CPM Is the Industry Standard for Construction Project Scheduling
Critical Path Method (CPM) scheduling is the primary tool used to plan and manage construction project timelines. It identifies which sequence of activities controls the project completion date - the "critical path" - and where there is flexibility in the schedule. The ARE PjM division tests both the conceptual understanding of CPM and the ability to read and interpret simple CPM diagrams.
CPM was developed in the late 1950s by DuPont and is now standard practice on virtually all significant construction projects. Understanding CPM helps architects manage design schedules, communicate with contractors and owners about project timelines, and evaluate schedule impacts when changes occur.
Core CPM Concepts
Activity
An activity is a discrete task with a defined duration and resource requirement. Examples: "Submit SD documents for owner review" (duration: 2 weeks), "Excavation and grading" (duration: 3 weeks). Activities are connected by dependencies - which tasks must be complete before others can begin.
Network Diagram
CPM schedules are displayed as network diagrams (precedence diagrams) showing activities as nodes and dependencies as arrows. The diagram maps the sequence of all project activities from start to finish. The longest path through the diagram from start to finish is the critical path.
Critical Path
The critical path is the longest sequence of dependent activities from project start to finish. Any delay to an activity on the critical path directly delays the project completion date. There is no flexibility (float) on the critical path. The project duration equals the sum of durations on the critical path.
Float (Slack)
Float is the amount of time an activity can be delayed without delaying the project. Activities not on the critical path have float. Total float is the amount an activity can be delayed without delaying project completion. Free float is the amount an activity can be delayed without delaying any successor activity. Activities with zero float are on the critical path.
Simple CPM Example
| Activity | Duration | Predecessors | Float |
|---|---|---|---|
| A: Site prep | 2 weeks | None | 0 |
| B: Foundation | 4 weeks | A | 0 |
| C: Steel erection | 6 weeks | B | 0 |
| D: MEP rough-in | 3 weeks | C | 2 |
| E: Exterior skin | 5 weeks | C | 0 |
| F: Interior finishes | 4 weeks | D, E | 0 |
Critical path: A → B → C → E → F = 2+4+6+5+4 = 21 weeks. Activity D has 2 weeks of float (can be delayed up to 2 weeks without affecting completion).
Fast-Track Scheduling
Fast-track scheduling overlaps design and construction phases that would normally be sequential. For example, the structural foundation package might be put out to bid while the architect is still completing design of the upper floors. This compresses the overall project timeline but increases risk: if design changes affect already-bid packages, change orders result. Fast-tracking is common in CMr and Design-Build delivery methods.
Key Exam Points
- Critical path = longest path; no float; any delay affects completion.
- Float = schedule flexibility on non-critical activities.
- Total float: delay before affecting project completion. Free float: delay before affecting a successor.
- Fast-tracking compresses schedule by overlapping design and construction - increases change order risk.
- CPM requires a complete list of activities with durations and dependencies.
Study PjM on AREprep
AREprep’s PjM flashcards cover every concept on this exam with spaced repetition, and the practice exams mirror the real question formats so the actual test feels familiar.