Different market areas set different requirements on software intensive products. A careful domain analysis yields the predictable variance that can be managed at architectural and design levels. But, the unpredictable variance that is detected only during the maintenance phase is usually managed at the implementation level due to the high costs of the system redesign or re-architecting. Thus, most legacy systems contain a lot of unpredictable variance in the source code for example in the form of cluttering it with pre-processor directives. Weak overlay technique is a promising reverse engineering approach to abstracting, comprehension, maintaining and redesigning legacy software containing various types of variance.