Many people have run away with the idea that delay caused by a client forgives delay caused by a contractor. It doesn't
As you will be aware, the causes of delays to projects are often difficult to disentangle. Separate and distinct events can appear to coincide, making it difficult to establish whether either or both are causing delay. This type of situation, commonly referred to as "concurrency", poses particular problems for those who assess extension of time entitlements.

The Society of Construction Law in its delay protocol published this year explains that "concurrency is a contentious issue both because there are differing views on the correct approach to it … and because there are differences about the meaning of concurrency itself". The result has been disagreement across the industry as to how properly to deal with concurrency when assessing how much extra time to give a contractor.

Some apparently regard the decision in Henry Boot vs Malmaison Hotel as the equivalent of a get out of jail free card when it comes to such situations. This view appears to be based on the comment in Mr Justice Dyson's decision that "if there are two concurrent causes of delay, one of which is a relevant event [that is, a legitimate cause of delay], and the other is not, then the contractor is entitled to an extension of time for the period of delay caused by the relevant event notwithstanding the concurrent effect of the other event".

On the basis of this comment, it is often argued that any and every relevant event that occurs during the course of a project automatically serves to relieve contractors of liability for delays that have been caused by their own performance. In a contractual situation, these arguments are misguided, as they ignore one crucial aspect – the fundamental need to demonstrate cause and effect as a matter of fact and not as a matter of theory.

It is important to be clear as to what is meant by the phrase "concurrent causes", as used in Malmaison. The phrase does not refer to situations in which two unconnected events occur at the same time. It only refers to situations where two or more events – a relevant event and a contractor default event – both delay the progress of the works at the same time as a matter of fact.

Establishing this in anything other than a simple situation requires a detailed investigation of the factual circumstances surrounding the delay. Concurrency of effect will only exist where this investigation concludes that it is genuinely impossible to separate the effect on progress of the works caused by each of the competing events. Both events can then fairly be said to have equal responsibility and as a result the contractor is entitled to an extension of time for the relevant event, regardless of its own default.

However, in order for an extension of time to be granted, it is necessary for both events, on the facts, to be equally responsible for a delay. This is different to the situation where two events, although each potentially able to cause delay if occurring at different times, when occurring simultaneously operate so that one event (for example, plant breakdown or lack of labour) creates a degree of float within which the other event takes place without itself causing delay. A detailed factual enquiry will establish whether it is only one of the two events that was actually the cause of the delay, notwithstanding the apparent concurrency.

This is made clear in Royal Brompton Hospital vs Hammond & Others. The decision here refers to an example of a relevant event that, because it occurs when work was being delayed by the contractor's own difficulties, "in fact … made no difference" to progress and "simply has no effect upon the completion date".

In another example, the judge described how the contractor's own delay effectively created float that swallowed up any impact of the employer's late handover of part of the site.

In this situation, any concurrency of effect was more apparent than real. On the facts, the relevant event of late handover of part of the site simply did not cause a delay, so there was no entitlement to an extension of time.

When judges refer to "concurrent causes" and "events operating concurrently", they are referring to events in respect of which cause and effect has already been demonstrated – if necessary, by detailed factual investigation and analysis.