Start-to-Start (SS) Logic

Start-to-Start logic is appropriate when two activities must begin before either can finish — for example, a drainage trench that needs to start being backfilled as each section is excavated, or a design review that can begin while the final sections of the design document are still being produced. SS relationships allow planners to model productive overlapping without creating artificial dependencies that add unnecessary duration to the programme. When combined with a lag, SS can represent a situation where the successor must wait a defined period after the predecessor starts — for example, 'piling can start 2 weeks after mobilisation begins'.

SS relationships are powerful but require care in analysis. They affect the forward pass calculation differently from FS relationships: the early start of the successor is tied to the early start of the predecessor, not its finish. This means that extending the duration of the predecessor does not necessarily delay the successor's start — which can make the schedule more resilient in some scenarios but can also mask logical dependencies that should be driving dates. A schedule with heavy use of SS logic can be harder to analyse manually and is more prone to producing unexpected critical path results.

A common misuse of SS relationships is applying them where a FS relationship with a negative lag (lead) would be more transparent. Using SS + lag is generally cleaner than FS with a negative lag, because it makes the overlapping relationship explicit rather than hiding it in the lag value. In practice, DCMA 14 flags negative lags as non-compliant (leads), so SS + lag is the preferred modelling approach for overlapping work. Always document the rationale for any SS relationship — reviewers will ask, and a well-explained SS relationship is far more defensible than one that appears to have been applied to make a milestone date work.