Critical path method for construction: a worked example
The critical path is the sequence of linked activities that determines the earliest finish in a simple unconstrained schedule. Identify it from dependencies and durations, not from whichever activity looks most important.
Start with the assumptions
The example below uses finish-to-start dependencies, one working calendar, no lag, sufficient resources and no imposed date constraints. Durations are illustrative working days. Real schedules need their own calendars, constraints and resource review; the arithmetic below is not a promised construction programme.
Build a small dependency network
B and C can run in parallel after A. D cannot start until both are complete. This is a dependency example, not an instruction to begin site work without the necessary approvals or safe work arrangements.
| Activity | Duration | Predecessor |
|---|---|---|
| A: approve detail | 2 days | None |
| B: procure fixture | 5 days | A |
| C: prepare installation area | 3 days | A |
| D: install fixture | 2 days | B and C |
| E: inspect installation | 1 day | D |
Calculate the two paths
Path A → B → D → E takes 2 + 5 + 2 + 1 = 10 working days. Path A → C → D → E takes 2 + 3 + 2 + 1 = 8 working days. Under the stated assumptions, the first path controls the finish. C has two days of total float: it can move within that allowance without changing the calculated finish. B has no such allowance in this example.
Test a delay rather than colouring a bar
If B increases from five to seven days, the controlling path becomes twelve days. If C increases from three to four days instead, the finish remains ten days because C still finishes before B. If C increases to six days, its path becomes eleven days and controls the finish. This is why the critical path should be recalculated after changes rather than treated as a fixed label.
Check the model against the site
- Confirm that each dependency describes a real requirement.
- Use the actual working calendar, including access restrictions.
- Check that crews and equipment are not assumed to be in two places at once.
- Retain the baseline and record actual progress at a stated date.
- Explain constraints and review near-critical paths, not just the longest one.
- Have the planner review recovery options; shortening a bar does not make the work feasible.
Turn the schedule into a decision list
For each near-term controlling activity, record the owner, required input and decision due date. If a delivery is at risk, request a confirmed forecast and assess the effect before changing the opening commitment. The blank dependency register below helps collect inputs; it is not a scheduling calculator. For deeper review, consult the GAO guide and the responsible project planner.
Common questions
- What is the critical path in construction?
- The critical path is the sequence of linked activities that determines the earliest finish in a simple unconstrained schedule. It is found from dependencies and durations, not from which activity looks most important.
- What is total float?
- Total float is how far an activity can move without changing the calculated finish. In the worked example, preparing the installation area has two days of float while procuring the fixture has none.
- Does the critical path change during a project?
- Yes. When durations change the controlling path can move, so the critical path should be recalculated after changes rather than treated as a fixed label.
Activity dependency register
A blank CSV you can open in Excel or Google Sheets. Adapt the fields to your project. No signup required.
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