Slope
Completely flat roofs typically present weather proofing problems, so the rafters, or joists, of some flat roofs are laid at a slight angle for drainage. If this is the case, the slope of the roof is also important.
Load
In some localities, provisions need to be made for high winds, snow and earthquakes when specifying the size of rafters for a flat roof. The American Wood Council provides span tables for joints and rafters under different load conditions.
Example
If the rafters of a flat roof need to span 18 feet, with a deflection limit of their length divided by 360 (L/360), they need to be 2 inches by 10 inches or 2 inches by 12 inches, depending on the type and grade of timber used. These dimensions take into account a snow load of 20 pounds per square foot (psf).