Defining the Start-to-Finish [SF] Relationship
By definition, a Start-to-Finish relationship occurs “when the completion of the successor activity depends on the start of the predecessor activity”. The Figure above shows examples of SF relationships.
Occurrence of SF Relationships in a CPM Network
Although SF relationships are uncommon in practice, schedulers may nonetheless encounter them when reviewing CPM schedule networks. When such relationships are identified, resolving them is essential to achieving a clear understanding of the network’s underlying logic. Examining and clarifying SF links reveals the intent embedded within the schedule and ensures the network accurately reflects activity dependencies.
Avoiding Real-World Examples
Because the Forward and Backward Pass Total Float algorithm does not interpret activity names, this discussion intentionally avoids real-world examples. Introducing such examples, or attempting to justify their use, would only add unnecessary complexity to the analysis.
Sample Network Analysis
For simplicity, all activities in the CPM network model shown in Figure 2 are assigned to a 7-day calendar. In the sample model, WBS node “Path 2” shows that Task E is a successor to Task F through an SF relationship. The Critical Path runs through Tasks A, B, C, and D in WBS node “Path 1,” while Tasks G and H, shown in WBS node “Path 3,” each carry a Total Float value of 4 days.
Although Tasks E and F follow one another with respect to their Early Start and Early Finish dates, Task E carries a Total Float of 10 days, while Task F carries 3 days of Total Float. The SF relationship does not affect Task E’s Free Float, which remains 6 days. Because Task H’s Total Float is 4 days, Task E’s Total Float of 10 days reflects the more constraining path through Task F.
Step 1 – Resolve the SF relationship by replacing it with an equivalent Finish-to-Finish [FF] Relationship.
To resolve a SF relationship in a CPM network, first identify where it occurs. In this sample model, the SF relationship exists between Task E and Task F, where Task F is the predecessor and Task E is the successor.
In Figure 2 above, note that Task B is the driving predecessor to Task F through a Finish-to-Start [FS] relationship. Because Task F’s Early Start depends on Task B’s Early Finish, and Task E’s Early Finish is controlled by Task F’s Early Start (due to the SF relationship), it follows that Task E’s Early Finish is indirectly controlled by Task B’s Early Finish. The SF relationship between Task F and Task E can therefore be removed and replaced with an FF relationship between Task B and Task E, as shown in Figure 3 below.
Notice that the Total Float and Free Float values remain unchanged for all activities following this change, confirming that the CPM network stays intact and that both conventions are equivalent.
Where a lag value exists, carry it over to the FF relationship replacement. For example, an SF relationship with a 5-day lag becomes an FF relationship with the same 5-day lag.
Step 2 – Resolve the FF relationship by replacing it with an equivalent Finish-to-Start (FS) Relationship.
The next step is to resolve the Finish-to-Finish (FF) relationship between Task B and Task E. With no lag present, this scenario is classified as “FF between tasks with no lag.” This condition is discussed in detail in the article:
“The Finish-to-Finish Relationship Trap” by Lou Gonzalez. According to this article, because (a) Task B is related to Task E by an FF relationship with no lag, and (b) Task E is a predecessor to Task H by an FS relationship, the FF relationship between Task B and Task E can be safely replaced by establishing Task B as a predecessor to Task H through an FS relationship. The results of this change are shown in Figure 4 below.
Please note that after recalculating the CPM network, the following updates were made to Task E:
- The Early Start was revised from: 14-Apr-26 to 09-Apr-26.
- The Early Finish was revised from: 20-Apr-26 to 16-Apr-26.
- The Total Float increased from: 10 Days to 15 Days.
- The Free Float increased from: 6 Days to 11 Days.
As outlined in the referenced article, because neither Task B nor Task E governs the Early Start of Task G, the appropriate relationship for both with respect to successor Task G is FS with zero lag.
Where a lag exists — for example, where Task B is a predecessor to Task E with an FF lag of 3 days — consider splitting Task E into two tasks (E1 and E2), with E1 lasting 4 days and E2 lasting 3 days. Task E1 then becomes a predecessor to Task E2 through an FS relationship, and Task E2 becomes a predecessor to Task B through an FS relationship. Refer to the article “The Finish-to-Finish Relationship Trap” by Lou Gonzalez for more details on this procedure.
Conclusion
SF relationships are uncommon and generally discouraged in CPM network development, as their use can introduce unnecessary complexity. Nevertheless, they may appear in a CPM network, particularly due to technical considerations or unresolved dependencies between milestones.
SF relationships typically obscure the true logical connection between activities. Applying the step-by-step approach described above helps to clarify the underlying relationship within the CPM network structure.
While resolving SF relationships can sometimes be accomplished with fewer steps, a systematic procedure such as the one outlined above is preferred, as it minimizes the risk of errors when revising CPM logic.
Clearly identifying and documenting any changes made to the schedule logic is essential for transparency and justification. One solution is to run Zümmer’s Comparison Report #10, “Added Deleted Predecessors (By Activity ID),” as shown in Figure 5 below. The report documents the removal of the SF relationship between Task F and Task E, which was replaced by the more appropriate Finish-to-Start (FS) relationship between Task B and Task H.
