What type of roofing underlay should I use?
Roofing underlays come in two main types, typically defined as being either low-resistance or high-resistance. The right product choice is critical in protecting a roof and the building below from rainwater ingress, condensation and draughts. It is also necessary to ensure compliance with building standards and regulations.
Choosing the right underlay and ventilation for a project will depend on:
- The roof type – Warm, cold and hybrid roofs have different requirements.
- The roof pitch – Typical roofs have pitches between 10° and 75°. If the roof pitch is outside this range, it could affect the ventilation strategy.
- The roof covering – The amount and type of roof ventilation will depend on whether the roof covering allows airflow and if the roof is boarded.
- The design of the property – Properties with well-sealed ceilings will have different ventilation requirements.
With different options of roofing underlay, and so many variables to consider, how do you know which product is right for a project? The first step is to understand the types of roof underlays that are available. Then, you can narrow your selection by considering installation requirements and building regulations, as well as the unique features and benefits that certain products offer.
The different types of roof underlays
According to BS 5534:2014+A2:2018, the code of practice for slating and tiling for pitched roofs and vertical cladding, and BS 5250:2021, the code of practice for the management of moisture in buildings, there are two main types of roofing underlays:
- High-resistance underlays (also known as non-breathable underlays or non-vapour permeable underlays) and
- Low-resistance underlays (also known as breathable or vapour-permeable membranes).
Phil Wallis Technical Manager at Marley, explains: “The main difference between these two types of underlay is whether water vapour can migrate through the material. If it can, it means that the vapour can be passively removed from the property, helping reduce the risk of condensation, damp and mould.”
Let’s look at each type in more detail to understand their differences, how they impact roof systems and their potential impact on roof ventilation.
What are high-resistance roof underlays?
High-resistance roof underlays are non-breathable and non-permeable, meaning no air or water vapour can pass through them. This category includes traditional bituminous products, as well as impermeable roofing membranes made from modern materials like polypropylene.
High-resistance roof underlays are often perceived as the more cost-effective and functional option, and they provide an effective secondary barrier against wind pressure and weather penetration.
When high-resistance roof underlays are used, ventilation must be carefully considered. In most cases, a combination of low-level eaves ventilation and high-level ridge ventilation is recommended as a minimum requirement.
High-resistance underlays are often a good choice in situations where otherwise roof constructions may require counter-battens to ventilate the batten void.
Why choose low-resistance roofing underlays?
There are two types of low-resistance roofing underlays:
- Vapour permeable – Also known as VP roof underlays, which allow water vapour only to pass through.
- Air and vapour permeable – Also known as air-open roof underlays, these allow both air and water vapour to pass through.
Phil warns that it’s a common mistake for vapour permeable membranes to be used as the sole means of roof ventilation. He adds: “British Standards and the National Housebuilding Council (NHBC) both stipulate that high-level ventilation must be used with these underlay types to ensure sufficient ventilation to assist with carrying water vapour out of the building.”
When it comes to air permeable (air-open) membranes, these are acknowledged in BS 5250 and NHBC standards and may be used in roof constructions without ridge or high-level ventilation, provided they meet strict performance criteria and have third-party certification.
In most cases low and high level ventilation is recommended as a robust solution towards managing and removing moisture that may accumulate in the roof cavities. For additional information and specification guidance, please contact the Marley technical team to discuss your project requirements.
More than ventilation: what else do the British Standards say about roof underlays?
When discussing roof underlays, BS 5534 and BS 5250 also provide the following guidance:
- Ensure underlays are installed with a maximum 15mm drape – This prevents the transfer of wind loads (ballooning) to the roof covering.
- Avoid leaving underlays exposed – Since over-exposure to UV light or freeze/thaw conditions could lead to degradation of underlays, BS 5534 now states that underlays should not be exposed for more than a few days in certain conditions.
- BS 5250 states that designers must ensure that the manufacturer’s stated water vapour resistance values for LR vapour-permeable underlays are suitable for the application. These types of underlay should also comply with BS EN 13859-1 or have third-party accreditation, such as a BBA certificate.
Choosing the right roofing underlay from Marley
Marley’s range of high-performance roofing underlays includes both vapour-permeable membranes and non-breathable membranes. All of our roofing base layers are compatible with our complete roofing system, suitable for every UK wind zone and fully compliant with relevant standards including BS 5534.
Because product performance is so closely linked to the quality of the materials used, our roofing underlays are manufactured using the best materials and the latest processes. The result is strong, durable, high-performing products that provide years of excellent service.
Our roofing underlays have also been designed for quick and easy installation. We believe that even small details can make a big difference to our customers, so just some of the features you’ll find on our products include:
- Integrated tape for sealing underlay laps
- Printed guide lines and measurements that provide a visual reference for alignment and lapping distances
- Supplied in 1m-wide rolls for ease of handling
- Available as part of our bespoke roofing system
- BBA certified
To learn more, browse our range of roofing underlays or contact a member of our technical team for guidance on choosing the right one for your next project.