Nominal Loads

The nominal loads for design should be according to the applicable code or specification under which the structure is designed or as dictated by the conditions involved. In the absence of a code or specification, the nominal loads should be those stipulated in the American Society of Civil Engineers Standard, Minimum Design Loads for Buildings and Other Structures, ASCE 7. The following loads are used for the primary load combinations in the AISI Specification:
D = Dead load, which consists of the weight of the member itself, the weight of all materials of construction incorporated into the building which are supported by the member, including built-in partitions; and the weight of permanent equipment
E = Earthquake load
L = Live loads due to intended use and occupancy, including loads due to movable objects and movable partitions and loads temporarily supported by the structure during maintenance.
(L includes any permissible load reductions. If resistance to impact loads is taken into account in the design, such effects should be included with the live load.)

Lr = Roof live load

S = Snow load
Rr=  Rain load, except for ponding
W = Wind load
The effects of other

The effects of other loads such as those due to ponding should be considered when significant.
Also, unless a roof surface is provided with sufficient slope toward points of free drainage or adequate individual drains to prevent the accumulation of rainwater, the roof system should be investigated to assure stability under ponding conditions.