What are roof vents and their role in a complete roof system?

exploaded render of Marley ventilation

What are roof vents and what is their role in a complete roof system?

Roof vents are installed into a roof build-up for several reasons, to allow fresh air to enter, circulate through the structure, and then exit again, preventing moisture and condensation from accumulating within the property.

Moisture and condensation in a building can pose a serious problem. A poorly ventilated building can lead to the development of mould and rot with the potential to cause serious damage to the roof structure, anything stored in the loft space, as well as the building in general. Damp conditions can also have a negative impact on the health of the people occupying the building. Therefore an important consideration when designing a building is ensuring the structure incorporates adequate ventilation.

The issue of moisture control within buildings is covered by BS 5250:2021 - ‘Management of moisture in buildings’. This British Standard states that buildings must be designed so that the structure of the building and the health of the occupants are not affected by moisture. The document provides guidance and advice on how to minimise moisture and harmful condensation within buildings and is a valuable resource. In July 2021, several revisions were made to the document, influenced by changes to modern building design as well as the effects of climate change and the drive towards energy conservation.

How do roof vents support BS 5250?

BS 5250:2021 provides details on ventilation requirements in a roof to ensure adequate air flow throughout the roof structure. The ventilation recommendations vary depending on the design and intended use of the roof space, the pitch of the roof, location of the insulation, the type of ceiling, the use of air/vapour control layer (AVCL) and the type of roof underlay used. It is important to consider the minimum roof ventilation requirements at the initial building design stage. Roof vents are available in a wide range of options and solutions to ensure that roof ventilation can be achieved to meet the minimum recommendations of the standard.

How to ventilate a roof

Roof ventilation typically comprises a combination of continuous eaves (low level) ventilation, using proprietary soffit or over-fascia ventilators and continuous ridge (high level) ventilation, commonly achieved using proprietary ventilated ridge systems. Using this combination, a reliable flow of fresh air is able to enter the roof at the eaves and is exhausted at the ridge, which will help to prevent the build-up within the structure. Ventilation may also be achieved by the inclusion of tile vents and a number of products are available to work with different roof types.

How many vents does my roof need?

The number and frequency of vents installed is determined by the type of vent being used as well as the design of the roof. Most roof vent manufacturers provide technical data for their products which include figures confirming the airflow, area of ventilation per vent and the corresponding spacing of each vent to achieve the equivalent continuous ventilation gaps, to meet the recommendations of BS 5250:2021.

Roof vents and breathable underlay - When using a breathable underlay, do you need to include roof ventilation?

Although some manufacturers of breathable membranes state the product can be used without ventilation, it is considered best practice to ventilate roofs in all situations at eaves (low level) and ridge (high level). In accordance with BS 5250:2021 'Management of moisture in buildings‘, particular consideration should be given to the use of close fitting roof coverings that are not considered to be air open and which do not allow sufficient air movement through the laps and joints. To avoid the risk of trapped moisture in the batten cavity, the design of the roof should incorporate a 50mm ventilated batten cavity above the underlay and below the roof covering.

Marley manufactures a range of roof vents designed to meet your functional and aesthetic requirements, to view the full range, please click here.

When designing a roof, the best approach is to specify a complete roof system which incorporates all the components necessary to ensure compliance with standards. This complete roof system approach ensures certainty of performance and compatibility from the start.

Here at Marley we can supply a complete roof system, enabling architects to specify and design a roof that is both visually appealing and fully meets the requirements of building regulations. Every component within the Marley roof system has been designed and tested to ensure they work together, giving specifiers peace of mind and confidence in the system performance.

To enquire about which roof vents can fit within your own whole roof system, please contact us here.

Category: Roofing