Roof Giant's Breather Membranes FAQ

What are breather membranes?

Breather membranes (otherwise known as breathable membranes) are used within external wall and roof constructions in order to prevent damp in buildings. BS5250 (clause 8.4.1.1.3) recognises two types of underlay, with breather membranes being defined as “type LR / breathable underlay has a water vapour resistance of less than or equal to 0.25MNs/g”.

Breather membranes are water-resistant but vapour permeable which means that they can protect your building from damp related issues while releasing any surplus moisture vapour into the air. They are also able to prevent penetration by external environmental factors including snow, wind and rain and they keep out contaminants such as dust and dirt.

One common problem with homes, especially new builds, is condensation. This can be caused by a number of factors, and a poorly fitted and installed breather membrane can be a contributing factor. Breather membranes are installed to the outer side of the insulation and each roll should contain detailed fitting instructions. You may also need to check the ventilation requirements of the membrane will be met before you begin the installation.

Some breathable membranes are also approved as being air open as well as vapour permeable. These underlays are the most breathable on the market and are particularly suitable where high levels of condensation are to be expected (such as the drying out period of new build properties), or for more complex roofs where traditional ventilation options may be problematic.

Are breather membranes waterproof?

Breathable membranes are often water-resistant or waterproof and can therefore be used as a temporary covering. However, it’s recommended that the roof covering is installed as soon as possible.

How do I choose a breather membrane?

In order to choose the right product for your project, think about the exposure levels of the membrane, your ventilation requirements, how much you will need, your budget and what performance level you require.

Here at Roof Giant, we stock a wide range of breather membranes that are heavy-duty, highly durable and permeable yet will provide immediate weatherproofing against wind, rain and snow. We also have a range of products which are made in Great Britain and have BBA (British Board of Agrément) approval.

How do I install a breather membrane?

As mentioned, most breather membranes will come with fitting instructions, and it’s vital to check ventilation requirements for each roof. There are some key steps to ensuring effective installation.

The eaves

A UV resistant membrane or felt support tray should be fitted. The bottom edge of the membrane often extends into the gutter, but exposure over time can lead to degradation (in fact most modem underlays are not intended for this purpose). Felt support trays do exactly as their name suggests, while providing drainage directly into the guttering.

Laying & draping the membrane

Always follow the instructions provided, but generally the membrane is laid parallel to the eaves, with the printed side facing outwards. As mentioned above, the membrane should overlap at the bottom by approximately 150mm.

It is usually recommended that the membrane should drape slightly between the rafters by approximately 10-15mm. This allows small channels for any moisture to run off efficiently, reducing the risk of ingress. Adding temporary clout nails to the top of the membrane will allow you to check it’s fitted correctly, making sure they’re above where the second length of membrane will overlap. Adding slates or tiles will provide a secure fitting.

Work up the roof to finish the job, following manufacturer instructions relating to the overlap required. Additional battens can be installed if the overlap doesn’t coincide with an existing batten. They can be added over the top of the membrane, ensuring the required drape still exists.

Other things to consider

Ridge ventilation - if this is present, the membrane should be cut on each side of the ridge to ensure this is still clear. If no ventilation is present, the breather membrane should go over the ridge and overlap on each side.
Verges - with dry fix verges, membranes should be laid so they extend past the face of the gable. With a wet verge, it should overlap onto the masonry by a minimum of 50mm.
Valleys - the membrane should be extended by a minimum of 300mm on each side when measuring from the centre. At abutments, the underlay should be turned up by at least 100mm to create a secondary barrier.
Penetrations - the breather membrane should be cut and folded up against any penetration. Cut an asterix shape for round penetrations and, if required, use tape to stop it folding back down.

What is the difference between a vapour control layer (VCL) and a breather membrane?

In terms of their positioning within construction, breather membranes are positioned to the outside of the insulation and vapour control layers (VCL) are positioned to the inside of the insulation. While VCL can minimise the amount of warm moist air from entering the construction, breather membranes allow moisture particles to escape from the inside whilst acting as a weather barrier.