A roof isn't simply a layer of tiles or slates keeping the rain out. Beneath the external roof covering sits a series of components designed to help protect the building from moisture, condensation and the unpredictable British weather.
One of the most important of these components is the roof breather membrane.
Modern breather membranes are lightweight sheets installed beneath roof tiles or slates and above the insulation or roof structure. They provide a secondary layer of protection against water entering from outside while allowing water vapour from within the building to escape.
But how does a breather membrane actually work, where should it be installed and which type should you choose?
In this guide, we'll explain what roof breather membranes do, why they're used and what you should consider when choosing one for your project.
What Is a Roof Breather Membrane?
A roof breather membrane is a water-resistant but vapour-permeable sheet installed beneath the external roof covering.
On a typical pitched roof, the membrane sits underneath the tiles or slates and provides an additional protective layer between the external roof covering and the structure below.
Its job is essentially twofold.
It helps prevent water that gets beneath the primary roof covering from reaching the roof structure while also allowing water vapour to pass through the membrane and escape.
This combination is what makes modern breathable membranes particularly useful in insulated pitched roof construction.
What Does a Roof Breather Membrane Do?
Although roof tiles and slates provide the primary protection against the weather, small amounts of water can occasionally get underneath them.
This could be caused by:
- Wind-driven rain
- Snow
- Broken or displaced tiles
- Water entering during severe weather
- Temporary exposure during construction
The breather membrane provides a secondary line of protection.
Any moisture reaching the membrane can be directed towards the eaves rather than entering the roof structure below.
At the same time, the membrane's vapour-permeable construction allows water vapour from inside the building to pass through it.
This helps manage moisture within the roof construction.
Why Does a Roof Need to Breathe?
Everyday activities inside a property produce water vapour.
Cooking, showering, drying clothes and even breathing all contribute moisture to the air inside a building.
Warm air containing water vapour can move upwards through the building towards the roof.
If that moisture reaches a cold surface and cannot escape, it can condense into liquid water.
Over time, excessive moisture within a roof structure can contribute to problems including:
- Condensation
- Damp
- Mould growth
- Moisture affecting insulation
- Timber deterioration
- Reduced thermal performance
A breathable roof membrane forms part of the overall strategy used to manage this moisture.
However, the membrane shouldn't be considered in isolation. Ventilation, insulation, airtightness and the overall roof design all play a role in controlling condensation.
Is a Breather Membrane Waterproof?
Roof breather membranes are designed to resist water penetration from the outside while remaining permeable to water vapour.
This is an important distinction.
A breather membrane isn't intended to replace tiles, slates or another primary roof covering. Instead, it provides a secondary protective layer underneath them.
The tiles or slates remain responsible for providing the main weatherproof surface of the roof.
The membrane provides additional protection if moisture gets past that outer layer.
Where Is a Roof Breather Membrane Installed?
On a typical pitched roof, the breather membrane is installed over the roof structure and beneath the battens, tiles or slates.
A simplified roof build-up might look like:
Roof tiles or slates
Roof battens
Breather membrane
Rafters and insulation
Internal ceiling
The exact construction will vary depending on the roof design and insulation arrangement.
The membrane is normally laid horizontally across the roof, starting near the eaves and working upwards towards the ridge.
Adjacent sections overlap so that water travelling down the membrane remains on the outside rather than passing through a joint.
Always follow the membrane manufacturer's installation instructions regarding overlaps, fixing, detailing and ventilation.
Do You Need a Breather Membrane Under Roof Tiles?
Breather membranes are widely used beneath roof tiles and slates in modern pitched roof construction.
They provide a secondary barrier against wind-driven rain, snow and other moisture that may penetrate beneath the external covering.
Whether a particular roof construction requires a specific type of underlay or ventilation arrangement depends on factors including:
- Roof design
- Roof pitch
- Insulation position
- Ventilation strategy
- Type of roof covering
- Building Regulations
- Manufacturer requirements
For new roofs and substantial roofing work, the specification should therefore be considered as part of the complete roof design.
What Is the Difference Between a Breather Membrane and Traditional Roofing Felt?
Traditional bituminous roofing felt was historically used beneath tiles and slates as a secondary weather-resistant layer.
Modern breather membranes perform a similar protective function but are designed to allow water vapour to pass through the material.
They are also generally much lighter than traditional bituminous felt.
Potential advantages of modern breather membranes include:
- Lightweight construction
- Easier handling
- Vapour permeability
- Water resistance
- Convenient roll formats
- Straightforward installation
- High strength relative to their weight
This is why breathable membranes have become commonplace in modern pitched roofing systems.
