What is Slack Time?

Slack time is an interval that occurs when there are activities that can be completed before the time when they are actually needed. The difference between the scheduled completion date and the required date to meet the critical path is the amount of slack time available. The project manager should always be aware of where slack time exists in a project, since this time can be used to reshuffle the schedule to support the critical path.

Example of Slack Time

As an example of slack time, a construction company is building a new office complex with a scheduled completion time of 12 months. Within this project, one of the activities is the installation of office windows, which is scheduled to start in month 6 and finish by month 8. However, based on the project’s critical path, the windows are not actually needed until month 10 to begin interior work such as drywall installation and painting. This creates a slack time of 2 months for the window installation task. In other words, although the windows are planned to be finished by month 8, they could be delayed without affecting the overall project deadline, as long as they are completed by month 10.

This slack time gives the project manager flexibility. If labor or materials for the window installation are delayed, resources can be shifted temporarily without putting the entire project at risk. Alternatively, the slack time could be used to reassign workers to other critical tasks or reduce overtime costs.

Slack Time Best Practices

You should keep track of the trend in available slack time for each task. If the trend is declining, it can indicate that work is taking longer than expected. By spotting these trends, you are in a better position to investigate and determine why work issues are occurring. This can result in the timely reallocation of resources to maintain a reasonable amount of slack time within the system.