Concurrency & Competing Causes
Untangling Responsibility in Construction Delay
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April 13, 2026
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Concurrent delay remains one of the most controversial and misunderstood concepts in construction delay analysis. While practitioners accept that concurrency affects entitlement to time and money, consensus is lacking on what constitutes concurrent delay, how to identify it, and how to treat its consequences.
- What constitutes concurrent delay?
- How should concurrency affect entitlement to extensions of time and compensation?
The Concept of Concurrent Delay
Concurrent delays occur when two or more delay events occur simultaneously, each caused by different parties, the Contractor, the Employer or external factors.
If an Employer’s risk event (excusable delay) overlaps with a Contractor’s risk event (non-excusable delay), both may extend the project timeline, though one may have greater impact.
The central question is not simply whether events overlap in time, but whether each constitutes an effective cause of delay to completion.
SCL Protocol Definition
The SCL Protocol defines ‘true concurrent delay’ as “the occurrence of two or more delay events at the same time, one an Employer Risk Event, the other a Contractor Risk Event, and the effects of which are felt at the same time.”1
However, true concurrent delays are rare. Accordingly, the SCL Protocol provides a flexible definition where events need not overlap in time “but the effects of them are felt at the same time.”2
In both cases, concurrent delays require critical delay, which means both events must independently contribute to the same delay in completion and “an effective cause of Delay to Completion (not merely incidental to the Delay to Completion).”3 Accordingly, both delays must both affect the critical path.
The key wording in the SCL Protocol definition is the ‘effective cause of the Delay to Completion’. If an Employer Risk Event occurs after a Contractor Risk Event but runs parallel without causing the same period of critical work delay, it is not concurrent. “Concurrent delay only arises where the Employer Risk Event is shown to have caused Delay to Completion or, in other words, caused critical delay (i.e. it is on the longest path) to completion.” 4
If one delay has substantially greater impact, it is the dominant cause; the other, even if overlapping, may not be deemed concurrent.
AACE Definition
The AACE Recommended Practice No. 10S-90 proposes five definitions of concurrent delays, which are similar to the latter definition of the SCL Protocol.5 For concurrency, delays must:
- Overlap during the same time period, but they do not necessarily have to happen at the same precise moment.
- Affect the project completion date.
AACE provides a flexible, realistic approach by recognizing delays need not precisely match in timing.
Legal Practitioners’ Views
John Marrin KC advocates a flexible approach defining concurrency as “a period of project overrun which is caused by two or more effective causes of delay which are of approximately equal causative potency.”6
Approximately equal causative potency recognises that only causes of comparable significance should be treated as concurrent; minor causes may properly be disregarded.
According to Marrin, concurrent delays require three key criteria:
- Both Employer and Contractor risk event are effective causes of delay to completion;
- Each contributes independently to the same delay to completion; and
- Events need not occur simultaneously.
Conversely, HHJ Richard Seymour KC provides a narrower definition of concurrent delays, emphasising on the timing and the effect of the delays on the project’s completion.7 In Royal Brompton Hospital NHS Trust v Hammond, HHJ Seymour distinguished between:
- Work already delayed when a further relevant event occurs but makes no difference to completion; and
- A situation where two events occur in parallel, either independently causing delay.
Only the latter constitutes true concurrency. This aligns with the “first-in-time” principle, whereby the delay event that occurs first in time is generally considered the cause of the delay, even if other events occur later that overlap in time.
While Seymour’s approach has been challenged in some jurisdictions, it has been supported in others.
These differing perspectives illustrate why concurrency remains contentious: the disagreement extends beyond terminology to fundamentally different understandings of causation.
How Concurrent Delay Affects Entitlement to Extension of Time
The Malmaison Approach
The Malmaison approach, established through UK case law, holds that when concurrent delays occur, the contractor is entitled to an EOT for the excusable delay period.8 The employer cannot claim liquidated damages during this period despite contractor-caused delays.9
The SCL Protocol’s is directly influenced by the Malmaison approach. It states that “where concurrent delay has been established, the Contractor should be entitled to an EOT… The Contractor Delay should not reduce the amount of EOT due to the Contractor as a result of the Employer Delay.”10
Most UK case law applies these principles when resolving concurrent delay disputes. This approach is grounded in the prevention principle: an employer should not benefit from its own act of prevention.
