Products > Roof

RIBA Core Curriculum
- Design, construction and technology
- Legal, regulatory and statutory compliance

Knowledge level
- General Awareness

Condensation, caused by water vapour coming in to contact with cold surfaces which then forms water droplets, can be harmful to buildings and cause issues such as mould and mildew growth. This course examines how to control and avoid excessive condensation from building up within a roof structure by providing passive airflow from the outside of a building and by limiting airflow from within the living spaces.

In this section
- Building Regulation requirements
- British Standard 5250: 2011
- Traditional Scottish practices

            
                    
Navigate through each step of the CPD using the left and right arrows to review the content. At the end of each section are some questions, these are required if you wish to obtain a certificate upon completing the course.                     
                                     
            
    

Building Regulations

Building Regulation Part C sets out what must be achieved; i.e. roofs should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial Condensation’.  It then goes on to say that one way of meeting its requirements is by following the guidance given in BS 5250.

Clause 6.2 c
‘Roofs should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial Condensation’

Clause 6.10
‘A roof will meet the requirement if it is designed and constructed in accordance with BS 5250’

BS 5250 is one way of complying with the requirements of Building regulation Part C.

There is some very good advice in this Standard; for example, remember that building occupants may not always use the building on the way it was intended, so err on the side of caution and provide robust solutions.

Example use case: A family with several young children may generate more condensation than a single person and may push the system beyond its limits, particularly in winter, if they are at home all day and want to keep the house very warm, with no draughts – so they keep the windows closed – and are hanging clothes on radiators to dry etc.

 

As with Building Regulation ‘C’, BS 5250 recognises that temporary condensation may occur during adverse climatic and internal conditions (e.g. when it’s very cold outside with little or no air movement, warm indoors with no windows open).

Obviously, any condensation cannot be severe enough to cause damp or staining on internal surfaces and must disappear before damage to the structure can occur

BS 5250: 2011 key summary

  • Minimise the amount of warm moist air entering the construction to reduce condensation risk
    • Moisture should be prevented from entering the roof by removing it at source and/or restricting the transfer of internal air into the roof.  Guidance on how to design and install well ceilings and air and vapour control layers is provided in BS 9250
  • Condensation in a cold pitched roof should be removed by ventilation to the outside air, assisted by wind action
    • How to ventilate the roof space will be cover later in this course

BS 5250: 2011 key summary

  • The amount of ventilation in a cold roof can be reduced if a vapour-permeable underlay is used
    • Guidance on how to design and install well ceilings and air and vapour control layers is provided in BS 9250
  • Ventilation of a warm roof is not required if an AVCL is provided at ceiling
  • Most vapour -permeable underlays are airtight, but some are air permeable. BS 5250 does not deal with these products

Whilst following the guidance given in BS 5250 is one way of complying with Building Regulation C, an alternative approach is to follow a recognised third party certification of a product or process.

For example, some vapour permeable underlays are also classed as ‘air permeable’ and as such the manufacturers claim that these can be installed without roof space ventilation.  However, it is important that the guidance given in the BBA certificate is carefully followed.

Table H1
BS 5250: 2011: Table H1 – COLD ROOF
A) An additional high level vent 5 mm x longest horizontal dimension of roof should be provided where:
The pitch exceeds 350; or
The span exceeds 10 m; or
The roof is a lean-to or monopitch
B) Alternatively, high level vent 5 mm x longest horizontal dimension of roof should be provided

BS 5250 Table H1 shows the minimum recommendations for low level ventilation requirements for a cold roof.

Regardless of whether an impermeable or vapour permeable underlay is used, some ventilation is required at low level.

Similarly, regardless of whether a normal ceiling or well-sealed ceiling is installed, some ventilation is still required at low level. Albeit less if the underlay is vapour permeable and there is a well-sealed ceiling.

 

 

BS 5250 does allow the use of high level ventilation instead of low level ventilation where the underlay is vapour permeable and there is a well-sealed ceiling.

It also important to note that if using impermeable underlay, additional high level ventilation should be provide if the roof pitch exceeds 35 degrees or the span exceeds 10 metres.

Best practice, unless you can be very sure of the long term integrity and efficiency of your design,  is to install eaves ventilation in a cold roof construction, even with a vapour permeable underlay.

Traditional Scottish roofing practice
Plywood, chipboard, OSB, etc, should be treated for design purposes as a high resistance underlay, regardless of the actual underlay type

Traditional Scottish roofing practice
Softwood sarking, typically 150mm wide with 2mm gaps between each board, can be treated for design purposes as a low resistance underlay

On a roof where plywood or other similar boarding has been used, this must be treated as an impermeable underlay with regard to ventilation requirements.

But where softwood sarking is used, typically 150mm wide with small gaps between each board, this can be regarded as air permeable and so used in conjunction with a low resistance underlay it can be treated for design purposes as a low resistance underlay.

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CPD Q&A

You’ve reached the end of the CPD. To make sure you’ve taken on board the key learnings of this course, please fill out the quick multiple choice Q&A below. This will certify that you have completed the CPD and provide you with an email certificate, which, if the course is accredited, you can share with RIBA.

Question

Which of these is not covered by BS 5250 temporary condensation?

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Question

Which is a key point of guidance given in BS 5250 to meet Building Regulation Part C?

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