Extending an existing pitched roof? 

Using a vapour permeable underlay

does not negate the need for traditional ventilation 

Greater awareness now exists of situations where reliance on vapour permeable underlays only is unacceptable to adequately evacuate moisture from the roof space. 

Vapour permeable underlay that has a low water vapour resistance (less than 0.25MN.s/g) is commonly used under tiles on cold pitched roof constructions to provide the sole means of air and vapour evacuation from the roof space. To be acceptable the vapour permeable underlay must be a material that in service conditions is able to adequately limit the risk of condensation in roofs. 

An announcement by the NHBC has helped clarify the understanding of why alternative or supporting ventilation should be incorporated. Cavity Trays of Yeovil has products to satisfy the NHBC directive having previously extended its range of ventilation products and advises designers and contractors to be mindful of the following circumstances where reliance on a vapour permeable underlay can be insufficient and unacceptable: 

1)  Where tight fitting tiles or slates are used. 

2)  Where the vapour permeable underlay is fitted tightly across the rafters and no counter battens are fitted. 

3) Where an existing roof that is already traditionally ventilated is extended. 

4) Where the building does not comply fully with the requirements of Approved Document F ventilation or with BS5250 regarding the control of condensation in buildings. 

5) Where the cavity is open into the roof space. 

6) Where the roof configuration introduces areas where uniform evacuation is not possible. 

7) Where any requirements accompanying the underlay are not fulfilled, such as sealing all penetrations where they pass through the roof covering or from within the building into the roof space. This includes where down light punctuate the ceiling. 

8) Where the NHBC advises that the use of a vapour permeable underlay only is unacceptable, regardless of whether that membrane has BBA certification. 

At the end of 2010 the NHBC announced the amounts of roof space condensation witnessed in dwellings less than two years old had been horrendous. Accordingly, with effect from 1st January 2011 the NHBC has required high-level ventilation in pitched roof spaces regardless of whether advised otherwise by the vapour permeable underlay manufacturer. With its’ range of ventilation products increased, Cavity Trays of Yeovil is able to offer traditional and supporting ventilation solutions for the building envelope. 

A spokesperson qualified: 

‘The NHBC announcement has prompted builders to re-evaluate the situation. Fortunately high-level ventilation is easy to incorporate and can be normally be introduced at the ridge or at high points within the roof slope, so there are immediate solutions to the NHBC initiative. We have also been advising contractors to be aware of the situation that applies when extending an existing pitched roof. The fact that one must not extend a roof incorporating traditional roofing ventilation with a vapour permeable underlay only has surprised many contractors who had not realised the combination  can nullify the intended performance. We now offer 40 types of ventilators in numerous sizes and colours. We are usually able to provide a matching or harmonizing style to satisfy compliant compatibility’. 

A new ‘ Book of Wise Decisions – Best Practice Guide 2011/12’ is available now upon request and contains the latest range of ventilation solutions as well as details of trays, dpc’s, flashings, membranes, closers and associated construction details.