What Does Vapour Permeable Mean?
A vapour-permeable material allows water vapour to pass through it.
Water vapour consists of extremely small molecules carried within the air.
Liquid water droplets are considerably larger.
The structure of a breathable membrane is designed so that water vapour can pass through while liquid water is resisted.
This allows moisture from within the roof construction to move outwards while helping prevent external water from travelling in the opposite direction.
Are All Roof Breather Membranes the Same?
No.
Although different products may perform the same basic role, their specifications can vary considerably.
When comparing roof breather membranes, you may see differences in:
- Weight
- GSM
- Tensile strength
- Tear resistance
- Vapour resistance
- Water resistance
- Roll width
- Roll length
- Coverage
- Number of layers
- Manufacturer certification
The appropriate membrane will depend on the demands of the project.
A lightweight membrane may be perfectly suitable for many standard roofing applications, while a heavier and more robust membrane may be preferred where greater strength and durability during installation are desirable.
What Does GSM Mean on a Breather Membrane?
When shopping for roof membranes, you'll frequently see specifications such as 92gsm, 115gsm, 135gsm or 155gsm.
GSM stands for grams per square metre.
It describes the weight of the membrane material across one square metre.
For example, the Powerlon UltraPerm range includes membranes at several different weights:
- Powerlon UltraPerm Lite - 92gsm
- Powerlon UltraPerm Standard - 115gsm
- Powerlon UltraPerm Premium - 135gsm
- Powerlon UltraPerm Max - 155gsm
Generally, moving up through these grades provides a heavier and more robust membrane.
However, GSM shouldn't be used as the only measure of performance.
Tensile strength, tear resistance, vapour performance, water resistance and certification should also be considered when comparing products.
We'll cover this subject in more detail in our upcoming guide to What Does GSM Mean on a Breather Membrane?
How Do You Choose a Roof Breather Membrane?
The first step is to check the requirements of your particular roof construction.
Consider:
- Whether the membrane is suitable for your roof type
- Required strength and durability
- Vapour performance
- Water resistance
- Roof pitch
- Exposure
- Installation method
- Required coverage
- Roll dimensions
- Relevant standards and certification
You should also consider who will be installing it.
For larger roofing projects, a more robust membrane may make handling and installation easier, particularly when the material will be exposed to movement during construction.
Does the Roll Size Matter?
Yes, although roll size doesn't normally change the fundamental function of the membrane.
Breather membranes are available in different widths and lengths to suit different projects.
For example, some Powerlon UltraPerm membranes are available in a choice of 1m and 1.5m wide rolls, as well as different roll lengths depending on the product.
A wider roll can cover more roof area with fewer horizontal runs, potentially reducing the number of laps required.
Smaller rolls may be easier to transport, carry onto the roof and handle in tighter working areas.
When comparing roll sizes, consider both:
- Total square metre coverage
- Practical handling during installation
The best roll size isn't necessarily the largest one available.
How Much Breather Membrane Do I Need?
Start by calculating the total surface area of the roof slopes.
However, ordering exactly the same square metre coverage as the calculated roof area is unlikely to be sufficient.
You also need to account for:
- Overlaps between membrane runs
- Detailing around ridges
- Valleys
- Hips
- Abutments
- Chimneys
- Roof windows
- Cutting and general wastage
The amount of additional membrane required will depend on the complexity of the roof.
A simple dual-pitch roof will generally require less additional material than a roof containing several hips, valleys and penetrations.
Do Breather Membrane Joints Need to Be Taped?
Whether joints need to be taped depends on the membrane, roof design and manufacturer's installation requirements.
Compatible membrane tapes can be used to seal overlaps, joints and penetrations where required.
Using a tape designed for membrane applications is important because it needs to remain securely bonded to the membrane and perform throughout the life of the installation.
The Powerlon range includes compatible Powerbond membrane tapes designed for sealing and jointing applications.
Always check which tape is recommended for the specific membrane and application rather than using a general-purpose building tape.
Choosing a Powerlon UltraPerm Roof Breather Membrane
The Powerlon UltraPerm range provides several options depending on the level of strength and robustness required.
Powerlon UltraPerm Lite 92gsm
Powerlon UltraPerm Lite is the lightweight option within the UltraPerm range.
At 92gsm, it provides a cost-effective breathable roofing membrane for applications where a lighter membrane is appropriate.
It's available in different roll formats, allowing you to select a size suited to the scale of the roofing project.
Powerlon UltraPerm Standard 115gsm
Powerlon UltraPerm Standard increases the membrane weight to 115gsm.
