Construction error accounts for billions annual budget loss in world’s construction industry and it is common for inspectors to find missing structural member during construction auditing. This scenario alters intended functional requirement of certain portion of a structure and demands application of external strengthening mechanisms to make it fit for use. Due to the great advantages such as high strength to weight ratio, corrosion resistance, durability, ease of transportation, low maintenance costs and faster installation time, etc., carbon fiber reinforced polymer (CFRP) become very popular strengthening technique as the way of fixing construction errors. It gives insights to practicing and design engineers’ alternative ways of solving real life challenges. This study numerically investigated the response of RC slab under missing perimeter beam scenario and studied the use of CFRP sheets as external strengthening mechanism to restore load carrying capacity and increase stiffness of RC slab. A powerful software in field of engineering, ANSYS was used to perform a structural 3D non-linear finite element analysis (FEA) of RC slab models. The accuracy of the FEA method was verified using experimental test results in literature. In terms of ultimate load and crack pattern, the FEA results were compared and showed good agreement with experimental test results. The response of RC slab under external beam missing scenario was investigated by disregarding perimeter beam in validated FEA and found that 35KN ultimate load carrying capacity of it was lost. In order to investigate the effect of CFRP sheet external strengthening mechanism such as sheet layer number, sheet width, sheet spacing, sheet running direction and fiber orientation in structural behavior of RC slab, parametric study was performed. A total of twenty-seven CFRP sheet strengthened slab specimens were simulated in five parametric variables of study. The FEA results indicated that due to CFRP sheets the restored ultimate load carrying capacity of RC slab that was lost due to missing perimeter beam scenario ranges from 18.3 to 127.7%. Whereas, the increased in pre-crack and post-crack stiffness ranges from 6.7 to 21.1% and 36.1 to 92.3% respectively. Also, the ductility of RC slab was enhanced due to CFRP sheet.