Why the contract form matters
Contractors frequently work across projects procured under different standard forms — an NEC4 ECC for a public infrastructure contract in one jurisdiction, an NZS 3910 for a government building project in New Zealand, an AS 4000 for a private-sector development in Australia, and a JCT Design and Build for a UK developer client. Each form takes a materially different approach to what counts as weather that entitles the contractor to relief.
The entitlement test differs. The benchmark standard differs. The notice timing differs. The consequence of late notice differs. And the quantum of relief — time only, or time and money — differs.
Understanding these differences matters at three stages: at tender, when pricing the weather risk allowance; during the works, when deciding whether to issue a notice; and when building the EOT claim, when the analysis must be structured around the specific clause rather than a generic weather narrative.
Quick comparison at a glance
| Dimension | NZS 3910 | AS 4000 | NEC4 ECC | JCT DB 2024 |
|---|---|---|---|---|
| Entitlement trigger | Weather conditions that could not reasonably have been anticipated by an experienced contractor | Weather conditions that a competent contractor could not reasonably have anticipated at the time of tender | Weather measurements that, at the project location, occur less than once in ten years (90th percentile) | Exceptionally adverse weather conditions |
| Benchmark standard | Historical norms at the specific project location — implied but not explicitly prescribed | Historical norms at the specific project location — similar implied standard to NZS 3910 | Explicitly prescribed: 10-year return period (90th percentile) based on the weather measurements listed in the contract data | No prescribed statistical threshold — "exceptional" assessed against prevailing norms for the location and time of year |
| Notice timing | As soon as practicable after the event becomes apparent; specific notice obligations attach under clause 10.3 | As soon as practicable; written notice required within a reasonable time | Early warning as soon as contractor becomes aware; notification of compensation event within 8 weeks of becoming aware | Written application to the Employer's Agent as soon as reasonably practicable and within 2 months of it becoming apparent |
| Consequence of late notice | May reduce or extinguish entitlement at Engineer's discretion; generally not a complete bar | May reduce entitlement; contract is generally not a complete bar but late notice weakens the claim | If notification is more than 8 weeks late, the contractor is not entitled to the change in the prices, key date or completion date | Entitlement to extension of time can be assessed at Employer's Agent's discretion, but late notice is a risk factor |
| Relief available | Extension of time only; additional cost generally not recoverable as a weather EOT event | Extension of time; additional cost may be recoverable if the weather event meets the test and cost provisions apply | Extension of time and additional cost (compensation event); the contractor is paid the forecast cost impact | Extension of time only; exceptionally adverse weather is a relevant event, not a relevant matter (no associated loss and expense) |
| Hardest threshold to meet | NEC4 — the 10-year return period is mathematically precise and requires historical data. It excludes all weather that is merely unusual or inconvenient. | |||
NZS 3910 — New Zealand Standard
NZS 3910:2013 Adverse weather relief
Primary clauses: 10.3 (Delays), 10.3.1 (Extension of Time), Schedule 1 (Special Conditions of Contract)
The entitlement test
NZS 3910 provides for an extension of time where the contractor has been delayed by weather conditions that could not reasonably have been anticipated by an experienced contractor at the time of tender. The test is objective: what would an experienced contractor have anticipated, given the location, the time of year and publicly available historical data?
This is not a test of what the specific contractor anticipated. It is a test of what a reasonable, experienced contractor would have anticipated. A contractor who did not check historical rainfall data for a coastal site in a wet month cannot escape the test by arguing they did not foresee the rain.
Historical weather norms at the specific project location. The contract does not prescribe a specific return period, but the implied standard is the range of conditions a contractor would ordinarily allow for in their programme and weather risk allowance.
The contractor must notify the Engineer as soon as practicable after the delay becomes apparent. Clause 10.3 requires a written notice of delay identifying the cause, the affected activities and the anticipated impact on the completion date.
Extension of time only. The contractor recovers time but not additional cost as a direct consequence of the weather event. Cost recovery may be possible in other circumstances (e.g. Engineer's instruction) but not as a direct weather EOT entitlement.
Late notice does not automatically bar the claim, but the Engineer has discretion to take account of the timing. A contractor who notifies months after the event, with no contemporaneous records, is in a significantly weaker position than one who notified promptly.
