THE FUNCTION OF ROOF UNDERLAYS
Roof Underlays are required in virtually all modern tile and slate pitched roof construction and perform a number of key functions.
- Secondary line of defence
Very few tile or slate roofs are completely effective at resisting wind driven rain and snow, even when laid within their pitch and headlap limits. The underlay is designed to provide additional protection to the internal roof and building, by arresting any moisture that gets through the roof covering and safely drain it away to the gutters or other rainwater systems.
- To reduce wind uplift on the roof covering
Wind that blows over a pitched roof generates positive and negative wind pressures which can, in extreme conditions ,cause roof covering damage. The roof underlay helps to reduce the wind loading applied to the roof covering by absorbing a proportion of the wind load, especially when insulation is at horizontal joist level. The upward deflection of the roof underlay under maximum negative pressure must be small enough to avoid contact with the underside of the roof covering. This helps reduce the wind pressure on the roof covering and so the risk of damage. Underlays that cannot adequately resist the imposed wind loads for given locations in the country and required batten gauges cannot be used without significant risk to the roof covering
- As a temporary protection layer before the roof covering is applied
It is often necessary for the roof underlay to act as a temporary protection layer before the roof covering is installed. This is especially true of large roofs and where protection of insulation and building fabric is important, in the case of re-roofing projects. It is good practice to keep this period of time to a minimum especially in summer months when the intensity of UV radiation is at its peak. Virtually all roof underlays are polymeric based and all are vulnerable to UVA and UVB degradation if left exposed for long periods of time. Underlays should be suitably stabilised to resist both UV and heat ageing for their design life.