Preconstruction — Scheduling & Phasing

CPM Schedule

A logic-linked schedule that calculates which chain of activities controls the finish date.

Quick Answer

A CPM schedule uses the critical path method to model a project as a network of activities linked by logical dependencies, each with a duration. The method calculates the longest path of dependent activities, which determines the shortest possible project duration. Activities on that path are critical: any delay to them delays the whole project.

The Full Picture

The critical path method exists because a project isn't a simple list of tasks but a web of dependencies, and you can't tell which tasks matter most just by looking. CPM turns the project into a network so the schedule itself calculates which sequence of work actually controls the completion date, rather than leaving it to intuition.

Mechanically, each activity gets a duration and is linked to others by relationships (usually finish-to-start). A forward pass through the network computes the earliest each activity can start and finish; a backward pass computes the latest each can occur without delaying the project. The difference is float. The chain of activities with zero float is the critical path, and its length is the shortest time the project can be built.

In practice, the CPM schedule tells a project team where to focus. Effort spent accelerating a non-critical activity buys nothing; effort on a critical activity shortens the whole job. As work progresses and actuals are entered, the critical path can shift, so teams re-run and review it regularly rather than treating the first calculation as fixed.

For preconstruction, a CPM schedule built during planning tests whether the intended duration is realistic and exposes the sequences that will drive the field. Quality matters: schedules stuffed with hard constraints, missing logic, or open-ended activities produce a misleading critical path. Assessments like the DCMA 14-point checks exist precisely to catch those defects.

Real Examples

→Forward and backward pass: The scheduler runs the network to find early and late dates for every activity, and the zero-float chain (excavation to foundations to steel to deck) emerges as the critical path.
→Focus of acceleration: The team ignores a request to speed up interior painting (which has three weeks of float) and instead compresses steel erection, the activity actually on the critical path.
→Shifting critical path: A concrete delay consumes the float on the structural chain, and the critical path shifts to the MEP rough-in sequence, which now controls the finish date.

Common Misconceptions

People assume: The critical path is the list of the most important or expensive activities.

Actually: It's the longest chain of dependent activities by duration, the one that controls the finish date. A cheap, short task can be critical, and a huge, costly one can have float and not be critical at all.

People assume: A CPM schedule's dates are fixed once it's built.

Actually: CPM is a live calculation. As durations, logic, and actual progress change, the schedule is re-run and the critical path can move. A CPM schedule that's never updated stops reflecting the real project quickly.

Does MeltPlan Solve This?

Not directly

Not directly. Building a CPM network, linking logic, and calculating the critical path is exactly the kind of scheduling work MeltPlan doesn't do. MeltPlan focuses on preconstruction document and estimating tasks like drawing review, takeoff, and bid leveling. For CPM scheduling you'll want a dedicated tool such as Primavera P6, Microsoft Project, or Asta Powerproject.

Frequently Asked Questions

How does the critical path method work?

Activities with durations are linked by logical dependencies into a network. A forward pass finds early start and finish dates; a backward pass finds late dates. The difference is float, and the chain of activities with zero float is the critical path, which sets the shortest project duration.

What software is used to build a CPM schedule?

Primavera P6, Microsoft Project, and Asta Powerproject are common. They store the activity network, run the forward and backward passes automatically, and recalculate the critical path as durations, logic, and actual progress change.

What makes a good CPM schedule?

Complete logic (every activity has predecessors and successors), minimal hard constraints, realistic durations, and regular updates. Frameworks like the DCMA 14-point assessment check for exactly these quality issues, such as excessive lags, constraints, and missing logic.

What's the difference between a CPM schedule and a bar chart?

A bar chart (Gantt) shows activities on a timeline but may not encode dependencies. A CPM schedule links activities by logic and calculates the critical path, so it shows not just when work happens but which delays would push the finish date.

Related Terms

Critical PathThe chain of dependent activities whose total length determines the project finish date.Float (Schedule)The slack an activity has before its delay starts to push other work or the finish date.Baseline ScheduleThe approved, frozen version of the schedule that progress is measured against for the life of the job.Master ScheduleThe top-level, whole-project timeline of major phases and milestones from design to closeout.Schedule CompressionShortening a project's duration without reducing scope, by crashing or fast-tracking the work.Preconstruction ScheduleThe timeline for all the design, bidding, and procurement work that has to finish before construction can start.

More Preconstruction — Scheduling & Phasing Terms

Sources

  1. Project Management Institute (PMI) — Practice Standard for Scheduling
  2. Defense Contract Management Agency (DCMA) — 14-Point Schedule Assessment
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