BS 5250:2021 – Management of Moisture in Buildings. Code of Practice

The end of July saw more than the start of the summer holidays, it also heralded a full revision of BS 5250:2016.  The new document, BS 5250: 2021 recognised that buildings are under increasing strain from moisture for two main reasons: climate change and desire for energy conservation.

Just in this last year, there have been numerous occurrences of heavy rain (causing flooding across the UK), storms, and even snow in late March in parts the country – during a time where daffodils are usually expected to bloom!  This means that the resistance to water ingress at the roof level will needs to match the increased pressures associated with more extreme and persistent weather.

The revised standard addresses the issue of additional moisture-related issues, such as high humidity levels, roof leakages, and rain incursion.  It covers all states of water, as liquid, gas, and solid as well as the interactions between these states of water in the building.  The standard refers to all buildings, whether new or existing, domestic and non-domestic, from construction to occupancy.

As well as being restructured for more cohesion and clarity, the revised BS 5250:2021 standard considers the combined impact of individual methods of moisture control, rather than discussing them in isolation.

In 2017, BSI released a whitepaper: ‘Moisture in Buildings: an integrated approach to risk assessment and guidance’.  This whitepaper influenced the latest iteration of BS 5250 in that both technical documents recognise the difference between the expected performance of a building in the design stage compared to the actual performance in the completed build.  This difference could be attributed to faults made during construction, excess moisture generated during the building’s normal use, or residual moisture from the development process.

Key Takeaways Related to Roofing from BS 5250

BS 5250:2021 explains how underlays can help with roofing ventilation.
  • Focus on the junctions between building parts (thermal bridges), which includes the eaves, and are at a higher risk of condensation due to the presence of cold surfaces.
  • More of an emphasis on roofing components and the problems of moisture, as well as specific and clear guidance for cold pitched roofs.
    • This includes describing the role of air permeable roofing membranes and clear guidance when using low resistance underlays, as well as how they can help provide suitable ventilation in the roof space.
    • Rooflights and ensuring good quality workmanship in the detailing around these features is also included.
  • Details calculation methods for assessing the risk of moisture in the roof for pitched and flat roofing.
  • Stresses the importance of ensuring air tightness in cold roof voids to prevent the ingress of moisture-laden air.

Despite these expansions and additions, a vast majority of the guidance from the previous version, BS 5250:2011+A1:2016, is still applicable to building developers and designers.

Hambleside Danelaw’s Roofing Solutions

Danelaw manufacture a wide range of pitched roofing ancillaries, many for roof ventilation, and can answer your questions regarding our products and how they can help comply with the recommendations of BS 5250.  To explore our range of products take a look at our website, and please do get in touch with us at