Context: Software decay is a key concern for large, long-lived software projects. Systems degrade over time as design and implementation compromises and exceptions pile up.Goal: Quantify design decay and understand how software projects deal with this issue.Method: We conducted an empirical study on the presence and evolution of code smells, used as an indicator of design degradation in 220 open source projects.Results: The best approach to maintain the quality of a project is to spend time reducing both software defects (bugs) and design issues (refactoring). We found that design issues are frequently ignored in favor of fixing defects. We also found that design issues have a higher chance of being fixed in the early stages of a project, and that efforts to correct these stall as projects mature and the code base grows, leading to a build-up of problems.Conclusions: From studying a large set of open source projects, our research suggests that while core contributors tend to fix design issues more often than non-core contributors, there is no difference once the relative quantity of commits is accounted for. We also show that design issues tend to build up over time.