Although buffers of inventories, time, and capacity are commonly recommended to mitigate variability in construction, they abstract away the role played by human agency. This study argues for slack as a socio-technical complement to buffers for dealing with variability. The investigation is based on two case studies conducted in construction projects that adopted the Last Planner® System. Data collection focussed on understanding how slack practices and resources (SPR) were used in production planning and control, and was based on observations, analysis of documents, and interviews. Findings revealed 57 instantiations of slack practices and 8 types of slack resources. Several of these SPR diverge from what are traditionally called buffers, highlighting how the concept of SPR gives visibility to a wider range of variability coping mechanisms. Thus, it is important to make SPR explicit so that managers can reflect on why SPR are necessary, understand how they relate to each other, and assess their unintended consequences. Five propositions are presented, encompassing: how to identify SPR; the variety and general- or context-specific nature of SPR; and the value of maintaining SPR. These propositions contribute to risk management in production planning and control.