COP v24.12:Trash;
The flashing cover of a cladding rib height of less than 20 mm is insufficient to ensure weathertightness.
When a parapet wall extends above the apron it can cause additional turbulence, and will also increase the water catchment and in these areas . O ne rib cover or 150 mm is regarded as insufficient to weather either a longitudinal or transverse barge or apron flashing.
When ribbed or trapezoidal roof cladding finishes at the wall or an abutment, the aesthetic appearance of a longitudinal narrow single rib apron flashing may be unacceptable and in these cases the cover should be increased. Cover dimensions for claddings other than profiled metal are given in 7.3 Flashing Cover
R1 Street awnings or roofs where limited people can gain access either from adjacent structures or from the ground only, must be designed to ressist minimum point load of 1.8 kN.
NZS 3604 is an acceptable solution for compliance with the NZBC for light timber frame buildings not requiring specific design. It contains prescriptive dimensions for purlin spacings and fasteners , based on maximum design wind speeds of Low ( 32 m/s ) , Medium ( 37 m/s ) , High ( 44 m/s ) , Very High ( 50 m/s ) , or Extra High ( 55 m/s ) .
Some limitations of the scope of NZS3604 are:
- Timber frame construction .
- Height from lowest ground to the highest point on the roof not to exceed 10 m .
- A snow load not exceeding 1.0 kPa ( although Section 15 of NZS 3604 does provide additional criteria for 1.5 kPa and 2.0 kPa snow loads).
- private dwellings, hostels, hotels and nurse's homes ,
- factories with restricted floor loadings , and
- institutional and educational buildings with restricted floor loadings .
- buildings dedicated to the preservation of human life ,
- buildings which may host crowds ,
- publicly owned buildings containing high value contents , and
- curved roof construction .
NZS3604 uses conservative figures to arrive at a design load. Th e NZ Metal Roof and Wall Cladding Code of Practice uses the same philosophy to prescribe a value known to perform , either historically or by test , and will comply with the NZBC.
The price for this conservatism is small in comparison the overall cost of the whole of the building, and unless the building is specifically designed, designers and contractors should not deviate from the loading requirements of this section of the Code of Practice.
Classification of Wind Zones in NZS 3604 are specific to the site, because the buildings covered by this standard are limited in size. Design tables (but not design wind speed) include a local pressure factor of 1.5 over the entire structure rather than varying factors according to the position on the roof. ( see section 3.3.2 )
( S ection 3.2)
A point load is supposed to be representative of a workman with a bag of tools, which is calculated at 112 kgs = 1.1 kN. As required by AS/NZS 1170.1 the point load for restricted access roofs is taken as 1.1 kN over an area of 100 mm diameter in the case of a person ; or in the case of a superimposed load, such as an air conditioning unit, the area of contact if the load is not directly attached to the structure. A point load on a roof is always positive or downward (+).
Fully supported parapet and internal gutters should comply with section 8.4.4. It should be :
- laid to a minimum fall of 1 in 200 – 12 mm in 2.4 m;
- designed to discharge rainfall of 200 mm/h without flooding;
- not less than 50 mm deep at the highest point;
- designed to discharge the rainfall clear of the building if the outlet becomes blocked; and
- designed with expansion drips at a minimum depth of 50 mm .
The width of a rain water head or sump must be equal to that of the gutter.
The rain water head must have an overflow area equal to or greater than the area of the gutter at a level below the height of the sole of the gutter. (see drawing 8.6.1.)
Expansion joints must be provided to accommodate the thermal movement at the gutter high points between outlets. (see drawing 8.4.4.)
The roof side of a parapet gutter upstand should be joined to the roof sheeting by means of a drip edge flashing. (see drawing 11.6.1)
Paint protects metal roof and wall cladding from environmental deterioration and improves its appearance. . P aint can be either factory or field-applied , but t his section covers only factory applied systems . F or information on field applied coatings , 15.8 Overpainting
Painted coatings on a steel substrate that are coated with a metallic coating and then continuously paint ed and oven cured are known as pre-painted or coil coated. Following pre-treatment, a corrosion inhibiting primer and top-coat is applied to the out side surface and a primer coat and/or a backer coat is applied to the reverse side. This process is continuous and provides for a quality of paint finish not equalled by any other process. (see drawing 2.2.7.)
Prepainted Zinc and Zinc/Aluminium coated steel and Aluminium
Materials comprising the building envelope, although required to meet particular design criteria, should not be considered in isolation.
There are many types of corrosion that affect metal claddings but they can be divided into two main types:
- atmospheric for a metal in isolation
- electrochemical or galvanic, where the metal reacts with another metal or material
The various manifestations of corrosion within these two types are categorised in a different way, cannot be clearly defined and almost always overlap . Frequently the different types of corrosion typical of different metals and alloys do not develop separately, but are interdependent .
The nature of the interaction between various metals and electrolytes is influenced by the many permutations of the environment, the degree of pollution and the pH of the electrolyte.
It is the responsibility of both the designer and the Roofing Contractor to ensure that they do not cause corrosion by incorrect use of materials.
The major factors that affect metal corrosion are:
- t he metal ,
- t he atmosphere ,
- t he rain , and
- p ollutants
Sacrificial protection and passivity can be mutually exclusive and passivity can be so well developed that sacrificial activity is suppressed. An AZ coating does not provide the same degree of sacrificial protection as a Z, ZA or ZM coating, which does not have the non-loss passivity of aluminium.
The differences between primarily zinc and primarily aluminium coatings lies in the balance between passivity and sacrificial protection, and for roofing and cladding products in most conditions, an AZ coating provides satisfactory sacrificial protection of cut edges although not as well as a zinc-based coating. The introduction of magnesium into coatings containing aluminium is intended to enhance the cut-edge corrosion resistance of the coatings to provide better all-round performance.
As the requirements of the NZBC are performance based, it is necessary to make a subjective assessment specificallyconcerning durability of the building elements covered by this Code of Practice. While it could be assumed that roofor wall cladding can be easily accessed and therefore easily replaced, the same cannot be assumed for any flashingswhich may be embedded in plaster or could not be removed without the removal of monolithic claddings. Someflashings are half hidden and as this portion would not be subject to inspection or maintenance its failure could leadto structural degradation and therefore would not comply with the NZBC. Because replacement could be classifiedas a major reconstruction, the flashing material durability requirement is 50 years. This requirement also applies tounseen flashings and secret gutters.
The NZBC does not take into account the cost or aesthetics of replacement, so while replacement period would complywith Code, it may be not considered acceptable by the client or owner. Any pre-painted cladding will change colourover time and the replacement would be visible, and the cost of replacing one long length sheet from a distantsupplier would be disproportionate to its value.
All metal roof and wall cladding and accessories should be designed and installed to comply with the durability requirementsof the NZBC, but the economic and aesthetic consequences of replacement should also be considered.