The construction sector has become a fundamental axis of development for society. Given the complexity of the sector, it would seem important to understand how its activities are developed. Resilience engineering with its tools allows understanding complex socio-technical systems, to achieve the capacity to cope with unforeseen events that are related to variability. The main objective of this study is to identify the variability of system performance in the concreting of a building slab. The methodology used in this article is based on the “Functional Resonance Analysis Method” (FRAM) and, considering its principles and phases, it would allow to analyze the global resonance of the complex socio-technical system, considering the incidence of the variability of a function in subsequent functions. Twenty-four main system functions have been determined in the concreting of a building slab. Variability has been identified, considering simple phenotypes of time and precision, giving priority to human functions, with an analysis of endogenous and exogenous variability. FRAM as a Resilience Engineering tool makes it possible to recognize system functions and identify the variability of system performance in the concreting of a building slab, making the functional couplings visible in the daily work performed.