Technical tips: underlays and ventilation

When it comes to advice on roofs, one of the most common contractor queries received by Marley’s technical services helpline is on underlays and ventilation. Jamie Riddington from the technical team shares best practice advice on the topic.

Some ‘grey’ areas still remain when it comes to the use of ventilation with underlays, particularly surrounding their use with clay, concrete and slate tiles. This uncertainty partly arises from the fact that the industry terminology of a ‘breathable underlay’ is somewhat misleading, as it suggests ventilation is not required. This, combined with the fact that some manufacturers state that their underlays don’t require additional ventilation, means it is hardly surprising there is so much confusion.

The British Standard BS 5250 does not consider the situation where it is proposed to provide no ventilation to the roof void. As such, our recommendations are in line with this British Standard, that roofs will always require some form of supplementary low and high level ventilation, regardless of what underlay is used.

Follow BS 5250

Always comply with BS 5250:2011+A1:2016 ‘Code of practice for control of condensation in buildings’ and BS 9250:2007 ‘Code of practice for design of the air tightness of ceilings in pitched roofs’. These standards set out what ventilation is required for cold and warm roofs.

Don’t rely on breathable underlay as the sole means of ventilation

Despite misconceptions, breathable underlays should not be solely relied upon when installing clay and concrete roof tiles and slates, and additional roof ventilation is also recommended.

As well as the breathable underlay, roofs will always require some form of supplementary low and high-level ventilation in accordance with BS 5250. This can be achieved using an array of solutions, including eaves ventilation, ventilated dry ridge systems, ridge vents or multiple tile vents to provide the recommended minimum ventilation.

This is also a tried and tested approach which avoids placing the heavy long-term burden of effective roof ventilation on one single element. The cost of adding ventilation components is relatively small in comparison to having to go back and repair condensation damage.

Some manufacturers state that the performance of their underlays negates the requirement for additional ventilation. However, this remains strictly under the control of third party certification (e.g. BBA) and is outside the scope of BS 5250. It is therefore risky to assume that a fully breathable underlay is adequate on its own without checking other factors, such as how well sealed the ceiling is and the air permeability of the roof covering.

Interlocking tiles do need additional ventilation

Because there is a small amount of air openness with interlocking tiles, there is a myth that you don’t need additional ventilation if you are using a breathable underlay. However, that is not the case. For the purposes of BS 5250, all variables in the roof build-up, including the external covering such as standard interlocking tiles, must be considered.