The underlay is a key part of the weathertightness of the roof. It is fixed under the roof tiles. When rain is driven up under the tiles by strong winds then the underlay catches the water and channels it down to the gutter.
Older underlays are bitumen-based which are both water and airtight. Bitumen based underlays eventually rot, however, and are a significant cause of leaks in ageing roofs.
Modern underlays tend to be made of polypropelene and are likely to be vapour permeable (or “breathable”). This means that they allow water vapour to escape upwards from the loft space to be channelled away from the house but do not allow moisture back in the other direction.
These underlays will not rot with age in the same way as bitumen based products. You should note that even if you are using a vapour permeable underlay you will still need to consider adequate ventilation to the roof space – you cannot rely on the underlay alone to remove all moisture from the roof space.
