Start-to-start (SS)
A start-to-start (SS) dependency means the successor cannot start until its predecessor has started. It lets two tasks run in parallel, often with a lag so the second begins a few days after the first.
SS suits work that can follow closely behind other work, such as validating data while it is still being migrated. It is written as 3SS, or 3SS+2 for a two-day lag.
Because the link only looks at the predecessor's start, a predecessor that starts on time but finishes late does not move an SS successor. That is a common surprise when people expect every link to pass a delay on.
Worked example
| Task | Duration | Depends on | Scheduled | Total float | After the delay |
|---|---|---|---|---|---|
| 1. Data migration | 10 days | None | Days 1 to 10 | 0 days | Days 1 to 14 |
| 2. Data validation | 8 days | 1SS+3 | Days 4 to 11 | 0 days | Unchanged |
The delay: Data migration takes 4 days longer.
Data validation starts 3 days after Data migration starts, on day 4, and both run in parallel. If the migration takes 4 days longer, it finishes on day 14, but validation still starts on day 4: only a late start would move it.
Open this example in the free delay simulator to change the durations, the links or the delay and watch the schedule move.