Practical implications
The absence of a prescribed statistical threshold makes NZS 3910 claims inherently more subjective than NEC4. The contractor must argue, and often demonstrate with historical data, that the conditions fell outside the range of what was reasonably anticipatable. In practice this means assembling historical data for the site location and showing that the measured conditions were materially worse than the historical norm for that month and location.
The Engineer (who administers the contract) has significant discretion. Well-presented, contemporaneous evidence — a weather log matched to historical data for the same month across multiple years — is far more persuasive than a retrospective narrative.
AS 4000 — Australian Standard
AS 4000–1997 Adverse weather relief
Primary clauses: clause 34 (Extensions of Time), clause 35 (Latent Conditions — not weather specific)
The entitlement test
AS 4000 provides for an extension of time where the contractor has been delayed by weather conditions that a competent contractor could not reasonably have anticipated at the time of tender. The standard is closely analogous to NZS 3910 — it is an objective test based on what a competent contractor in the market would have foreseen, not a purely statistical threshold.
Unusually severe conditions, events outside the historical norm for the location and time of year, or conditions that materially exceed what was allowed in the contract's weather risk pricing — all of these are candidates. But the burden of proof sits with the contractor.
Implied historical norms for the specific project location. Australian standard form practice has developed a body of adjudication and case law supporting the use of Bureau of Meteorology historical data to establish what a competent contractor would have anticipated.
Written notice as soon as practicable. AS 4000 clause 34 requires the contractor to give notice of any cause of delay and the anticipated impact. The notice obligation is a genuine contractual condition, not a mere administrative formality.
Extension of time. Unlike NEC4, the baseline AS 4000 does not make weather an automatic cost recovery event. Additional cost may be recoverable in specific circumstances (e.g. where the weather event also constitutes a Principal's risk event under the specific contract), but this is not the default position.
Not a complete bar in most cases, but late notice significantly weakens the claim. Where a contractor fails to notify within a reasonable time, the Superintendent has limited basis to assess the impact, and contemporaneous evidence is thin or absent.
Practical implications
The AS 4000 entitlement test is substantively similar to NZS 3910 — both use a "could not reasonably have anticipated" standard without prescribing a statistical threshold. In practice, this means the quality of the contractor's evidence is the determining factor: historical data, contemporaneous records, a clear link between the weather event and the affected programme activities, and a notice issued at the right time.
The subjectivity of the test is both an opportunity and a risk. An experienced Superintendent will be familiar with regional weather patterns. A contractor who argues that ordinary summer afternoon thunderstorms in Queensland were unforeseeable will not get far. A contractor who can show that a sustained wet spell fell materially outside the 20-year average for that month at that location stands on much firmer ground.
NEC4 ECC — the most precise threshold
NEC4 ECC Weather compensation events
Primary clauses: clause 60.1(13) (Compensation Events), clause 61 (Notifying Compensation Events), Contract Data Part 1 (weather measurement thresholds)
The entitlement test
NEC4 is the most mathematically precise of the four forms. A compensation event for weather arises when the weather measurements recorded at a named weather measurement point (specified in the Contract Data) are less likely than once in ten years — that is, they fall at or beyond the 90th percentile of the historical distribution for that weather variable, in that month, at that location.
The contract specifies the weather variables to be measured (for example, the cumulative monthly rainfall, the number of days with rainfall exceeding a stated threshold, the monthly average wind speed or the number of air frost days), the measurement point, and the source of historical data. The contractor does not get to choose the variables or the dataset after the fact — these are fixed in the Contract Data at execution.
Explicitly prescribed: 90th percentile of the historical distribution for the named weather variable at the named measurement point. Ten years of weather measurements or a longer period if specified in the Contract Data. This is the clearest and most objective threshold of any of the four forms.
Two-stage. First, an early warning notice as soon as the contractor becomes aware of any matter that could affect cost, completion or meeting a key date. Second, formal notification of the compensation event within eight weeks of becoming aware of it. The eight-week deadline is hard.
Extension of time and additional cost. A weather compensation event under clause 60.1(13) entitles the contractor to an assessed change in the prices (cost recovery) and a change in the completion date (time relief). This is the most complete form of relief of any of the four standard forms.
