Too hot to build? The growing challenge of heatwaves in construction contracts

July 2026 was the hottest month recorded in the UK so far this year and the second warmest July since records began (and we are currently in the midst of the 5th heatwave this summer!). 

There is no fixed UK maximum temperature at which construction work must stop. In the absence of this and as temperatures continue to rise, the construction industry faces increasing challenges relating to productivity, site safety, equipment performance and material quality. These issues can also give rise to significant legal, commercial and operational risks, some of which we have set out below. 

Extension of Time claims

For projects affected by extreme heat, contractors may seek an Extension of Time (EOT) on the basis that, for example, it’s too hot for their employees and/or sub-contractors to work or undertake certain aspects of a project, causing delay. However, entitlement will depend on the contractual terms.

• JCT contracts

Under standard form JCT contracts, delay caused by "exceptionally adverse weather conditions" may constitute a Relevant Event, entitling a contractor to an extension of time. However, the term is not defined, and no temperature threshold is prescribed, making heat-related claims difficult to establish. 

•  NEC contracts

NEC contracts take a more objective approach. Weather-related compensation events may arise where specified contractual thresholds are exceeded, typically by comparing actual weather data against a 10-year average. Depending on the Contract Data, contractors may be entitled to both an Extension of Time and additional costs.

As heatwaves become more common in the UK, it may become increasingly difficult to characterise such events as exceptional. Parties should therefore carefully consider how heat-related risks are allocated under the relevant contract. 

Making a successful claim

Meeting a contractual weather threshold is unlikely to justify relief by itself. Contractors will also need to demonstrate compliance with notice requirements, establish a clear causal link between the heat event and any delay, quantify the impact, and show that reasonable mitigation measures were taken.

Causation is often the most challenging aspect. Unlike flooding or heavy rainfall, the effects of heat are less visible and can be difficult to evidence. Robust record-keeping is therefore essential. Site temperature data, weather reports, photographs, progress records and evidence of mitigation measures will all assist in substantiating a claim and reducing evidential disputes.

Contractors can even support their claims using weather reports that the JCT have created in collaboration with the Met Office. These include a Weather Planning Report, which provides one-in-ten-year and long-term average weather data for project planning, and a Downtime Report, which measures actual weather conditions against those benchmarks during the project to help substantiate Extension of Time claims.

Future-proofing contracts

Many construction contracts still rely on historic weather patterns when allocating risk. As extreme heat events become more frequent, those assumptions may no longer be appropriate.

Parties should consider bespoke contractual provisions that:

• Define temperature thresholds that trigger entitlement to relief
• Specify notice and evidential requirements
• Set out expected mitigation measures
• Address the cost consequences of heat-related disruption

Consideration should also be given to obligations concerning design life, climate resilience and ground conditions, which may be increasingly affected by rising temperatures.

Looking ahead

Extreme heat is becoming a foreseeable project risk rather than an exceptional event. While standard form contracts provide some relief mechanisms, they offer limited protection and can be difficult to rely on in practice. Clear risk allocation, careful contract drafting and comprehensive project records will be key to managing heat-related disruption and reducing the potential for disputes.

For any queries or legal advice on construction projects, please contact Ross Galbraith on 0191 211 7999 or [email protected] or Abi Chorlton Wilson on 0191 211 7786 or [email protected].