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The decision to model a structure using a single block or several blocks depends on ease of configuring walls and your intentions regarding of the generation of lateral loads on roofs.
If a structure is to be designed for wind loads generated by the software using the low-rise method, the designer has no choice but to define the structure using only one block.
Multiple blocks allow for defining different numbers of levels for portions of the structure, so that roofs at different heights are possible.
In the case of small jogs in the building, it is normally not necessary to define separate blocks for these jogs. Instead, the exterior walls of a block can be subdivided and moved to create the jogs. This approach allows the same roof geometry to be applied over the entire block and its jogs, as shown in Figure 2.
Figure 2 – Block with small jog over roof
If a block is contained entirely within one side of another block it is not necessary to be too concerned about the amount of overlap between the blocks, as walls are not created in the interior and the program automatically joins the initial roofs. However, if the blocks are diagonally opposed, the precise placement of the blocks is important to properly position exterior walls and because the roofs in this case overlap.
Since blocks cannot be added or deleted once you enter Walls Action and the walls are generated on the first level, you must be careful to create all the blocks you need for wall creation before proceeding. Blocks can be added in Roof Action for the purposes of adding new roofs.