Three Common Scheduling Mistakes Beginners Make in Primavera P6 and How to Avoid Them

In theory, all four exist for a reason. In practice, however, Start-to-Finish relationships are rarely appropriate and should usually be avoided.

Why Start-to-Finish Is a Problem

Consider a simple example:

If you assign a Start-to-Finish relationship such that Site Preparation starts and Excavation finishes at that same moment, the logic is effectively saying:

“The moment site preparation begins, excavation must already be complete.”

In real construction scenarios, this is almost impossible. It creates a situation where a successor finishes before its predecessor has meaningfully progressed. While beginners often encounter this logic while “experimenting” with relationships, it usually introduces confusion rather than clarity.

If you ever come across a Start-to-Finish link in your schedule, whether by accident or inheritance, don’t hesitate to question it. More often than not, there is a better alternative.

What to Use Instead

Most construction activities work best with Finish-to-Start relationships. For example, finish excavation, then begin pipe installation; that logic is intuitive and easy to track.

That said, there are realistic cases where activities overlap. On large projects, site preparation can last several months, and excavation may begin before preparation is fully complete. In such cases, a Start-to-Start relationship may be reasonable.

Even then, caution is needed. Activities may not start on the same day, which is where lag or lead comes into play. A small lag (for example, +5 days) might reflect mobilization time. A small lead (negative lag) might allow partial overlap.

Still, these adjustments should be used sparingly. Many clients and organizations explicitly restrict lag and lead usage because excessive offsets can obscure the logic of the schedule.

As a scheduler, part of your responsibility is to explain the implications of these choices to the project team:

The second mistake usually appears during schedule updates, not initial planning.

Constraints, such as Start On or After or Must Start On, can be tempting, especially for users transitioning from Microsoft Project. They seem like a quick fix when dates don’t align with expectations.

A Common Scenario

Let’s say:

At this point, many schedulers immediately apply a Start On or After constraint of November 20. Technically, it works, and the activity moves. But something subtle (and risky) happens in the background.

You’ve just broken the logic flow.

By forcing the date, you override the natural relationship between this activity and its predecessor. Even if upstream activities change, this one remains fixed. Over time, constraints like this can:

The final issue relates to milestones, which are often misunderstood or underutilized.

Milestones represent points of achievement, not work durations. They should clearly indicate when a phase starts or finishes, such as Commence Engineering, Engineering Complete, or Foundation Complete.

Common Errors with Milestones

A frequent mistake is linking milestones without first establishing proper activity logic. When that happens, milestones lose their meaning and simply inherit arbitrary dates.

Instead, milestones should be linked after the schedule logic is complete.

How to Do It Properly