Exploring the Role of Pitched Roof Underlay

Pitched Roof Underlays - Maguire Brothers

The primary reason for using a pitched roof underlay is not as many think, to provide secondary waterproofing protection, it is to help reduce the risk of wind uplift.

This is achieved by equalising the air pressure above and below the roof coverings during periods of strong wind. In these situations, without an underlay, the higher pressure within the roof/loft and the lower pressure on the outside, creates lift, pushing the roof coverings up. Once exposed, the force of the wind does the rest.

Providing a secondary waterproofing layer, to direct any condensation which forms on the back of the roof coverings or driven snow and rain in extreme conditions, is an additional benefit. The use of roof underlays has now been adopted throughout the UK construction sector.

Historical Methods.

Before membranes were used the same benefit was provided, albeit not as well, by other means. These included the use of ‘sarking’ boards (early roof underlay membranes were called ‘sarking felt) where the top of the rafters were boarded over and the roof coverings fixed directly to the boards (more common with slates) or to battens which were fixed to the boards (more common with ‘nibbed’ tiles).

In very old traditional buildings ‘torching’ was often used. This method, which can be traced back hundreds years and involved the application of lime mortar, reinforced with animal hair, usually horse or cow, to the underside of the head-laps of double lap tiles or slates. It is no longer recommended but may be required on listed buildings and conservation projects.

Early Roofing Underlays – The Transition to Modern Membranes.

The first ‘sarking’ felts were bitumen based and reinforced with a hessian matrix. They were eventually enshrined by the standard BS747:1F. Later reinforced plastic membranes were introduced.

These membranes started to be introduced at the same time that people were greatly increasing the amount of roof insulation and just as the boom in central heating was also taking off. This increased the concentration of water vapour inside buildings. Neither of these membranes were air or vapour permeable and they created problems with condensation. This created a need for roof space ventilation. This is why we often see unsightly, plastic, mushroom shaped protrusions on roofs.

But Just as we were getting used to seeing roofs covered with these unsightly plastic ventilators, along came ‘breather membranes’.

The Different Types Of ‘Breather Membranes’.

There are two main types of breather membrane. They are:- ‘vapour permeable’ and ‘open air and vapour permeable’.

Vapour permeable underlays are designed to provide additional roof air circulation and are used alongside roof ventilation systems. These types of underlays offer an effective solution to optimising air-flow and reducing the risk of heavy condensation within the building space.

Open air and vapour permeable underlays can be used on their own as a form of ventilation for cold roof structures with little or no additional ventilation products.

But be careful not all products which claim to be open air and vapour permeable are. Before specifying or installing the membrane check that it has been third party approved and has the relevant BBA approval stating that it is ‘air open and vapour permeable’ and does not require additional forms of ventilation.

Balancing Cost and Effectiveness.

Although they are more expensive than the other forms of roofing underlays, breather membranes are effective and in the case of ‘open air and vapour permeable’ underlays, do not usually require other unsightly ventilation products and offer a simple and easy solution, especially on complex roof slopes.

This article, as with all other articles we produce, is for guidance purposes only. It does not constitute formal advice and should not be relied upon as such. For bespoke, unbiased advice relating to your commercial roofing project please contact us and we would be pleased to assist.

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