Timely completion of dam and hydroelectric power plant (HEPP) projects is indispensable for the countries constructing them due to their economic, political, and social impacts. Robust and stable schedules should be created at the beginning of these projects in order to realistically estimate project durations considering uncertainties and variations. This paper proposes a buffer sizing methodology based on fuzzy risk assessment which can be used to calculate time buffers accurately for concrete gravity dam and HEPP projects by considering the vulnerability of activities to various risk factors as well as their interdependencies. A generic schedule is developed and 89 potential causes of delay/risk factors are identified for the concrete gravity dam and HEPP projects. Risk assessment is conducted at the activity level. The inputs of the model are frequency and severity of risk factors, and the output is estimated time buffer as a percentage of original duration. Implementation of the model is illustrated by an example project. Results show that outputs of the model can be used for scheduling, estimation of time buffers, and risk management of concrete gravity dam and HEPP projects. Although the model and its outputs are specific for concrete gravity dams, the buffer sizing methodology based on fuzzy risk assessment can easily be adapted to other types of construction projects. (c) 2017 American Society of Civil Engineers.