The Apportionment Approach
Some jurisdictions, particularly civil law systems or other common law traditions, favor apportionment, dividing responsibility according to relative causative significance.
In John Doyle, Lord MacLean emphasized that, when there are concurrent delays in a construction project, “responsibility for the loss can be apportioned between the two causes, according to their relative significance.”11
This approach analyzes each delay event's impact on completion and allocates responsibility accordingly. The contractor may receive an EOT or compensation for the employer-caused period, even if overlapping with contractor delay.
Liquidated damages may be adjusted based on each party’s proportional delay contribution. For employer-caused portions, liquidated damages might not apply.
Unlike Malmaison, where the Contractor is fully relieved of the employer-caused delay, apportionment could conflict with the prevention principle if the contractor does not receive a full EOT.
Concurrent Delays – Effect on Entitlement to Compensation for Prolongation
An EOT for excusable delays extends the contractual completion time, relieving the contractor from liquidated damages. Time-related costs (overhead, labor, site costs) are typically allocated to the party responsible for the delay.
Both SCL Protocol and AACE recognise that where excusable delay is concurrent with Contractor-caused delay, the Contractor generally cannot recover prolongation costs. Concurrency grants time, but not money.12
An exception arises where the Contractor demonstrates identifiable additional costs caused solely by the Employer’s delay and would not have been incurred but for that event.13
The Malmaison approach emphasizes the principle that a contractor is not entitled to claim compensation for prolongation costs incurred during the extended period. Contractors must provide adequate evidence that the excusable delay, for which the EOT is granted, directly caused financial losses.
Under the apportionment approach from John Doyle, time-related costs are apportioned like delays. Lord Drummond Young considered that “the causative significance of each of the sources of delay and the degree of culpability in respect of each of those sources, must be balanced.”14 Accordingly, prolongation costs are apportioned based on the (i) causative significance of each of the sources of delay and (ii) the degree of culpability in respect of each of those sources.
A main criticism of the apportionment is that it can undermine the prevention principle. If the contractor does not receive a full EOT, it may still be penalized despite employer contribution to the delay. This is in contradiction with the principle that the employer should bear the responsibility for their own actions that prevent the timely completion of the project. It is for that reason that several jurisdictions have suggested that the apportionment approach should be rejected.
Conclusion
The concepts of concurrent delays in construction projects are inherently complex due to differing views, often leading to disagreements and challenges in determining entitlement to an EOT and compensation for prolongation. Primary factors influencing decision-making are the contractual terms, local laws and case-specific facts.
Given the diversity of legal approaches, parties should address concurrency expressly in their contracts. Clear definitions and agreed consequences can significantly reduce disputes.
Footnotes:
1: Society Of Construction Law Delay And Disruption Protocol, 2nd edition (February 2017), at para. §10.
2: Ibid., It is worth noting that in the 2002 edition of the SCL Protocol, this definition was slightly different. This edition did not require both delay events to contribute independently to the same delay in completion: “the effects of them are felt (in whole or in part) at the same time”. This definition has evolved in the 2017 edition, which provides a more nuanced definition. This demonstrates the controversy on this subject, even within the same document.
3: Supra 1, para. §10.
4: Ibid.
5: AACE International Recommended practice No. 29r-03, page 102.
6: John Marrin QC, “Concurrent Delay Revisited” (February 2013) at page 2.
7: Royal Brompton Hospital NHS Trust v. Hammond (No. 6) [2000] EWHC Technology.
8: Henry Boot Construction (UK) Limited v Malmaison (Manchester) Ltd (1999) 70 Con LR 32, at para. §13.
9: Ibid., at para. §18.
10: Supra 1, at para. §10.
11: John Doyle Construction Ltd (in Liquidation) v Erith Contractors Ltd [2021] EWCA Civ 1452, at para. §16.
12: Supra 1, at para. §12, Supra 5, at page 99.
13: Supra 1, at para. §14.
14: City Inn Limited v Shepherd Construction Limited [2007] CSOH 190, note 4, para. §167.
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Published
April 13, 2026
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