There are many different kinds of underlay available on the market. They can be split into two different categories, as described in BS 5534 or BS 5250, the British Standard for Slating and Tiling and the Code of Practice for the Control of Condensation:
- High resistance: non-breathable underlay
- Low resistance: breathable underlay (vapour permeable)
Non breathable underlay
Non breathable underlay includes traditional bituminous and impermeable plastic products. These are often seen as affordable and functional, designed to provide an effective secondary barrier against wind pressure and water penetration.
Non breathable underlays are used in the following instances:
- Where traditional ventilation methods are preferred
- Low level eaves ventilation used in conjunction with high level ventilation - often at the ridge-line - has been proven to be an effective means of ventilating the roof space for many years and many people still prefer this simple and effective method.
- For 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 - and this is commonly achieved through the use of counter-battens.
Where counter-battens are used, there is no additional benefit in using a breathable (low resistance underlay), therefore it is commonplace to use simply use a high resistance non-breathable underlay.
These newer types are generally more expensive than standard underlays, but as well as providing a barrier to wind pressure and water penetration, they also offer some benefit with regards to roof ventilation.
In BS 5534 and BS 5250 they are referred to as ‘Low Resistance’ underlays (vapour permeable).
It is important to recognise that there are two types of breathable underlay in the marketplace - vapour permeable, and vapour and air permeable.
Vapour permeable (VP) is the most common type of breathable underlay and has often been used (mistakenly) as a sole means of roof ventilation. In reality, product performance is varied and linked closely to the quality of material used. In all instances, the British Standard and NHBC stipulate that high level ventilation must also be used with these underlay types, to ensure sufficient cross-flow ventilation to carry water vapour out of the building.
Air and vapour permeable (air open) underlay is very expensive and is predominantly a specification-only product.
The British Standard BS 5250 does not recognise this specific form of underlay or the associated manufacturer claims that it can be used as a sole means of ventilation. In order to substantiate their claims, manufacturers of this underlay type, have usually had to commission independent testing to prove the performance of their underlays. Whilst segments of the market - particularly house builders - often choose to adopt these products particularly for ventilation simplification, they are still relatively new to the market and their long term performance is yet to be proven.
BS 5534 provides recommendations for selecting a roofing underlay. BS 5534 was revised in August 2015 to ensure that newer lightweight underlays (breathable and non-breathable) are securely installed.
Unlike their traditional bitumen coated predecessors, the newer lightweight underlays, if not secured properly, have the potential to balloon when exposed to wind pressures and ‘balloon’ in the roof space. This can place a load on the underside of the roof covering with the potential to dislodge it.
The underlay may be either fully supported or unsupported (draped), should be of adequate strength, durable and resistant to water penetration. Unsupported underlay should also be resistant to wind uplift and ‘ballooning’.
In the case of LR vapour permeable underlays, the designer must ensure that the manufacturers’ stated water vapour resistance values are in accordance with BS 5250 and Condensation Risk Analysis. These types of underlay should comply with BS EN 13859-1, or have Third Party accreditation, such as a BBA certificate.
Specifying underlay is step 6 in our guide to roof design considerations. Download the full guide here.