The slope of your flat roof is essential to making sure water drains quickly and smoothly. Design considerations for the overall aesthetic of the building must be balanced with this functional need. A drop of 1/4 inch every 12 inches (or 1/4:12) should be fine for most structures. This means that for every 12 inches of roof, there should be a drop of 1/4 inch.
Adequate Slope for Drainage
Some builders use a ratio of 1 in 40, meaning that for every 40 inches of roof span there is a drop of 1 inch in the pitch of the roof. This is usually done as a means of adding extra insurance to roof drainage. It is more than is necessary to give adequate drainage, however.
The general standard for appropriate flat roof slope is 1/4 inch every 12 inches. This slope will allow for adequate drainage of water from the roof. Do be certain to plan how your roof will shed water, for example into gutters or built drains, as slope alone is not enough.