VM ZINC® and water or vapor In general, the presence of water inside metal roofing and cladding complexes accelerates corrosion and thus reduces the life span of the work. VM ZINC® reacts to the main components of the atmosphere particularly in the presence of H2O (water), SO2 (pollution), NaCl (sea spray) and CO2 (CO2). The first reaction of titanium zinc to oxygen and water is the formation of zinc hydroxide Zn(OH)2, which, when there is sufficient concentration of CO2 in the air, allows the protective layer of patina or zinc hydroxycarbonate to form. It is therefore important that the surfaces of the VM ZINC® are ventilated so that there is a sufficient supply of CO2 to allow the formation of the protective patina. This applies particularly to the underside of the VM ZINC®. VM ZINC® PLUS, a VM ZINC® titanium zinc with a protected underside, meets the requirements related to a lack of sufficient ventilation of the underside of the titanium zinc. This product is mainly intended for warm roof applications, where an insufficient supply of CO2 to the underside of the titanium zinc prevents the formation of the patina. It is also intended for solid wood or reconstituted panels the compatibility of which is compromised by the accumulated effects of humidity and the type of treatment. Several factors contribute to the appearance of water on the underside of titanium zinc: . poor management of condensation phenomena, . poor management of water tightness, . not enough care taken with installation. Management of condensation Good management of condensation is necessary when choosing components for a roofing or cladding complex. Condensation occurs in the following situations: . Differential between the temperature of metals (residual radiation) and the day/night air temperature cycle (natural dew). . Migration of water vapour through the layers of the building envelope materials from warmest to coolest environment. The use of vapour retardant and/or waterproof membranes helps to limit the transfer of water vapour from the warmest to the coolest environment but does not eliminate it completely. Designing a building envelope complex according to recognised good practice should avoid the formation of condensation on the inside surfaces of the building in air change conditions that meet minimum requirements. Management of water tightness The combined action of rain and wind are natural climatic occurrences which must be taken into account when deciding the degree of water tightness of the building envelope. With regard to roofing applications this concerns in particular: 1. the methods for joining and/or overlapping the roof components, 2. junctions where the surface covered meets breaks in the roof geometry, chimneys, lift extrusions, skylights, upstands against walls, 3. valleys. * Snow loads In regions where the average temperature is below 0°C for a minimum period of three weeks snow lies. Moreover, cycles of freezing / melting contribute to the transformation of the snow into ice which results in: 1. an increase in the density of the snow, 2. an increase in the hydrostatic pressure of the water trapped inside the transformed snow, thus compromising the water tightness of the roofing systems either by simple infiltration through overlapping elements, or by increasing capillary action through crimped components. Therefore, special design should be chose in order to control such constraints.