1F Felt: Roofers Return To Under Tile Felt.

As we better understand the impact of changes to the design of the building envelope as a whole, some experts are predicting a return to traditional 1F felt.

When breather membranes replaced 1F as the top-selling roof felt, they were considered the wonder product. The fact is that today, most roofers would not consider anything else. As we begin to understand some of the limitations of breather membranes, many roofers and architects are predicting that roofers will return to traditional under-tile felt.

Benefits of Under Tile 1F Felt

1F Felt being installed on a new roofAs well as reducing the impact of wind lift, roofing felt is designed as a secondary barrier to rain and snow. Your roof tiles or slates should be doing most of the hard work. The real benefit of roofing felt kicks in when something goes wrong.

If the worse happens, and a slate or tile is damaged, roofing felt will channel the rain harmlessly down to the gutter. Roofing felt also minimises pollutants, dust and insects from entering your attic. Great news for those wanting to safely store their bits and bobs (and that strange wooden mask that Aunt Mildred brought back from her travels), out of sight in the attic!

Traditional undertile felt (slaters felt), manufactured from hessian fibres coated with bitumen was used on thousands of properties over many decades. Offering long life, good protection, value for money, slaters felt was also considered simple to install.

Then a double whammy hit. Changes to building design started to improve air-tightness. Houses started to become warmer and less ventilated. At the same time, lifestyle choices started to create more steam (think showers, cookers and washing machines).

Roof Tile Vent
Some tile vents are just plain ugly.

All this warm, water-laden air had to go somewhere – up. Entering the roofspace, the warm air would hit the cold bitumen felt and then condensate. The result: Damp attics, rotting timbers and calls to insurance companies.

The simple solution was to ventilate. Manufacturers started to produce an enormous variety of tile vents, slate vents, ridge and fascia vents. The downside – they cost money, can be complicated to install, and some look downright ugly.




Breathable Membranes Not Perfect After All?

So the breather membrane was born. Offering similar protection, breather membranes function a bit like a Gortex jacket. They prevent water from getting in, but small holes in the skin of the breather membrane allow all that moisture-laden water to dissipate out of your roof space. Or so we thought.

Breather membranes were originally sold as roofing felt that needed no additional ventilation. The reality, according to the National Home Building Council who insure most new builds, is something else. They state breather membranes should be correctly installed to protect the sheathing and frame from moisture, allow water vapour from within the frame to pass into the cavity.

The truth (as always),  probably lies somewhere in between. Many roofs fitted with a breather membrane, get along just fine without the need for any additional ventilation. However, in some situations, breather membranes become overloaded with moisture. Seemingly random events combine to create this problem. New build properties becoming occupied in cold winters are the primary concern. North facing roofs seeing less sunlight in winter, poor sealing of ceiling lights, complicated roofs located over kitchens or bathrooms are others.

Following even better changes to the design of houses, many manufacturers are now stipulating that their breather membranes should be installed with additional ventilation (the very thing that they hoped to replace). Others require taping at walls or eaves, or the use of plastic vent strips. That’s fine if your builder or roofer understands the detail. But what if they don’t?

Another impact of the use of Breather Roof Membrane occurs when bats decide to use your roof to roost. The simple fact is that bats and breathable roof membranes just don’t mix. The fine fibres of a breathable membrane fluff up like cotton wool, entangling the bats with dire consequences.

Roofers Return to Tradition Felt

So why the return to traditional bitumen based roofing felt? We spoke to some of our local roofers and architects and asked for their opinions.

  • 1F felt reduces wind lift.
    Bitumen felt significant reduces wind lift. Because of this, slates and tiles are much less likely to be blown away during strong gales.
  • Bat friendly felt.
    Roof felts manufactured from hessian / bitumen are much less likely to entangle bats.
  • Undertile felt is easy to install.
    Simply unroll and nail onto the rafter.
  • Breather Membrane requires additional ventilation.
    If you’re going to fit additional ventilation, why not just install 1F anyway.
  • Heritage projects stipulate original or traditional materials.
    Hessian weave bitumen 1F roofing felt is still manufactured in virtually the same way as it was 100 years ago.
  • Durable:
    Tried and tested over many decades.

Your view:

Has the wheel turned back towards traditional slaters felt? Do you use or have preferences on one type of roofing felt or another. Please share your views on breathable membranes and the return of 1F felt?

Which Is The Best Breathable Roofing felt?

To ensure that you buy the best breathable roofing felt for your project, you should first consider what particular stresses and strain your felt will be exposed too. The best membrane for one project, may in different circumstances, perform poorly on another. Let me try to explain.

Breather Membrane Benefits

Breathable roofing felt, or breather membrane is considered a vital component of all new builds, extensions and re-roofing projects. It offers significant advantages over traditional 1F slater’s felts:

    • Allows water vapour to exit your roof space, preventing condensation from dripping back in.
    • Stops wind-driven rain and snow from being blown into your roof through gaps between tiles and slates.
  • Enables air to circulate, discouraging the formation of mould and rot.
  • Reduces or removes the need for expensive/ugly air vents.
  • Lowers heat loss, saving you money and lower carbon emissions.
  • Provides a temporary roof, during the building’s construction.
  • Minimises the likelihood of slates and tiles being blown off your roof (by reducing the impact of wind lift).
  • Acts as a barrier to insects, dust and airborne particles.

