Through construction contracts, clients and contractors are involved in principal-agent relationships. These relationships are argued to be adversarial and characterised by two main features of contracting parties, as follows: (1) different goals, and (2) different attitudes to risk. Agency-related problems contribute to the project participants concentrating on risks associated with their own parts of projects instead of joint risk management (JRM). Fewer studies have discussed JRM, although risk-sharing is a key issue in principal-agent relationships and agency-related problems must be effectively addressed to establish strong collaboration. Thus, the aim of this paper is to investigate how project participants address problems associated with principal-agent relationships and thereby enhance cooperative relationships (and JRM). The empirical findings are based on two longitudinal case studies and show that strong collaboration is a multidimensional activity that is crucial for JRM. Effective JRM is highly dependent on strategies for fostering relationships and collaboration (such as cooperative procurement, establishment of common goals, and open communication), and should be implemented together with these strategies.