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.
The person who cuts a hole greater than 600 mm x 600 mm in the roof is responsible for safety precautions to prevent falling through. A hole of this size is regarded as a hazard under the Health and Safety in Employment Act. Safety mesh complies with this requirement.
Once a hole is cut in self-supporting metal cladding, the continuity provided by the sheeting is destroyed, and the ends of the sheeting should then be regarded as simply supported or as end spans. (See 3.5.8A Continuity.)
The end spans should also be reduced to a minimum of two-thirds of the intermediate span to perform as specified for point load or U.D.L. To fix the roof cladding, the end spans require additional fixing support upside and downside of the penetration.
To overcome the tension induced in the cladding when cutting a hole in a sprung curved roof, additional fixings should be used to hold the roof cladding in position at the front and back of the penetration.
Often, the responsibility of the roofing contractor does not include the installation of the over-flashings or the weathering of the unit within the penetration. However, it is his responsibility to inform those persons who are concerned of the likelihood of deterioration from runoff if inappropriate materials are used.
It is also the responsibility of the roofing contractor to ensure that for all the work covered in his contract, the correct selection of materials, good design, and good installation practice is provided as a warranty can be voided by poor design or workmanship.
A roof warranty of penetration flashings may be voided if they are installed by other trades, or not in accordance with the requirements of the manufacturer's warranty or this Code of Practice. The COP strongly recommends that designers include all roof penetrations within the scope of work of the roofing contractor.
The back curb of penetration flashings manufactured from metallic coated steel and which have no fall will collect dirt and debris. Due to the presence of continued moisture, the subsequent ponding can cause deterioration and premature corrosion. When this flashing design flashing is manufactured from metallic coated steel, it is not covered by a manufacturers warranty. Therefore, a diverter or cricket design with a back curb and a minimum fall of 1.5° should be used.
When the penetration design provides for the minimum fall and is maintained as described in section 13.0. of this Code of Practice, the penetration is covered by the normal manufacturer's warranty.
The designer and the roofing contractor should ensure that the methods and materials used will satisfy the durability requirements of the NZBC. The specifier and contractor should follow any manufacturer's specific installation requirements, any certification or appraisal conditions, and this Code of Practice to comply with the NZBC. The durability compliance will only be met if the maintenance requirements of this Code of Practice are also followed.
Even if the designer has failed to detail the requirements, the responsibility remains with the contractor, who has a duty of care to fulfil the requirements of the NZBC and this Code of Practice, unless a written agreement has been made to the contrary.
Designers and tradespeople should have an understanding of the mechanism of the corrosion process to comply with the requirements of this section of the Code of Practice. See 4 Durability.
Corrosion is defined as the reaction of a material with its environment, resulting in measurable alteration which may impair the performance of a component or an entire system. Corrosion can result from direct contact with another metal or substance, or as the result of run-off, fall-out, poor design or installation that would allow ponding. In some cases, metal can corrode when it is either exposed to or denied contact with oxygen.
Where the roofing contract does not include the over-flashing or installation of the cowlings for a penetration, the designer and the installer assumes responsibility to comply with this COP, as the roofing contractor is not responsible for the work of others.
The profile capacity, the roof pitch, the catchment area and the Design Rainfall Intensity (DRI) should all be considered to ensure that the catchment area behind the penetration can be drained without flooding.
- the design rainfall is no greater than 100 mm/hr;
- the sheeting length is less than 40 m;
- the catchment is less than 20 m²;
- the profile is asymmetrical; and
- the pan is at least 100 mm wide.
A diverter or cricket penetration design should be used when:
- the width of the penetration exceeds 600 mm;
- the catchment is greater than 40 m²; and
- there is a likelihood of snow.
As rain flows faster on steeper pitches and will discharge at a greater speed, the capacity at the discharge area is determined by the roof pitch as well as the capacity of the pans, the height of the profile, and the catchment area.
Penetrations should ideally line up with the ribs of the sheeting. Because this does not happen often, the module of the roof cladding determines the width of the penetration.
The symmetrical profiles normally used on draped curved buildings to avoid panning, do not have sufficient rain carrying capacity to include large penetrations; consequently, the discharge should be shared over a number of corrugations or pans on a curved roof.
All metal roofs in N.Z. are required to have a minimum fall of 3° and this is not possible when using a drape curve design.
The back and side curbs of a penetration flashing act as a gutter draining the catchment behind the obstruction which often discharges onto the roof into one corrugation or pan of a profiled sheet.
The position, orientation, penetration design, and the roof pitch will determine how much the penetration obstructs the free flow of rainwater. A cricket penetration design will shed rainwater more efficiently than a flat back curb because it will reduce turbulence. See 17.7 Cricket Penetration Patterns.
The water carrying capacity of the roof cladding profile becomes critical when the penetration is wide or is distanced from the ridge. For this reason, rectangular penetrations should be placed lengthwise or at 45° down the roof, and a second penetration should not be placed in tandem down the roof slope as this can cause flooding of the profile at the lower penetration.
Where the use of metal cladding is unsuitable for multi-penetrations alternative materials should be considered. See 8.2.8 Alternative Materials.
Similarly, asymmetrical trapezoidal sections may require the discharge to be spread over more than one pan.
Where it is necessary to increase the penetration outlet to more than one pan of a trapezoidal profile, the side curbs can be tapered from the pan to the height of the rib as shown in 8.2.6D Penetration Discharge.
5 & 6 rib trapezoidal or trough
The discharge capacity is limited by the capacity of one side of the penetration flashing (half the total catchment). For Example, the maximum length of run for a penetration 1m wide using corrugate is 10m.
Five-rib trapezoidal with a 1 m wide penetration 15 m below the apex in a 100 mm/hr DRI area.
|Width||Length||DRI||Catchment per side|
Result—OK to Use
Corrugate with a 1.2 m penetration 10 m below the apex in a 140 mm/hr DRI.
|Width||Length||DRI||Catchment per side|
Result—Exceeds table. Discharge over two crests as per drawing 6.1.6B
If the catchment area is greater, the discharge is spread over two or more pans as shown in 8.2.6D Penetration Discharge
Although the design principles are the same for all metals, the method of sealing penetration flashings depends on the metals in use.
Galvanised steel, zinc, and copper can be soft soldered, but not aluminium, unpainted and painted Z and AZ coatings. The acceptable method of sealing these materials is using either a neutral cure silicone sealant or a butyl sealing tape. These sealants should only be used in conjunction with mechanical fasteners.
The penetration flashing should be made weathertight without relying on sealants as a first-line defence against water ingress. When a sealant is used to prevent the accumulation of dirt in the joint, the excess sealant can be removed by a plastic spatula or purpose made plastic scoop, because excess sealant collects dirt which can cause corrosion and it is unsightly.
Sealant should always be used in conjunction with mechanical fastening and be applied between the two sheets to be lapped before they are fixed. It is unacceptable to apply the sealant after the joint has been made. See 7.9 Sealants.