Programme float for construction projects

The concept of float (or 'slack') is used to describe the amount of time that an event or activity can be delayed without delaying the overall completion of the works. Float is calculated by subtracting the time necessary to perform a task from the time available to perform it.

Only tasks which are not on the critical path can have float. The critical path is determined by all the events for which the total float is zero.

Identifying float can be helpful allocating resources, where priority can be given to activities with less available float.

There are two important types of floats: total and free. The total float, Hij for activity (i,j), is the difference between the maximum time available to carry out the activity (Lj-Ei) and its duration (Dij); it represents the maximum amount of time the start date for the activity can be delayed, in relation to the early start without delaying the completion of the whole project:

Hij = Lj - Ei - Dij = Iij - Ei = Lj - Tij

In terms of the free float, it is assumed that all activities start as early as possible. In this case the free float, Fij for activity (i,j), is the excess of time available (Ej-Ei) over its duration (Dij); it represents the delay allowed for an activity without holding up the early start date for the initiation of another activity. An activity which has a positive total float may also have free float, but this can never be greater than that shown below:

Fij = Ej - Ei - Dij

NB The term ‘activity float’ refers to ‘the duration contingency directly related to a single activity built into the planned duration of that activity. Activity float is established simply by dictating an activity duration that is greater than the actual time needed to complete that activity.’ Ref Society of Construction Law Delay and Disruption Protocol, 2nd edition, February 2017, published by the Society of Construction Law (UK). https://www.scl.org.uk/resources/delay-disruption-protocol

Float is generally considered to be positive rather than negative (unless stated otherwise).

Negative total float refers to the time by which the duration of an activity or path has to be reduced in order to permit a limiting imposed date to be achieved. Negative float only occurs when an activity on the critical path is behind programme. It is a programming concept, the manifestation of which is delay. Ref The Society of Construction Law Delay and Disruption Protocol, 2nd edition, published in February 2017 by the Society of Construction Law (UK).

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