Finish-to-Finish (FF) Logic

Finish-to-Finish logic ties the completion of one activity to the completion of another. A common example is commissioning and testing: the testing activity cannot finish until all the installation it tests has finished, even if testing started early while installation was underway. FF relationships are also used for contractual milestone dependencies — a delivery milestone that cannot be signed off until all the supporting documentation is complete, regardless of when the documentation work started. On construction schedules, FF is sometimes used to model the relationship between fitting out and snagging: snagging cannot finish until the fitting-out it inspects has finished.

FF relationships interact with SS relationships on the same activity pair to create what schedulers call 'SS-FF hammock' logic — a pair of relationships that capture both the start dependency and the end dependency of two overlapping activities. This is the most rigorous way to model partially overlapping work and is preferable to using a single FS relationship with an inflated lag, which hides the true overlap. The SS-FF approach gives the schedule more analytical transparency and produces more reliable float calculations.

In practice, FF relationships are less common than FS and SS, and some planners avoid them because they make the schedule harder to read quickly. The cost of that simplicity is accuracy: replacing an FF relationship with a FS + lag is an approximation that will produce incorrect results wherever the predecessor duration varies. If the predecessor activity overruns, the FS + lag model will not correctly capture the knock-on effect on the successor finish. Always use the relationship type that genuinely represents the dependency, even if it makes the Gantt look more complex.