Code of Practice v3.0 Online
The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
COP v3.0:durability; environment
The durability performance of metal roof and wall cladding depends on the macro- and microclimates, airborne contaminants, and the material itself.
The macroclimate is the general environmental category where the building is situated.
The microclimate relates to the exact location of the building and the design or position on the roof or wall. Microclimate influences include geothermal fumaroles, rain sheltering, topography and ground roughness, prolonged wetness, and exclusion of oxygen. Internal microclimates can also occur as result of the particular use of the building.
Contaminants and pollutants are corrosive influences which can affect the cladding. These can include fertiliser, soil, leaf fall-out, exhaust fumes, industrial fumes, bird droppings and the build-up of debris. Influences such as chlorides near the sea, geothermal hydrogen sulphide (H2S) or man-made gases such as sulphur dioxide (SO2) accelerate the corrosion rate by increasing the conductivity of the electrolyte and changing its pH value.
Rain provides the moisture that acts as the electrolyte in corrosion cells. Rain varies in pH because it picks up various contaminants from the pollutants in the atmosphere. Acid rain can happen in geothermal areas due to the presence of hydrogen sulphide in the atmosphere.
At 0°C metal corrosion is minimal, because colder temperatures slow the reaction. The corrosion rate of some metals doubles with every 10°C rise in temperature given the same time of wetness and environmental conditions. However, in dry, warm environments the time of wetness is decreased by faster drying times, which has the opposite effect.
Designers should be aware of macro- and microclimates and the degree of contamination. They should design their building and select materials considering a combination of all these factors.
The major contributor to metal corrosion in New Zealand is sea spray. Sea spray contains a mixture of salts consisting of 2.5 to 4% sodium chloride and small quantities of magnesium, calcium and potassium chloride. These salts make water far more electrically conductive.
Sea spray, evaporation, and infrequent rain increase salt concentrations on exterior surfaces, particularly when it accumulates in unwashed areas.
4.5.2A Airbone Salt from Sea Spray.
Onshore winds, big swells, wide generation zone and rugged coastline make ideal conditions for the production of salt aerosol.
The distance airborne salt is carried inland varies significantly with local wind patterns. Salt deposits have been measured as far inland as Lake Taupo in the North Island. Geographic or man-made obstructions, such as trees or buildings, slow air velocity and allow the air to discharge some of its salt burden, which can make the environment less aggressive. Conversely, where there are few impediments to the free flow of air, severe marine influence can extend well inland.
In high humidity levels, or when wetted by condensation, marine salts absorb water and form a chloride solution. Therefore, the effect of salt spray is greatest in unwashed areas, where salts can accumulate over time.
Where the ends of roof cladding are exposed to contaminants such as sea salt or industrial pollutants, it is good practice to provide an over flashing which discharges into the gutter or spouting. (See A Gutter-Eaves Flashing.)
- It gives a measure of protection to the underside of the roof cladding and the underlay.
- It provides support for the roofing underlay which is subject to damage from wind and UV.
- When using PVC spouting, there is a gap between the spouting and the fascia caused by the thickness of the brackets. In coastal locations where the ends of roof cladding are exposed, this unwashed area becomes susceptible to corrosion. A gutter apron can minimise this risk.
- If there is no spouting or it has a low front.
- In severe environments, wind can drive contaminants up the ribs of exposed ends of roof cladding. Metal scriber flashings or filler blocks can be used to prevent or inhibit ventilation.
The over flashing should extend 50 mm into the gutter, and the underlay finishes on the down-slope of the flashing. If there is no over-flashing to the gutter, the underlay should be extended into the gutter by a minimum of 20 mm.
In some cases, the over flashing becomes a sacrificial flashing which can extend the life of the cladding. In such circumstances, the COP recommends making the flashing from aluminium.