In essence, we are asking these fabrics to do many things at once:

  • Breath (which require more numerous, or larger holes for air to pass through).
  • Waterproof (which require fewer or smaller holes to stop water from leaking through).
  • Strong (which requires more fabric).
  • Flexible (which requires less fabric).

Of course, no single fabric can do each of these things, all at once. This is why manufacturers are forced to target their products to particular strengths. Some choose to target the most breathable, others the strongest or most water-proof.

Thankfully we don’t have to rely on the manufactures telling us which product is best. Most breather membranes on the UK market are tested by the British Board Agreement (BBA). By studying the results of the BBA certificate, you will be able to work out which is the best breathable roofing felt.

Before wading in at the results, it’s worth looking at the BBA test in detail. This will really help you understand how well the material will function in the different areas it will be exposed to. And therefore to be able to pick the one that is best for your roof.

Breathability and Water Tightness

Best breathable roofing felt
The best breathable roofing felt has to deal with many complexities

Breathable roofing felt allows your roof to breath whilst preventing water penetration. It works in a similar way as to how a Gortex jacket protects an athlete from getting too hot and sweaty, whilst protecting them from wind, rain or snow.

Three parts of the BBA approval process test this functionality. 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 quickly a breathable membrane allows moisture-laden air 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 the figure the better (they will allow more water vapour to disperse more quickly).

Moisture (water) Vapour Permeability:
The measure of how much moisture passes outwards through 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 watertight. The BBA certificate tests for water tightness by measuring how high ahead 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

A nail in a roofing batten
A breather membrane must be strong enough to resist tearing

In addition to lasting the lifetime of the roof, a breather membrane has to be tough. This is so that it can withstand the installation and build process. It also has to last through repeated winter gales and hot summers.

During installation, the material must withstand accidental puncture and snagging against wood splinters, rough edges or nail heads. Where the felt has been punctured, 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 has a dramatic effect on your roof
The wind puts an enormous strain on your roof.

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 the 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).

Conclusion: What is the best breathable roofing felt?
There is no single breather membrane that is the best in every situation. This is because it is simply impossible to design and manufacture a membrane that is the best at everything (breathability, water-tightness & strength). Instead, manufacturers will optimise their products towards either strength/durability, vapour control or water-tightness.

Consider what specific pressure your breather membrane will be exposed too. Some properties will be more vulnerable to wind uplift (where a stronger membrane would be better). Other roofs may be exposed to a higher degree of water condensation (where a more breathable membrane would be suitable). Once you know this, your choice will become more obvious.

Our own list of favourites:

For New Builds / Breathability: Proctor Roofshield 

For Strength: Klober Permo Forte

For All Round Performance: Tyvek Supro


Has this article helped you choose? Do you have any experience you would like to share on what are the top breather membranes for roofs. Please share with us your views on what is the best breathable roofing felt!

What is a breather membrane?

In the same way that a gore-tex jacket protects a walker or a cyclist, a breathable membrane protects your building from wind, rain and condensation.

With the improvement of quality and the tightening of building regulations, modern properties are becoming ever more airtight. This is great for reducing cold drafts and your heating bills. However, a negative impact is that the steam created by cooking, showers and the very air you breathe, has no-where to escape but upwards into your roof.

When this water-laden air comes into contact with a barrier (i.e. your roof tiles), the water molecules trapped within the air condense back into the water – which will then start dripping downwards.

condensation_forming_on_a_glass_of_cold_beerCondensation becomes especially heavy if the barrier is colder than the water vapour. (This is the same principle that creates that lovely condensation on a glass of cold beer on a warm summers day!)

When moisture-laden air escapes into a roof constructed without a breather membrane, the water will condensate on the inside of the roof tiles. This water will then drip down into the eaves or onto the ceiling below. This issue is escalated during colder months and is especially an issue for new build properties where the wet trades (plaster, cement, etc) are drying out.

If this condensation continues to be left unchecked, your house is likely to suffer from the effects of mildew and rot. Apart from being visually unattractive, this may require the expensive repair or replacement of your roof timbers.

What is a breather membrane and how does it help?

A breather membrane is a thin sheet of fabric that sits above your insulation and rafters but underneath your roof slates. The construction of the fabric allows any condensation to rise up into the roof void but prevents the condensing water from dripping back down into your roof. Instead, it channels the water down into your gutter, preventing many problems.

Purists may point towards that old fashioned roofs made no use of membranes. However, this takes no account that natural ventilation is much reduced in newer properties. In addition, a breathable membrane also blocks airborne dust, pollen and insects from entering the attic – a room now considered as a vital storage area by many of us.

Does this article answer your questions? Do you have any comments that you would like to share? Join the discussion on what is a breather membrane!