If you are starting from scratch you have to decide the pitch. In South Africa we define the pitch in degrees. So a 5 degree pitch is pretty flat, and a 45 degree pitch is fairly steep.

Something to note when buying stock plans on the Net: In North America you refer to the slope of the roof as the ratio of the rise over the run. Usually in inches with the run being 12". So an American drawing will note the slope as 4 in 12, that is 4 inches in 12 inches.

When designing a roof, the pitch determines what material can and can't be used for the covering. For example if you want to use corrugated Iron sheeting, there is a minimum pitch specified by the manufacture, it may be 5degrees. Similarly with roof tiles it could be 15degrees. Check out the material specs before you go ahead.

If you live in a high rainfall area you need to be wary of almost flat roofs.