Start to Start Vs Finish to Finish Relations in P6

By: Haseeb Jamal / On: Jul 21, 2017 / Primavera, Project Planning

Start to Start:

In this relation, two activities start together i.e. the start dates of the two activities, linked through Start to Start relation, will be same although the said activities may end on different dates. This relation is denoted by SS.

This relation is used where we need two or more activities to start on same date. This will help us to speed up the project and we will attain more progress in less time. The demerit of this relation is that if we start any two activities in parallel, their resource demand per planning unit will increase, which in most of the cases causes issues.

Assume that we have to excavate earth for foundation in different areas (Z210, Z220 and Z230 and the total excavation is 500m³ i.e. 166.6 m³ per area) in a project. It is calculated that one labor can excavate 1m³ earth in a day. If a contractor has capacity of 50 labors and employs all labors for excavation (all labors in one area until it is completed) then the excavation for one area will require 166.6/50 = 3.33 days. If the activities are linked using Finish to Start then the total duration for 500 m³ excavation will be 4 + 4 + 4 = 12 days. This is shown in the figure below:

But if in the same case, we apply Start to Start Relationships between the two activities of excavation for Z210 and Z220 then we get the following logic:

We can see that the total labors required in both cases are same but in latter case (when Start to Start relation were used), the number of labors required per day (for first 4 days) increased. Now the contractor has capacity of only 50 labors, therefore, the second case will create issues by using over allocated resources although it saved 4 days. In Summary, we can say that SS relationships can be used to save the Project time but the resources should not be over allocated.

Finish to Finish:

Like Start to Start relation, this relation allows the two activities to execute in parallel. In this relation, two activities finish together i.e. the finish dates of the two activities, linked through Finish to Finish relation, will be same although the said activities may start on different dates. This relation is denoted by FF.

This relation is used where we need two or more activities to finish on same date. This will help us to speed up the project and we will attain more progress in less time. The demerit of this relation is that if we execute any two activities in parallel, their resource demand per planning unit will increase, which in most of the cases, causes issues i.e. resources are over allocated.