Critical path

The critical path method (CPM) builds the programme as a network of activities linked by dependencies. Each activity has a duration, and each link says what must happen before something else can start, such as formwork before the slab pour, or the slab pour before the next level's columns.

The scheduling software then works forward through the network to find the earliest each activity can start and finish, and backward from the finish date to find the latest each can start and finish without delaying completion. Activities where the earliest and latest dates are the same have no spare time. Linked together, they form the critical path.

On a typical commercial building, the critical path often runs through the structure, then facade or roof to get the building watertight, then services and fit-out on the last levels, then commissioning and handover.

Float is the amount of time an activity can slip without affecting something else.

• Total float. How long an activity can be delayed without delaying project completion. Critical activities have zero total float.

• Free float. How long an activity can be delayed without delaying the next activity that depends on it.

• Negative float. The programme cannot meet a required date as it stands. Time must be recovered or an extension sought.

• Near-critical paths. Sequences with only a little float. A small delay can make them critical, so they need watching too.

Who owns float is a common dispute. Some contracts say float belongs to the project, some to the contractor, and many say nothing. Check the programming clause and special conditions.

Most extension of time clauses only allow extra time when a qualifying delay actually delays the date for practical completion. In practice, that usually means showing the delay hit the critical path, or used up all the float on a path so it became critical.

That is why a properly linked, regularly updated programme matters. When a delay happens, the planner inserts it into the current programme and shows its effect on the completion date. Without a credible critical path, a delay claim is hard to prove and easy for the superintendent to reject.

The critical path also changes as the job progresses. Work finishes early or late, logic changes and new scope arrives. A critical path from the baseline programme may not be the critical path six months later.

• Unlinked activities. Activities with no predecessor or successor break the network, and the software cannot calculate a true critical path.

• Hard-coded dates. Constraints and fixed dates hide the real logic and can make the wrong activities look critical.

• Not updating the programme. An out-of-date critical path is weak evidence in an EOT claim.

• Ignoring near-critical work. Paths with a few days of float can overtake the critical path with one late delivery.