It provides a useful middle ground for general roofing applications where additional robustness compared with the Lite membrane is desirable.
Different roll sizes are available to suit varying roof areas and installation requirements.
Powerlon UltraPerm Premium 135gsm
At 135gsm, Powerlon UltraPerm Premium provides a heavier membrane option.
The increased weight makes it suitable for projects where a more robust roofing membrane is preferred while maintaining the vapour-permeable characteristics required from a breather membrane.
Powerlon UltraPerm Max 155gsm
Powerlon UltraPerm Max sits at the heavier end of the UltraPerm range at 155gsm.
It's designed for projects where a particularly robust breathable roofing membrane is required.
When deciding between the four options, don't simply assume that the heaviest membrane is necessary.
The aim is to choose a membrane with the appropriate performance for your particular roof and installation requirements.
Powerlon UltraPerm Range at a Glance
| Product | Weight | Position in Range |
|---|---|---|
| Powerlon UltraPerm Lite | 92gsm | Lightweight option |
| Powerlon UltraPerm Standard | 115gsm | General-purpose option |
| Powerlon UltraPerm Premium | 135gsm | Heavier-duty option |
| Powerlon UltraPerm Max | 155gsm | Most robust UltraPerm option |
Each product should be assessed against the requirements of the individual roof rather than GSM alone.
Frequently Asked Questions
What is the purpose of a roof breather membrane?
A roof breather membrane provides a secondary layer of protection beneath tiles or slates while allowing water vapour to pass through the roof construction. It helps protect against moisture entering from outside while supporting the management of condensation within the roof.
Does a breather membrane stop rain?
A roof breather membrane is designed to resist water penetration and provide secondary protection against rain or snow that gets underneath the main roof covering. It isn't intended to replace roof tiles, slates or another primary weatherproof covering.
Does a breather membrane stop condensation?
A breathable membrane can form an important part of a roof's condensation-control strategy by allowing water vapour to escape. However, condensation control also depends on insulation, ventilation, airtightness and the overall roof construction.
Which way up does a breather membrane go?
Installation orientation varies between products. Many membranes have a clearly identified external face, but you should always follow the manufacturer's instructions for the specific membrane being installed.
Can you leave a breather membrane exposed?
Breather membranes aren't intended to act as the permanent external roof covering. Some products may tolerate temporary UV exposure during construction, but permitted exposure periods vary. Check the manufacturer's technical information and cover the membrane as soon as practical.
What does 115gsm mean on a breather membrane?
115gsm means that one square metre of the membrane weighs approximately 115 grams. GSM is useful when comparing membrane weights but doesn't by itself determine overall product performance.
Is a heavier breather membrane better?
Not necessarily. A heavier membrane may provide additional robustness, but the best membrane is the one that meets the performance requirements of the roof. Vapour performance, water resistance, strength and certification should all be considered.
Do you need to tape breather membrane overlaps?
This depends on the membrane, application and manufacturer's installation requirements. Compatible membrane tapes may be recommended for sealing overlaps, joints and penetrations in certain roof constructions.
Explore Powerlon Roof Breather Membranes
At Trusted Building Supplies, we stock a growing range of Powerlon roof breather membranes suitable for a variety of pitched roofing applications.
The Powerlon UltraPerm range allows you to choose between different membrane weights and roll sizes depending on the requirements of your project.
Our range includes:
- Powerlon UltraPerm Lite 92gsm
- Powerlon UltraPerm Standard 115gsm
- Powerlon UltraPerm Premium 135gsm
- Powerlon UltraPerm Max 155gsm
- Powerbond membrane sealing tapes
- Specialist Powerlon roofing membranes
Whether you're completing a new roof, extension or refurbishment project, choosing a suitable membrane and compatible accessories can help create a properly designed and protected roofing system.
Why Choose Trusted Building Supplies?
Trusted Building Supplies is a family-owned independent builders' merchant supplying high-quality building materials to customers across the UK. We work with leading manufacturers and trusted suppliers to offer competitive prices, fast nationwide delivery and expert advice to help you choose the right products with confidence.
Whether you're a homeowner taking on a roofing project or a trade professional completing a larger installation, we're here to support you every step of the way.
Information Notice
This guide has been written by Trusted Building Supplies to provide general information and practical guidance for homeowners, DIYers and trade professionals. Roofing requirements can vary depending on the building, roof design, location and products being used.
Always follow the latest manufacturer's technical information and installation guidance, along with applicable UK Building Regulations and relevant standards. Seek advice from a suitably qualified professional if you're unsure about the design, ventilation, condensation control or suitability of a membrane for your project.
Last Reviewed: August 2026