Task 1 and Task 2 must start at the same time – they have a Start-to-Start dependency.
When the work of Task 2 cannot start until Task 1 starts, the tasks have a Start-to-Start (SS) dependency.
In the real world, SS dependencies generally occur because you want the work of two tasks to overlap. For example, "Level concrete" (Task 2) can't begin until "Pour foundation" (Task 1) begins.
Overlapping work also helps to reduce the overall duration of a project.
- If Task 1 will take 10 days to complete, and Task 2 will take 5 days to complete, and you set an FS dependency, the overall duration of the two tasks is 15 days.
- If you set an SS dependency, the overall duration of the two tasks is only 10 days (the time required by Task 1).
When two tasks have an FS dependency, but the work can overlap or be done in parallel, consider changing their dependency to SS to reduce the overall duration of your project.