Critical path and baseline: reading a construction schedule that slips

What the critical path method actually computes

The critical path method (CPM) takes the activities, their durations in working days and their logical links, and runs two passes. The forward pass gives each activity its earliest start and earliest finish. The backward pass, starting from the project finish, gives its latest start and latest finish. The difference is the total float: how many days the activity can slip without moving the project finish. Activities with zero float form the critical path.

Free float is the other number worth knowing: how many days an activity can slip without delaying the next activity. On a job site, free float tells the superintendent how much room a trade has before it gets in the next trade's way.

A small example

The critical path is A, B, D, E, G: 33 working days. Underground plumbing can slip four days before it delays the slab; site services have fifteen days of float. If the steel fabricator announces a five-day delay, the roof finishes five days late. If the plumber is three days late, nothing moves, but the plumbing has only one day of float left and deserves attention.

Getting the logic right

• Link every activity to at least one predecessor and one successor. An activity hanging loose has meaningless float, and the critical path stops being reliable.
• Prefer logic to imposed dates. A “start no earlier than” constraint is sometimes needed (a permit, a delivery), but a schedule full of fixed dates no longer computes anything.
• Use finish-to-start by default, start-to-start with a lag for overlapping trades (drywall can start two days after framing starts on a floor), and keep the reason for each lag.
• Schedule on a working calendar with the real holidays, construction holidays and weather assumptions of the region, or every duration is wrong by the same few percent.
• Keep activities short enough to be measured: as a rule of thumb, no longer than two reporting periods.

Baseline versus current schedule

The baseline is a frozen copy of the approved schedule: dates, durations and, for cost control, the budget spread over time. The current schedule keeps moving as the job progresses. Without a baseline, you only know where the job is going; with it, you know how far it has drifted and since when. It is also the reference for earned value: planned value is read from the baseline, not from the current schedule.

Many project managers keep two baselines: the initial one, never touched, for the record and for claims, and a current one, reset only after an approved change that changes the scope or the contract dates. Resetting the baseline just because the job is late hides the problem; resetting it after an approved extension is correct.

Reading a slipping finish

When the projected finish moves, ask three questions in order.

1. Which activity is driving it? Follow the critical path back from the finish until you find the activity whose dates moved. That is where the delay comes from, not necessarily where it shows up.
2. Is the critical path the same as last month? A new critical path means an activity that had float has used it up. It often points to a procurement problem (late shop drawings, long-lead equipment) rather than to the work on site.
3. Is the delay excusable? An owner's change, a late answer to an RFI or a differing site condition may justify a time extension; your own crew's productivity does not. Document it while the facts are fresh.

Then decide how to recover: resequence (work two floors in parallel), add a crew on the critical activity only, overlap with a start-to-start link, or accept the delay and notify. Crashing a non-critical activity costs money and gains nothing.

Schedule variance in days, not only in dollars

Earned value expresses schedule variance in dollars (SV = EV − PV). Site teams think in days. Report both: the finish variance in working days against the baseline, and SPI for the trend. Near the end of the job, trust the days: SPI drifts back to 1.00 even on a late project. For the weeks ahead, translate the critical path into a three-week look-ahead the foremen can act on.

Applying it in Teyvor

Teyvor's schedule computes the critical path on your working calendar as you link tasks in the Gantt chart, shows each task's float, and keeps two baselines, the initial one and the current one. The overview compares the projected finish with the contract finish and the baseline; when an approved change carries a schedule impact, it is applied only after you confirm it, so the baseline stays a deliberate decision.