Adverse weather during construction

Where there is a delay to a construction project which impacts on the completion date, and that delay was not caused by the contractor, it may constitute a relevant event (or compensation event), for which the contractor may be entitled to an extension of time and in some cases loss and expense.

Some contracts list exceptionally adverse weather conditions as one such event. Even if it is not specifically mentioned however, it may still qualify as a neutral matter (not caused by the employer or the contractor) that adversely affects the completion date.

The question then is what constitutes exceptionally adverse weather, rather than general adverse weather that the contractor should have allowed for in their price. Unhelpfully, contracts do not always define the term (for example JCT contracts), leaving it to the discretion of the contract administrator, and so opening up a potential source of dispute.

Where it is defined, it tends to be weather that is exceptionally adverse for that time and location. NEC contacts state that this is where the weather over a calendar month has occurred on average less frequently than once in ten years. This means that a short period of poor weather is unlikely to qualify, and that if poor weather crosses two calendar months it may not qualify even if it has lasted for a considerable time. The location and types of weather to be measured should be set out in the Contract Data.

Generally, information should be collected from a weather station close to the site (from which historical records are available), or on site and compared with met office data. This may include information such as; the amount and duration of rainfall, air temperature and duration, volume and duration of lying snow, wind speed and so on, depending on the nature of the works.

It is likely that this approach would also be accepted on projects using forms of contract in which there is no definition.

Even where exceptionally adverse weather is defined in the contract and can be shown to have occurred, it must still be proven that this was the cause of a delay and on some forms of contract, that the contractor has used their best endeavours to mitigate any delay. Again, this is a potential source of dispute, and so good record keeping is necessary, and the correct procedures of notification must be followed.

[edit] The 1-in-10-year test

NEC3 and NEC4 contracts make weather a compensation event under clause 60.1(13) where a weather measurement, recorded within a calendar month at the place stated in the Contract Data, is shown by the weather data to occur on average less frequently than once in ten years. The standard measurements listed in Contract Data part one are cumulative rainfall, the number of days with rainfall of more than 5 mm, the number of days with a minimum air temperature below 0 °C, and the number of days with snow lying at 09:00 GMT. Further measurements may be added. Only the difference between the recorded measurement and the 1-in-10-year value is taken into account in assessing the compensation event (clause 60.3 in NEC4), so weather up to that line remains the contractor's risk.

The 1-in-10-year value is a return period, not a ten-year average. For a given place, calendar month and measurement, it is the level exceeded in roughly one year in ten across a long record, normally the 30-year climate normal of 1991 to 2020 used by the Met Office and the World Meteorological Organization. With 30 years of record it is approximately the 90th percentile (the level beaten in three of the thirty years), or the 10th percentile for minimum temperature. It is usually well above the long-term average: a month 20 or 30 per cent wetter than average is often still below its 1-in-10-year value. An assessment based only on the most recent ten years does not meet the wording of the clause, which refers to an average frequency and so requires more than one ten-year period.

Because the value depends on the place, the baseline years and the method, a figure used in a claim should state all of them, together with the month's recorded measurement and the difference between the two. Values derived from a named weather station can differ from values derived for the surrounding area from gridded data such as the Met Office HadUK-Grid dataset; where the Contract Data names a station, the station record is the one the contract tests.

JCT contracts do not use a numerical test. Exceptionally adverse weather conditions are a relevant event, and whether a month was exceptional for that place and time of year is for the contract administrator to judge, commonly by reference to the long-term average and the 1-in-10-year value for that location.

References

NEC Users' Group, Frequently asked questions: weather data records. https://www.neccontract.com/support/faqs/weather-data-records

Met Office, HadUK-Grid gridded climate observations. https://www.metoffice.gov.uk/research/climate/maps-and-data/data/haduk-grid/haduk-grid

Met Office, Downtime Report for weather delays in construction. https://www.metoffice.gov.uk/services/business-industry/construction/downtime-report-for-extension-of-time

Rained Off, The 1-in-10-year weather value, explained, including monthly datasets of UK 1 km squares against their 1-in-10-year rainfall values. https://rainedoffcheck.co.uk/1-in-10-year-weather-value

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