Breather Membrane & Breathable Roofing Felt
A breather membrane is now considered a vital component of all new builds, extensions and re-roofing projects as they offer significant advantages over traditional slater’s felts:
- Allow water vapour to exit your roof-space.
- Prevent wind driven rain and snow from entering the roof and damaging your timbers.
- Enable air to circulate, discouraging the formation of mould and rot.
- Some (not all) remove the need for expensive / ugly air vents.
- Reduce heat loss, saving you money and lower carbon emissions.
- Minimise the likelihood of slates and tiles being blown of your roof (by reducing the impact of wind lift).
- Act as a barrier to insects, dust and airborne particles.
Whilst British Board Agrement (BBA) certification should be considered as a bare minimum requirement, active consideration should be made of the following:
Breathability and Water Tightness:
A breathable roofing felt prevents water penetration whilst allowing your roof to breath. (It works in a similar way to how a Gortex jacket protects an athlete from getting too hot and sweaty).
Moisture laden air rising from the living spaces is allowed to escape upwards through the breathable roofing felt. At the same time, wind-blown water and moisture laden air is prevented from entering the roof, as the roofing felt directs it harmlessly into the gutter.
The best felt is one that is able to combine the functions of allowing moisture laden air to pass outwards, yet preventing water from entering from the outside.
Three parts of the BBA approval process tests this functionality. A good breathable roofing felt will perform well across all three sections of the test.
Resistance to Water Vapour:
This section of the test checks how well a breathable membrane allows water vapour to escape your roof. To qualify as a low resistance (LR) membrane, it must have a vapour resistance of less than 0.25MNs/g. The lower figure the better (they will allow more water vapour to disperse more quickly).
Moisture (water) Vapour Permeability:
This test measures how much moisture passes outwards though the breathable membrane in 24 hours. A higher value is preferable.
Water Tightness:
The breathable felt must be able to channel water into your gutters. To achieve this, the top layer of the fabric must be water tight. The BBA certificate tests for water tightness by measuring how high a head of water can be contained, (the higher the result, the less likely the fabric is to allow water through). To ensure ‘real-life’ results, both aged and un-aged samples are tested. A W1 pass is a requirement for BBA certification.
Tensile Strength / Nail Tear Strength:
In addition to lasting the lifetime of the roof, a breather membrane has to be tough enough to withstand the installation and build process.
During installation the material must withstand accidental puncture and snagging against wood splinters, rough edges or nail heads. Where the felt it is punctured by nails, it must be tear resistant (to stop the puncture hole from growing in size).
Until slates or tiles have been installed, it is likely that there will be a period of time (from several days to several weeks) when the breathable roof membrane is the only weather protection on your building. During this time, it has to be strong enough to withstand the full force of the British climate.
BBA certification tests the normal tear strength of the fabric. A second test is performed to review how strong the fabric is after it has been punctured by a nail or a sharp point.
The better it performs in these tests, the more likely it is to last longer – both as a temporary roof covering during installation and as a protective ‘second skin’ in the roof.
Tear strength is particularly important to professional roofers who need to lay large volumes of breathable roofing membranes quickly and without fuss. This is also of interest to the beginner roofer, who may find the fitting process very awkward the first time around.
To strengthen their products, some manufacturers even add a fourth layer grid system which acts both to strengthen the fabric and prevent nail holes and snags from spreading.
UV / Temperature Stability:
A breathable roof membrane / vapour permeable underlay is designed to be installed on your roof many days or even weeks before the final covering of slates or tiles. However a vapour permeable underlay will deteriorate with sustained exposure to the effects of UV light and temperature.
Wind Loading:
Wind blowing over and through your roof-space creates areas of high and low pressure. With an unprotected roof, slates and tiles are at risk from blowing out from the roof at the point where these low and high pressure differences are greatest.
You can reduce the likelihood of your slates or tiles blowing out of your roof by minimising the strength and speed of wind passing through your roof. A heavier or stronger breather membrane is more likely to restrict wind better and is less likely to split. You do not want to reduce airflow completely (it will stop the roof from breathing).