When it comes to constructing or renovating a roof with metal cladding in New Zealand, one crucial component often overlooked until problems arise is the underlay. An awesome overview document from metal roofing manufacturers (published in 2011) titled “The Underlay Dilemma” highlights the critical role underlays play in the long-term performance and durability of your roof. I especially loved the overview of the different (dissolving!) underlays I’ve seen and was curious about in New Zealand roofs. The green underlay is almost certainly a solvent based fire retardant kraft paper which shrinks when wet which is why it looks like it has dissolved.
The primary function of a roof underlay is straightforward: to manage any water that might penetrate the primary roof cladding, whether due to leaks or condensation, while allowing moisture to pass through it as vapour. However, the dilemma arises from the need to balance this with proper ventilation and preventing the build-up of moisture, which can lead to corrosion of the metal cladding and structural damage. As we have better insulated roofs there is less heat to keep the roof space warm and condensation from any air leaks and even from external ventilation air is increased.
Underlay needs to store, ventilate and block water:
- store condensation by absorption -especially overnight as temperature differences between the outside air and the radiation (night sky) cooled underside of the roof can lead to moisture forming
- ventilate moisture vapour to allow trapped moisture a way to ventilate out and,
- block outside rain and wind driven water and direct it into drained and ventilated cavities.
It’s crucial to select and install underlays in accordance with the New Zealand Building Code requirements to ensure compliance and the longevity of the roof. And don’t forget correct installation matters. Even the best underlay will fail if not installed correctly. Attention to overlaps, penetrations and detailing is paramount to creating a watertight and durable secondary layer of protection.