How to install permeable membranes right first time

How to install permeable membranes right first time

Nick King, Portfolio Manager at Klober UK explains the steps builders need to take to make sure permeable membranes are installed right first time.

Whether it’s for a roof on a new-build home, an extension on a property, or simply looking to fix mould and damp issues, builders may find that they need to turn to air and vapour permeable membranes to help provide ventilation into the roof space.

Installing these membranes correctly is critical to ensure they perform as intended, and there are six key steps to take when fitting these roofing products. Besides correct installation, choosing the right membranes from the outset can help builders save time and costs too.

Step 1 – Check Certification

Before installing, contractors should check the credentials of the membrane they are planning to use, so the correct ventilation requirements can be considered.

Non-breathable membranes will require additional ventilation whereas breathable membranes, such as Klober’s vapour permeable Permo Forte 145, will aid the ventilation process, and not require as much additional ventilation. Membranes that are both air and vapour permeable, such as Klober’s Permo Air 160, are often selected as they can totally negate the need for any additional ventilation.

It is advisable to double check the specific information or certification of the membrane that is being used before installation.

Step 2 – Fit Eaves Ventilation Products 

After checking the membrane’s credentials, builders will need to evaluate what eaves ventilation system is needed for the roof, and ensure that any eaves ventilation products being used, for either cold or warm roof construction, are fitted correctly.

Eaves ventilation products including fascia vents, roll out rafter trays and underlay support trays, should be fitted at the same time, and are available through Klober. 

Step 3 – Install the Membrane

Klober membranes should always be installed with the printed side facing upwards, and they come with a headlap guideline to help with the installation process. When installing the membrane in an unsupported situation, such as in a typical cold roof loft space, it is important to lay it so that a drape can be created between the rafters. A maximum of 15mm is sufficient to allow any water that may find its way to the top of the membrane to run to the eaves and into the guttering.

When installing the first layer of membrane, make sure it is laid parallel with the eaves, so that the bottom edge of the product finishes in line with the over fascia vent or fascia board. It should also be noted that modern day membranes do not tend to be permanently UV stable. Membranes should not be draped into the gutter as this is the job of the bottom lip underlay support tray.

Continue with the following runs of membrane, using the printed headlap guideline to correctly position the membrane in relation to the roof pitch.

Step 4 – Use Headlap Guidelines

To provide an example, BS 5534 tells us that for a roof pitch of around 35 degrees, a 150mm headlap would be required. BS 5534 requires the headlap of the membrane to be secured and the zonal performance to be declared by the manufacturer. There are three ways of addressing this, depending on the individual project requirements.

The headlap can either be secured underneath a batten, or a separate tape could be used. Permo Forte 145 for example, is suitable with a battened headlap for zones one to four whereas a tape would be required for zone five.

Step 5 – Enable Ventilation

If high level ventilation is required, then a Klober ventilated dry ridge system can be used. In this case, the membrane should be stopped short of the apex by 30mm, to allow passage for ventilation. If no ventilation is required, then the membrane can be carried over the apex if preferred. Battening can then commence.

Step 6 Battening

Battening secures the membrane to the rafters and will be followed by the roof finish, for example a concrete interlocking tile. Various accessories are available, including tapes and sealants, for nail penetrations and to help create an airtight seal around a variety of details.

A Leak-Free Solution

When fitted correctly and used as a secondary barrier behind the tiles or slates, air and vapour permeable membranes will not leak and will give the homeowner a well-ventilated space for years to come.

For more assistance with installing any of Klober’s products, visit

To watch a video that outlines the installation of roofing membranes visit How to Install a Roofing Membrane (

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