How to Select the Best Breathable Roof Membrane

Say goodbye to 1F Slater’s Felts, breathable roof membranes are the gold standard for new builds and re-roofing projects. In fact, this durable, waterproof membrane offers a number of advantages over traditional roofing felts.

Breathable Roofing Felts

As the name suggests, these membranes let your roof breathe. They work in much the same way as a Gortex jacket, allowing moisture from inside the roof to escape while protecting the building from the weather.

Performance is tested across three key areas:

Resistance to Water Vapour: This is a measure of how quickly a breathable membrane allows moisture to escape. A low-resistant membrane will have a vapour resistance of 0.25MNs/g or less. The lower the number, the quicker the waterproof breathable membrane lets water vapour disperse.

Water Vapour Permeability: The higher the value, the better for this one, which measures how much moisture leaves through the waterproof breathable membrane every 24 hours.

Water Tightness: Breathable felt has a very important job to do – it must channel water into your gutters. This means the top layer of the felt needs to be watertight. All membranes must pass this test to achieve a British Board Agreement (BBA) certification.

Are Breathable Roof Membranes Waterproof?

Breathable roofing felt is completely waterproof. It is also guaranteed to last the lifetime of your roof, prevent condensation, and allow air to circulate.

The Best Roofing Underlayment for Your Project

Breathable roofing felts must be durable, breathable, strong, flexible, and waterproof. No one material can hit all of these points perfectly, so manufacturers target their products to particular strengths.

As such, choosing the right breathable roof membrane for your project depends on a number of factors. The location of the roof, its pitch, baton gauge, and the tile used are all important considerations, and the best breathable roof membrane for one project may be poorly suited to another.

The good news is that we don’t have to take a stab in the dark when it comes to selecting the right waterproof membrane. The BBA carry out independent tests on most UK manufactured membranes, and their certificate will help you identify which waterproof breathable membrane products meet your needs.

Extended Exposure

If your membrane is likely to be exposed to the elements before the tiles are fitted, then you need to choose a product that can handle it. Cromar Vent 3 Light Breathable Felt is not as durable as some of the heavier weight membranes, but it is UV stable for up to three months, making it the perfect choice for more moderate climates. If your project is in a windier area, consider the Cromar Vent 3 Classic, which has all the benefits of the lighter membrane with a higher tensile strength.

Unventilated Roofs

An unventilated roof can reduce heat loss by as much as 25% and will save time on-site, since there is no call for eaves and ridge ventilation. However, this does increase the risk of condensation forming in the roof space and means a membrane with a high water vapour permeability is essential. Klober Permo Air is the most breathable felt on the market and is the perfect choice for unventilated roof spaces or climates where condensation is likely to be a problem.

Nationwide Applications

High-performance products such as Cromar Vent 3 High Performance Felt and Redland Spritech 400 2S are suitable for nationwide applications. These strong, wind tight membranes can be used with almost all baton gauges and will protect the roof in even the most extreme weather conditions.

How Much Breathable Membrane Do I Need?

Estimating breathable roofing felt quantities can be tricky. A roofing calculator will give you a good idea of the size of your roof, which is the obvious starting point for calculating how much waterproof breathable membrane you need. Don’t forget to double check the baton garage on your roof; you’ll need to factor a 10mm drape between each baton into your calculations.

Roof pitch is another important consideration since it will determine how big the membrane lap needs to be. As a general guide, pitches under 14 degrees should have a lap of 150mm, while those over 15 degrees usually require a lap of 100mm. If your roof pitch is over 35 degrees, you may be able to reduce the lap to 75mm. Always check the manufacturer recommendations before purchasing your membrane.

Breathable roofing felt is typically priced by square meter, with rolls available in standard 50 meter lengths. At 1 meter wide, this equals 50 square meters of membrane. Some manufacturers also provide a second width of 1.5 meters, which gives a total of 75 meters squared.

Breathable Roofing Membrane Prices

Breathable roofing felt prices vary widely depending on their properties, and might range from as little as £0.54 per square meter (Cromar Vent 3 Light) right up to £3.30 per square meter for more specialist products like Klober Permo Air. If you’re still working out the finer details of your project, a budget of around £2 per square meter is a good starting point.

The Best Breathable Roof Membrane

Breathable roof membranes offer a host of advantages over traditional felt and far more choice when it comes to performance and suitability. By understanding the specific needs of your project, you can ensure you select the best breathable roof membrane for your roof.

How is breathable felt installed?

Monier Redland were kind enough to send their Technical Training Manager to the JJ Roofing Supplies training centre in Cricklewood to show us how it's done, explain how breathable felt works and a few best practice tips. The product used in this video is Spirtech 400 2S.