Construction Scheduling. The problem with Start to Start Relationships. Or, Why doesn’t a Delay to this Activity Duration push my Finish Date?

person-question-300x300Do you regularly use Start-to-Start SS relationships? Perhaps you add a lag to drive the start of the successor out a few days?

Have you had problems with this practice? Perhaps a delay to the completion of the predecessor activity is actualized, but does not impact the schedule?

This is a common practice, unfortunately. The best practice is to add an additional successor with a Finish-to-Start FS relationship, or even a Finish-to-Finish FF relationship. This will provide the logic that allows the delay to the completion of the activity to drive other work. Without that additional successor activity, what you have is an “open end” and the activity finish does drive any successive work. It could never complete and you would not see the problem in the logic.

This condition is simple to prevent. Just make it a practice to always add a FS or FF successor when using a SS relationship. The problem usually happens when you’re trying to recover time and start to schedule work concurrently. You simply forget to add the additional successor….

Personally, I like to develop my schedules with all FS relationships unless the plan actually calls for a SS condition. If I have the need to use SS relationships, I assume my work isn’t broken down into enough detail to allow me to sequence the work logically.

There are always “exceptions to the rule”, but this is one I try to follow.

What other methods have you found to successfully utilize SS relationships, with or without lags?

I’d love to hear what you think!

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Paul Epperson CCM, PMP, PSP, PMI-SP