Types Of Roof Ventilation Explained

Roof ventilation facilitates continuous airflow through the roof space, and prevents the build-up of warm moisture-laden air. Without adequate ventilation, several problems can arise, the most common being condensation and mould.

UK Building Regulations and guidance from the NHBC and LABC state adequate ventilation is a legal requirement in both new-build and renovation projects. Pitched roofs should be ventilated to limit the risk of interstitial condensation and be in accordance with BS 5250:2021

Air and vapour permeable roofing felts can be used to address the requirements of a wide range of roof configurations, but not all. There are situations where the mandatory airflow level can only be achieved by incorporating supporting measures. Far more moisture is allowed through breathable membrane than non-breathable, meaning that increased airflow is recommended for the most effective solution.

Small roof areas (3m2 or less) such as those over porches or bay windows are commonly the subject of a condensation risk analysis to ensure the risk of interstitial condensation is mitigated.

Similarly, where a continuous air and vapour control layer is impractical to install in larger pitched roofs incorporating insulation, the design should limit the risk of interstitial condensation by providing 25,000mm2/m eaves ventilation and 5000mm2/m ridge or high level ventilation should be provided below the underlay. A minimum 25mm clear ventilation pathway is required below the underlay.

 

Different Types of Roof Ventilation Solutions

Designed to address specific needs, the following examples permit the designer and builder to fulfil performance objectives and address aesthetic options/variations.
 

Fascia Ventilators

Fixed to the top edge of the fascia boards of a roof, Type OFV Over Fascia Ventilators allow air to flow in and out of the attic or roof space. Fascia cost is reduced as a smaller (less high) fascia is used, with the Type OFV substituting the final fascia rise. Not visible once the roof overhang and guttering are present, two models are available with 12,500mm² and 25,000mm² airflow ratings.

• The Type OFV is just one example from three fascia level ventilator types available to the designer.

 
 

Soffit Ventilators

When thermal insulation within the roof space of an existing property is increased, it is commonly necessary to also upgrade the ventilation. Ventilation apertures (70mm holes) are introduced through the soffit on opposing sides of the property to receive circular soffit ventilators (Type CSV). Once installed, air can flow into and out of the roof space (as long as the insulation laid within it is restrained from blocking the flow route where the converging roof timbers approach plate level). The installer may choose whether the Type CSV Circular Soffit Ventilators are fitted in a ‘see’ or ‘no see’ status.

Type CSV ventilators have a deflecting louvered face, providing positive air entry and insect screening. Unlike standard grille ventilators, rotating the Type CSV so the louvers slope downwards towards the property masonry face results in the ventilation openings being almost hidden from view, when viewed from the ground. By selecting a Type CSV in a colour to match that of the fascia, the soffit can maintain a continuous unbroken appearance.

Alternatively, fitted with the louvers facing the opposite direction – outwardly – provides a punctuated soffit appearance. The designer has a choice.
The Type CSV is available in seven colours and has an airflow rating of 2,100 mm², so fewer need be installed to provide the required performance level.

• The Type CSV is just one example from eight soffit ventilator types available to the designer.

 
 

Eaves Ventilators

Eaves ventilators allow air to enter the roof space from the eaves, balancing the temperature inside the attic with the outside environment to prevent condensation.
The Type EROV – Eaves Roll-out Ventilator is a flexible, easy-to-install ventilator that offers evenly spaced airflow along the eaves. Where accompanying insect protection is essential such as in an open-eaves detail, the Type OEVWF -Open Eaves Ventilator with Flyscreen provides effective pest control in addition to ventilation.

• The Type EROV and Type OEVWF are just two examples from six eaves ventilator types available to the designer.

 
 

Soffit Ventilator Strips

Strip soffit ventilators running along the soffit provide continuous uniform ventilation in and out of the roof space preventing the buildup of moist air. In addition to our standard, easy-to-install Type SSV Strip Soffit Ventilator, our range includes specialised strip soffit ventilators suited to meet the demands of more complex roof designs of both new and existing properties. The Type RASV Reversible Angled Soffit Ventilator features a reversible design for flexibility and installation at various angles, ideal for sloping soffits, whilst the Type USV has a hidden fixing feature making it suitable for both new and refurbishment/upgrading projects.

• The Type SSV, Type RASV and Type USV are just three examples from seven strip soffit ventilator types available to the designer.

 
 

Learn More

See the full range offered by Cavity Trays Ltd. A handy pocketbook – The Book of Wise Decisions and a Technical Manual Protecting the Building Envelope contain technical data and helpful guidance. Both are available in hardcopy form, without charge upon request. Alternatively, visit our resources page for more.

Remember, whilst standard products are suitable for most applications, some projects require bespoke solutions. Cavity Trays is able to manufacture custom ventilation solutions tailored to specific project needs. Unique architectural designs, historic building renovations, or new builds with unconventional roof structures. Contact Cavity Trays Ltd