Home Pitched Roofing Reducing moisture risk in pitched roofs

Reducing moisture risk in pitched roofs

by Jennie Ward

Stuart Nicholson, Roof Systems Director at Marley, looks at the recent update to BS 5250 and outlines advice for roofing contractors when it comes to reducing moisture risk on pitched roof projects.

Stuart Nicholson, Marley.

The launch of a fully revised British Standard, BS 5250, has highlighted the importance of reducing moisture risk and ensuring correct ventilation levels in pitched roofs. Previously focussed just on controlling condensation, the updated Standard now takes a whole building approach and has been broadened to include other moisture risks in buildings, such as rain penetration and roof leaks, humidity and high levels of ground water. It also recognises that there are differences between a building when it is designed and how it performs when it is built and in-use.

The revised BS 5250:2021 Standard came into effect on 31st July 2021 and has been re-named ‘Management of Moisture in Buildings – Code of Practice’. It reflects the growing understanding about managing moisture and the fact that some risks are increasing due to climate change and improved energy efficiency in buildings, with increased airtightness and insulation.

Excess moisture in a building can cause many problems, not only to the fabric of the building in terms of damp and timber decay, but also to the health of the occupants. In fact, a recent report commissioned by the Government highlighted the risk that poor ventilation plays in the spread of infections, such as Covid-19.

Pitched roofs are at particular risk of excess moisture because the natural movement of air in a building means moisture laden air, in the form of vapour, is likely to ascend to the roof space (unless steps are taken to make the ceiling, and any penetrations in it, as sealed and airtight as possible by following the guidance in BS 9250 ‘Code of Practice for design of the airtightness of ceilings in pitched roofs’). Even then, it is almost impossible to create a totally air-tight envelope, so contractors should install ventilation to the roof void, and the batten space in the case of an air impermeable roof covering, even if they are using breathable underlays.

We have been warning about the risks of interstitial condensation caused by inadequate roof ventilation for many years now. It is one of the main causes of roofing faults and call-backs for repairs. The updates to BS 5250 are a reminder of the critical role that roof ventilation plays as part of a whole building approach to reducing moisture risk.

Roofing contractors should familiarise themselves with the new guidance, which doesn’t introduce any major changes to pitched roofing but does give more clarity on some key issues. This includes clarifications about the use of breathable (LR) and non-breathable (HR) underlays, with recommendations and figures which illustrate the different ventilation requirements for specific roof types and roof coverings (air permeable and air impermeable). It also gives new guidance on the calculation methods for assessing moisture risk in roofs with a table for ‘as designed in-theory’ (ADT) and ’as built in-service’ (ABIS) conditions for various pitched, flat and composite roof types.

So, what should contractors do to minimise moisture risk in pitched roofs?

  1. Follow the new BS 5250 guidance at whatever stage you become involved in the roof design and installation. Liaise with other specification and construction stakeholders to ensure the roof design and proposed selection of materials satisfy current Building Regulations and the recommendations of BS 5250.
  2. Consider the impact of any roofing work on the rest of the building fabric in terms of moisture risk assessment, see table 4 in BS 5250:2021.
  3. If you are working on a refurbishment project, check the different requirements – e.g., change of use of building, structural changes.
  4. Also ensure you install the roof covering and its fittings and accessories in line with the latest BS 5534 ‘Slating and tiling for pitched roofs and vertical cladding-Code of practice’, and the BS 8612 ‘Dry-fixed, ridge, hip, and verge systems for slating and tiling – Specification’ Standards, which will ensure the roof is secure against increasingly volatile weather and prevent water ingress.
  5. As a minimum, get a new fixing specification for every project and ensure it is followed. Marley can also provide full roof system and NBS specification, which will ensure correct ventilation levels via their approved roof underlays and ventilation products.
  6. Don’t rely on a breathable (LR) underlay as the sole means of ventilation. BS 5250 continues to recommend ventilation of the loft space and sometimes the batten space, and so our recommendation is that roofs will always require some form of supplementary low, high or both, levels of ventilation, regardless of what underlay is used.
  7. Consult the roof covering manufacturer for their recommended package of materials. Consider using a full roof system from one manufacturer to ensure compatibility of roof components for an efficient, well-ventilated roof.
  8. For new and existing buildings, increased loft insulation can produce a condensation problem. Take steps to minimise thermal bridging by using eaves ventilation, a well fitted rafter roll to stop any insulation blocking the ventilation, and underlay support trays to maintain clear ventilation routes.

Find out more about Marley at www.marley.co.uk

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