Proving an adverse weather extension of time claim

Construction is a risky business

If you are a contractor, weather is one risk you cannot avoid. Before the project starts, you price the job, prepare the programme, make allowances, choose a methodology, organise labour, book plant and agree a completion date.

Then the project starts — and the weather does what weather has always done. It changes the story.

Rain stops earthworks. Wind stops crane lifts. Cold temperatures affect concrete pours. Heat affects productivity. Gusts make cladding, roofing, scaffolding and lifting operations unsafe. None of that is unusual. Every contractor knows weather can affect progress.

The question is not whether weather affects the work. The question is whether the contractor can prove it — properly, in contractual terms.

Why adverse weather EOT claims fail

An adverse weather claim should be one of the easiest Extension of Time claims to win. The event is external, measurable, date-specific and capable of being tested against the wording of the contract.

The problem is that contractors often try to prove it with fragments: a few site diary notes, a few photographs, a screenshot from a weather app, and a programme update that does not clearly identify the affected activities.

That is not a claim. It is a rescue mission — assembled under pressure, after the fact, without the contractual structure that gives it teeth.

Usually, it is not because the weather did not happen. It is because the contractor cannot prove the event properly. The evidence is incomplete, the notice was late, or the analysis does not match the contract's specific entitlement wording.

The four elements every weather EOT claim must address

A proper weather EOT needs to tell four simple things clearly and with evidence:

When those four points are evidenced clearly and coherently, the argument becomes much harder to refuse.

Understanding the contractual threshold

This is where many weather claims fail even when the weather was genuinely disruptive. The contract decides the test — not the weather report, not industry convention, not the contractor's experience of what "bad weather" means on site.

Different standard forms approach adverse weather differently:

• NZS 3910 (commonly used in New Zealand) provides a specific clause structure for adverse physical conditions, including weather. The clause typically references conditions that could not have been reasonably anticipated, tested against historical benchmarks.
• AS 4000 (Australia) provides for delay relief due to weather conditions that a competent contractor could not reasonably have anticipated at the time of tender.
• NEC4 provides for compensation events including weather conditions that occur less frequently than once in ten years — a statistical threshold that requires historical data to substantiate.
• JCT contracts provide for relevant events including exceptionally adverse weather conditions, which again requires a benchmark to establish what is "exceptional."

In every case, the claim must be tested against the specific wording of the contract — not a general argument that the weather was bad. Pasting the actual contract clause into the analysis and re-running the evidence against those specific thresholds is the only way to produce a claim the contract administrator can properly evaluate.

The notice problem — protecting your entitlement

Many weather claims are damaged before the claim is ever written, because the contractual notice was late, weak, or never issued.

The weather may be severe. The delay may be real. The impact may be obvious. But if the contract requires an early warning, a notice of delay, or notification within a stated period, failing to issue that notice on time can put the entire entitlement at risk — regardless of how bad the weather was.

This is not a technical objection that can be argued around. Most standard form contracts are explicit: late notice can extinguish the right to claim, even if every other element of the claim is solid.

All three of these reasons are understandable. None of them protect the contractor's entitlement. The notice that is not issued is the entitlement that is lost.

What a proper early warning notice contains

A contractual early warning notice for weather should typically include:

• Formal notice reference, date, parties and contract details
• The forecast or actual weather conditions giving rise to the notice
• The trade operations anticipated to be affected, and why
• The anticipated impact on the works and programme
• Proposed mitigation measures
• A request for direction from the Employer where appropriate
• A reservation of rights to an Extension of Time and associated cost claims

The route to a strong weather EOT claim

Building a weather EOT claim is not complicated if the evidence is gathered systematically throughout the project. The problem is that most contractors start building the evidence when the claim is needed — not when the weather event occurs.

1. Quantify the weather risk with data at tender stage. Use historical data for the specific project location to price realistic weather allowances by trade and month.
2. Plan around the forecast during the works. Use trade-specific forecasts to understand whether forecast conditions affect the planned operation — before mobilising.
3. Issue early warning notices when the risk appears. Issue formal notices when forecast or actual conditions trigger the contractual obligation — while the project record is live.
4. Record which activities were affected and why. Maintain contemporaneous records linking weather events to specific operations and planned programme activities.
5. Benchmark the actual weather against historical data. Test the measured conditions against the relevant historical benchmark at the specific project location.
6. Test the results against the contract clause. Re-run the analysis using the actual contract wording to identify qualifying events that align to the specific entitlement thresholds.
7. Show the effect on the programme. Demonstrate how the qualifying weather days affected the critical path to completion and produce the formatted claim document.

That is why adverse weather should be one of the easiest EOT claims to win — not because bad weather automatically gives entitlement (it does not), but because the evidence exists, the event is measurable, the dates are known, the thresholds can be tested, and the delay can be explained clearly.

How Construction Weather supports this process

Construction Weather is built specifically to support each stage of this process:

At tender stage

The Tender Planning tool calculates the average number of weather-affected days per trade operation, per month, using 10 years of historical data at the specific project location. Weather risk is never generic — rain in January does not affect a project the same way as rain in July, and wind does not affect internal fit-out the same way it affects craneage, roofing or scaffolding.

During the works

The 14-day trade forecast and trade advisories help the team understand what the forecast means for the actual planned operation. A normal weather app tells you it will rain. Construction Weather helps answer the more important question: does that rain, wind, heat or cold stop the operation?

The platform grades risk by trade operation using Green, Amber and Red advisories, with standards-referenced thresholds and mitigation guidance. This moves the discussion away from opinion and towards evidence. Not: "I think it was too windy." But: "The forecast wind and gust conditions exceeded the advisory threshold for this operation on this date, and the works could not safely or reasonably proceed."

When the contractual notice is required

Construction Weather generates a formal Early Warning Notice in Word format, using the project details, forecast conditions, affected trade operations, anticipated impact, mitigation measures and reservation of rights. The barrier to doing the right thing is removed. The notice is drafted. The Employer is informed. The risk is recorded. The claim story starts at the right time.

When building the EOT evidence

The EOT Evidence Generator benchmarks the project month against 5 or 10 years of historical data and identifies the qualifying weather days. Rainfall, wind, gusts, temperature and snowfall can be tested, categorised and turned into a formatted claim document, with exports available for supporting evidence.

Most importantly, the contractor can supply the actual weather clauses from the contract, so the analysis is structured around the specific entitlement wording — not generic statistics. The result is a draft document ready for the project team to review and supplement with programme evidence.

Weather may be unpredictable. Proving a delay does not have to be.

The route to a successful adverse weather EOT claim is straightforward when the evidence is gathered systematically: quantify the risk, plan around the forecast, notify when the risk appears, record the affected activities, benchmark the actual weather, test it against the contract, and show the effect on the programme.

Construction Weather helps turn weather from a site complaint into a properly evidenced contract story. The evidence is there. The dates are known. The thresholds can be tested. The delay can be explained clearly.

Weather may be unpredictable. Proving a weather delay does not have to be.