Metal roofs can be designed either as ventilated or non-ventilated structures.
When moisture gets into building materials, it may cause irreparable damage and have an adverse effect on adjacent components.
In the case of unventilated structures it is essential to prevent moisture being introduced during the construction phase. This calls for careful planning on site as well as the use of suitable protective covering in all cases.
Use of Underlays
Underlay is used under interlocking tile roofs for a number of different reasons. Following installation of the substructure its function is to protect the building from the weather and metal coverings from damage caused by the materials of the substructure. It is said that an additional special underlay reduces noise levels when it rains.
Sound measurements performed by consultant engineers revealed that classical roofing felt has virtually no effect on sound suppression – or none at all.
There is a wide range of underlays available, with some only coming into use more recently. The classical types found on international markets include reinforced and high temperature bitumen sheeting.
Structured underlays, for example RHEINZINK-VAPOZINC or AIR-Z have been used for over 15 years. Their structured mesh allows any unexpected moisture between the roof covering and substructure to dry out. At the planning and tendering stage requirements for underlay with RHEINZINK seam systems should be based on the technical needs of the construction project. This greatly depends on the climate conditions at the installation site and the method of construction to be used.
Installation of double layers of underlay:
It is sometimes observed that, for whatever reason, two sheets of underlay are laid flat one over the other - we do not recommend this. It results in a capillary effect between the underlays, so creating a significant reservoir of water between the two layers of the underlay system with any ingress of moisture, for example if installed damp or with leakage. This may then cause damage to the substructure or corrosion of the metal covering. The problem is further compounded if the upper layer of underlay absorbs water.