials don’t even have to be waterproof if you have enough redundancy. Thatched roofing and wood shakes are examples of materials that are not waterproof but nonetheless shed water.

So, the first principle of shingle roofing is: Keep the water running off the roof at the eaves. Anything that interferes with that principle introduces the possibility of a leak. Steep roof slopes are the foundation for an efficient water run-off. The lower the slope, the greater the risk that water can somehow back-up under the shingles.
For that reason no modern shingle manufacturer will approve the use of their materials in a shingle roof system on a slope below 2/12.