Roofing underlays are probably the most important factor when it comes to either getting a new roof or replacing your older roof. There are a variety of types available which can have a dramatic effect on the lifespan of your roof.

There are in fact so many styles available, how do you know which product is the right choice for your project?

The first step to selecting a roofing underlay is to understand the available types. 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

Although the styles vary, there is really only 2 types of underlays used generally on roofs: high resistance (non-breathable underlays) and low resistance (breathable or vapor-permeable membranes).

Know the differences between these underlay types and how they will work within your roof system, and also potentially impact on ventilation requirements.

Non-breathable underlays

These include both traditional bituminous products, as well as impermeable roofing membranes made from modern materials like polypropylene. They are often perceived as the affordable and functional option and are designed to provide an effective secondary barrier against wind pressure and water penetration.

Non-breathable roofing underlays are typically used for:

• Applications where traditional ventilation methods are preferred.
• Roof constructions where low-level eaves ventilation is used in conjunction with high-level ventilation, often at the ridge-line. This is a longstanding and effective means of ventilating the roof space and is often preferred due to its simplicity.
• Roof constructions that use batten cavity ventilation – some construction scenarios or product choices require the batten cavity to be ventilated, to guarantee a minimum of 50mm of free air ventilation between the eaves and the ridge. This is commonly achieved through the use of counter-battens, in which case, there is no additional benefit to using a breathable underlay.

While breathable membranes are becoming an increasingly popular underlay choice, they aren’t, by any means necessary. For many years, traditional roof constructions have used non-breathable underlays in conjunction with low level (eaves) and high level (ridge) ventilation systems. This traditional approach is both effective and proven.

In locations where roof constructions require counter-battens to ventilate the batten void, there is arguably no benefit to using a low-resistance underlay. In these instances, it’s common and cost-effective to simply use a non-breathable underlay – either lightweight or a reinforced bitumen membrane.

Breathable underlays

This is a newer type of product, and low-resistance underlays are generally more expensive than their non-breathable counterparts. However, they offer several benefits in addition to their breathability. For example, they’re lightweight, durable and easy to install, and they also provide some supplementary advantages when it comes to roof ventilation.

These products are known as low-resistance underlays (vapor permeable), it is important to recognise that breathable membranes come in two different types: vapor permeable and vapor and air permeable.

Vapor permeable (VP) is the most common type of breathable underlay. It is often used (mistakenly) as a sole means of roof ventilation. High-level ventilation must be used with these underlay types to ensure sufficient cross-flow ventilation to carry water vapor out of the building.

Vapor and air permeable (air open) membrane is very expensive and is predominantly a specification-only product. While some segments of the market – particularly builders – often choose to use these products, particularly for ventilation simplification, they are still relatively new to the market, and their long-term performance has yet to be proven.

Roofing underlays

Ensure underlays are installed with a maximum 15mm drape – This prevents the transfer of wind loads to the roof covering, which it was not designed to resist.

Avoid leaving underlays exposed – since over-exposure to UV light or freeze/thaw conditions could lead to premature failure of underlays, underlays should not be exposed for more than a few days.

Meanwhile, in the case of LR vapor-permeable underlays, designers must ensure that the manufacturer’s stated water vapor resistance values are suitable for the application.

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