The hardest bar of any of the four forms. If the contractor notifies more than eight weeks after becoming aware of the compensation event, the contractor is not entitled to the change in the prices, the key date or the completion date. The entitlement is lost — not reduced, not at the Project Manager's discretion. Lost.
The 90th percentile in practice
The 90th percentile threshold means that only the top 10% of weather outcomes for that variable and month — historically — will qualify. In a dataset with 10 years of monthly data, only the highest single value qualifies. In a dataset with 30 years of data, only the top three values qualify.
This is a genuinely high bar. Wet months that are merely worse than average do not qualify. Only genuinely exceptional months — the worst in a decade — qualify. This is not a flaw in the system; it is the intended design. NEC4's compensation event mechanism is intended to price weather risk into the contract at tender, with the contractor bearing ordinary weather risk and the client bearing truly exceptional weather risk.
Practical implications
The precision of NEC4's threshold is its greatest strength and its greatest risk. Contractors know exactly what they are claiming against. Project Managers know exactly how to evaluate it. The dispute is not about whether the weather was bad; it is about whether the measured data crosses the threshold.
This means the data quality is everything. The contractor needs the right dataset, from the right measurement point, for the right variables, covering the right period. A site weather station that was not specified in the Contract Data cannot substitute for the nominated measurement point. Historical data from the wrong location cannot substitute for data from the named point.
It also means the notice timing is everything. Eight weeks sounds generous. On a busy site, eight weeks passes quickly — and in practice, contractors often realise they have missed the window when they are assembling the EOT claim months later.
JCT — the most subjective threshold
JCT DB 2024 Exceptionally adverse weather
Primary clauses: clause 2.26 (Relevant Events), clause 2.28 (Extension of Time)
The entitlement test
JCT contracts provide for an extension of time where the works are delayed by exceptionally adverse weather conditions. The word "exceptionally" carries significant weight — normal adverse weather, wet conditions, cold spells and seasonal wind do not qualify. Only conditions that are genuinely exceptional for the location and time of year are relevant events.
JCT offers no statistical threshold, no prescribed benchmark and no prescribed dataset. The assessment is qualitative and fact-specific. Whether weather conditions are "exceptionally adverse" is ultimately a question of degree, informed by expert evidence, historical data and the specific circumstances of the works.
No prescribed statistical threshold. The test is qualitative — conditions must be exceptional in the context of the location and time of year. In practice, contractors and Employer's Agents often refer to historical Met Office data to establish what is "normal," with exceptional conditions being those that materially exceed the norm.
Written application to the Employer's Agent (or Contract Administrator under JCT SBC) as soon as reasonably practicable after it becomes apparent that the completion date is likely to be delayed, and in any event within two months of it becoming apparent.
Extension of time only. Exceptionally adverse weather is a Relevant Event under JCT but is not a Relevant Matter — it does not entitle the contractor to loss and expense. The contractor recovers time but no additional cost arising from the weather event itself.
JCT requires notice within two months of the delay becoming apparent. While the Employer's Agent retains some discretion to grant extensions on review, late notice is a material risk factor and can undermine the strength of the entitlement.
Practical implications
The absence of a statistical threshold makes JCT weather claims inherently more contested. The word "exceptionally" has to do a lot of work, and both sides will typically engage experts to argue what is and is not exceptional for the relevant location and time of year.
In practice, Met Office historical data plays a similar role under JCT as formal threshold testing plays under NEC4 — it provides the empirical baseline against which the contractor's claim is assessed. A contractor who can show that measured rainfall in a given month was, say, 180% of the 30-year average for that month at that location is in a far better position than one who relies on site diaries and photographs alone.
The two-month notice window under JCT is worth noting. Unlike NEC4's eight-week hard bar, JCT's notice provisions do not automatically extinguish entitlement, but the risk of late notice is real — particularly on long contracts where weather delay events accumulate and the programme impact may not be obvious until significant time has passed.
