What is BS 5250, and how does it relate to pitched roof design?

BS 5250 – Management and Moisture in Buildings (Revised: JULY 2021)

Introduction to Standard
The launch of the fully revised British Standard, BS 5250, aims to address all sources of moisture within buildings and the associated impacts to the building fabric and occupants’ health by preventing them from becoming breeding grounds for health hazards such as mould spores and spreading infections.
BS 5250:2021 replaces BS 5250:2011 + A1 2016 and gives new guidance on preventing moisture risk in buildings, including pitched roofs.

While former editions of the Standard focussed mainly on condensation, the new guidance has been broadened to include other moisture problems such as excessive humidity, rising damp, rain penetration and roof leaks. It also recognises that, in both new and existing buildings, the gap between the design on paper and the way it performs when it is built and in use may vary significantly, particularly due to modern building methods and climate change.

The revised Standard represents best practice in terms of assessing, controlling, and preventing the risk of moisture in buildings, giving both design and practical recommendations.

Why has the Standard been completely revised?
Moisture risk in buildings has changed considerably over the past few years as new methods of construction have been developed along with the changing functions of building type and the lifestyles of the occupants. This means that apart from dealing with an analysis of the various separate elements, the interactions between them or the effects on the whole building as a system must now be considered.

The latest revision to BS 5250 was also stimulated by the publication of a BSI White Paper ‘Moisture in buildings: an integrated approach to risk assessment and guidance’ in 2017, which acknowledges that the assessment of moisture risk in buildings requires a different approach than that previously adopted. It is now based on the knowledge that in both new and existing buildings, the gap between the design on paper and that of ‘as built’ may vary significantly. Consequently, there are few buildings without evidence of ‘in service’ effects of residual moisture from construction, moisture generated in normal use, and more seriously, moisture caused by building faults.

Additional changes to the revised BS 5250
When it comes to managing moisture in pitched roofs, much of the guidance from the previous BS 5250:2016 still applies. However, there are some changes in the revised Standard, including:

  • Clearer advice on the use of high resistance and low resistance underlays, as well as air and vapour control layers, when used with air permeable and air impermeable roof coverings.
  • New guidance on the calculation methods for assessing moisture risk in roofs in the form of a table relevant for ‘as designed in theory’ (ADT) and ‘as-built in service’ (ABS) conditions for various pitched, flat, and composite roof types.
  • Designers are alerted to minimise the risk of surface condensation on the ceiling of any occupied spaces by ensuring that thermal insulation is continuous and that thermal bridging at roof / wall junctions and around openings is minimised.
  • Design considerations should now take account of internal finishes and ceilings, particularly the importance of maintaining air tightness to prevent the transfer of most air into colder roof voids.
  • More focus on the importance of sealing openings and minimising thermal bridging at the eaves.

What are the sources of moisture?
Moisture can be generated internally by building occupants or enter the building from external sources. It can also be present within the construction if the building has not been allowed to dry sufficiently.

BS 5250 states that the designer should take account of the following moisture sources in buildings:

  • Water incorporated during the construction process (including precipitation)
  • Precipitation after construction
  • Water vapour arising from the occupants and their activities
  • Temporary condensation occurring when cold weather conditions are followed by warm, humid weather


The building fabric should provide a weathertight envelope to minimise or prevent the ingress of moisture from external sources, and BS 5534 deals with the construction of pitched roofs. The 2018 amendment to BS 5534 and the introduction of BS 8612 have sought to improve the product quality and standard of installation of dry fix roofing systems.

Internally, moisture is generated by building occupants and their activities. Perspiration, respiration, cooking, cleaning and washing can all generate significant quantities of moisture that needs to be removed from the building, usually by ventilation.

BS 5250 acknowledges that occupants rarely use a building in the manner intended by the designer. It recommends adopting “fail-safe” solutions to account for the likely difference between theory and reality.

In more detail - How will it affect the design of pitched roofs?
The natural movement of air within a building means that there is a permanent risk of moisture laden air in the form of vapour likely to ascend to the roof space unless steps are taken to make the ceiling and any penetrations in it as sealed and air- tight as possible. Even then, it is almost impossible to create a totally air-tight envelope, and so designers and builders should provide minimum levels of ventilation to the roof void, in combination with a vapour permeable underlay and air open roof covering to ensure there is minimal risk of condensation within the roof build up.

In terms of recommendations for the management of moisture in pitched roofs, much of the guidance from the last version of BS 5250 in 2016 still applies, with the recommendations for the various types of cold and warm pitched roof configurations using tiles and slates with high water vapour resistant (HR) or low water vapour resistant (LR) underlays being maintained. Further options have been added with an improved clarity to illustrate the airflow within roof voids and batten spaces. Proprietary site-assembled sheet metal roofs, pre-formed (composite) insulated roof panels, fully supported metal, structural insulated roof panels (SIPS), and hybrid pitched roofs with flat roof systems, are also included.

The Standard provides new guidance on the calculation methods for assessing moisture risk in roofs in the form of a Table relevant for ‘as designed in-theory’ (ADT) and ’as built in-service’ (ABIS) conditions for various pitched, flat and composite roof types.
Designers are alerted to minimise the risk of surface condensation on the ceiling of any occupied spaces by ensuring that thermal insulation is continuous and that thermal bridging at roof/wall junctions and around openings are minimised.
Design considerations now take account of internal finishes and ceilings, particularly the importance of maintaining air tightness to prevent the transfer of moist air into colder roof voids.

Recommendations for installing sealed ceilings and openings in ceilings to prevent moist air entering roof voids are highlighted, along with the importance of sealing openings in roofs (such as rooflights) and the risks of condensation occurring within a refurbished roof, where both changes of use and changes to the structure, must be considered.
For new and existing buildings, insulation levels can be high, with the added risk of poor workmanship leading to air leakage through the ceilings and higher risk of condensation in colder loft spaces. The recommendations are therefore to minimise thermal bridging, particularly at the eaves, improve airtightness and maintain ventilation routes at the eaves.
‘As built in service’ conditions place importance on the level of workmanship within the loft space (insulation, storage platforms), ceiling airtightness and their integrity throughout the lifespan of the building. This connective and systemic approach to the continuity of the internal thermal insulation and uncontrolled air leakage into cold roof voids is paramount in minimising the risk of interstitial condensation.

Recommended minimum free area of openings for loft space ventilation is provided in a simple Table which specifies the requirements for eaves and high- level ventilation expressed as free area in square millimetres per linear metre of eaves or high-level vents, dependent on roof pitch. The ventilation requirements for complex roof shapes, such as hipped or pyramidical roofs, can be calculated using a formula based on empirical research.

Warm pitched roofs and hybrid roofs are also considered, where the use of an air and vapour control layer and the ventilation of small cold roof voids is described for the various combinations of roof build up using HR and LR underlays and air permeable and air impermeable roof coverings.
Modelling software programmes are also referenced and available to help assess condensation risk, and are particularly useful for complex building designs.

From us at Marley

Stuart Nicholson, Roof Systems director: “It is important all roofing specifications are reviewed to make sure they are in line with the new guidance and calculation methods. If in doubt, specifiers can contact our technical team who can provide advice and full roof system specifications to ensure correct ventilation levels and reduce moisture risk.”

If you want to discuss BS 5250 on your next roofing project, contact your local Marley specialist or our Technical Team and they will help guide you every step of the way.

Read the full in depth review of BS 5250

Everything you need to know about roofing to standard