Notice requirements side by side
Across all four standard forms, the single most common cause of lost or reduced entitlement is late or absent notice. The timing and consequences differ, but the underlying principle is consistent: the contract administrator needs to be informed promptly so that the impact can be assessed, mitigation can be discussed, and the project record can capture the event in real time.
| Form | Early warning | Formal notification deadline | Consequence of late notice |
|---|---|---|---|
| NZS 3910 | As soon as practicable after the event becomes apparent | As soon as practicable — no hard deadline prescribed | Discretionary reduction by Engineer; not a complete bar |
| AS 4000 | As soon as practicable | As soon as practicable — no hard deadline prescribed | Weakens claim; not a complete bar but Superintendent's assessment is constrained |
| NEC4 ECC | Clause 15 early warning: as soon as contractor becomes aware | 8 weeks from becoming aware of the compensation event (clause 61.3) | Entitlement to time, cost and key date changes is lost — not reduced. A hard bar. |
| JCT DB 2024 | As soon as reasonably practicable after delay becomes apparent | Within 2 months of delay becoming apparent (clause 2.28.3) | Risk of reduced or refused extension of time; not an automatic complete bar |
Time only versus time and cost
One of the most practically significant differences between the four forms is whether a weather event entitles the contractor to cost recovery as well as time.
- NEC4 is the only standard form that makes weather a cost recovery event. A qualifying compensation event under clause 60.1(13) entitles the contractor to a change in the prices — the actual or forecast additional cost of the weather impact is assessed and added to the contract price. This makes the financial stakes of a NEC4 weather claim substantially higher than under the other forms.
- NZS 3910, AS 4000 and JCT provide time only. The contractor recovers the extension of time — protecting against delay damages — but the cost of weather disruption (additional labour, plant standing time, rework, acceleration) is generally borne by the contractor as part of the weather risk they assumed at tender.
This difference has significant implications for tender pricing. Under NEC4, the contractor can price a lower weather risk contingency in the tender, knowing that truly exceptional weather will be a compensation event. Under the other forms, the contractor must price all weather risk into the tender because there is no cost recovery mechanism for weather events, however severe.
What evidence does each form require
The evidence required to substantiate a claim follows the shape of the entitlement test under each form.
Under NZS 3910 and AS 4000, the contractor needs: measured weather data for the project location covering the claimed period; historical data for the same location and period of year to establish what a competent contractor would have anticipated; a contemporaneous record linking the weather event to the affected programme activities; and a notice issued at the time.
Under NEC4, the contractor needs: the measured weather data for the specified measurement point; the historical data from the specified source covering the specified period; a calculation showing that the measured value exceeded the 90th percentile for that variable and month; a programme showing the affected activities; and a notification issued within eight weeks of becoming aware of the event.
Under JCT, the contractor needs: measured weather data to establish that conditions were exceptional; a comparison against historical norms for the location and time of year to support the "exceptional" argument; a programme showing the impact on the critical path; and a written application issued within two months of the delay becoming apparent.
In all four cases, the quality and contemporaneity of the evidence is the deciding factor. A claim assembled months after the fact, from incomplete records and a retrospective weather report, is a weaker claim than one supported by notices issued in real time, site diaries that identify affected activities by name, and historical data benchmarked at the project location.
Choosing the right form — or working with what you have
Most contractors do not choose the standard form — the principal or the client's procurement team makes that choice. But understanding the implications of the form you are working under matters at every stage.
At tender, it affects how much weather risk allowance to build into the programme and price. Under NEC4, truly exceptional weather is a compensation event — the contractor can price a lower contingency. Under JCT, NZS 3910 and AS 4000, all weather risk sits with the contractor, so the allowance should be more conservative.
During the works, it affects when to issue notices. Under NEC4, the eight-week window is a hard deadline — the site team needs to be tracking potential compensation events in real time, not assembling the claim at completion. Under JCT, the two-month window gives slightly more time but still requires prompt action.
When building the claim, it determines the structure of the analysis. A NEC4 claim is a threshold exercise — did the measured data cross the 90th percentile? A JCT claim is a qualitative argument — were the conditions exceptional? NZS 3910 and AS 4000 sit between these, using historical data to establish the reasonable anticipation benchmark.
The clause is the claim
Weather EOT claims fail not because the weather was not genuinely disruptive, but because the contractor cannot show that the specific conditions met the specific test in the specific contract. Understanding the form you are working under — before the rain falls, not after — is the foundation of every successful weather delay claim.
The contract decides the test. The evidence must answer the contract's question. And the notice must arrive before the deadline, not after it.