COP:penetrations; penetration-installation-requirements

8.2 Penetration Installation Requirements 

8.2.1 Responsibility 


The designer must provide details for the additional support required for all penetrations greater than 600 mm x 600 mm. The weight of any penetration such as an air conditioning unit, or plant platform, must not be imposed on the roof cladding without additional support.
The removal of structural members must not occur without written permission supported by engineering calculation.
The person who cuts the hole in the roof must be responsible to ensure that sufficient additional support is provided for the roof cladding and the penetration flashing.
All holes greater than 300 mm x 300 mm cut in roof cladding must have additional structural support.

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.


8.2.2 Durability 

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.

8.2.3 Corrosion 

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.

All fittings, such as fans, fitted above a penetration flashing, must be made from compatible materials and there must be no runoff onto penetration flashings from incompatible materials.
Condensate from air conditioning units must not be discharged onto metal roof cladding, but must be separately drained to an inert gutter or downpipe.

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.


8.2.4 Catchment 

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 penetration side flashing may only restrict water flow by up to 50% of the normal pan width, and may only do so if:
  • 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.
Symmetrical sinusoidal and symmetrical trapezoidal profiles must have full pan width for free discharge. See 5.3.3 Roof Cladding Capacity.
When the total catchment area exceeds 40 m², the discharge must be spread over two or more pans to provide for additional drainage. When the catchment area includes the walls of adjacent buildings, it must be calculated as shown in 5.3.1 Catchment Area
In snow zones more than 500 m above sea level, only full pan width discharge must be used.

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.

No penetration must be made in the pitch area between 0° and 3° for trapezoidal or 8° for corrugate profiles on a curved roof.

8.2.5 Obstruction 


A roof or wall penetration such as a dormer or a vent acts as an obstruction to the rainwater flowing off the roof.

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.

Designers and roofing contractors often assume that one corrugation is sufficient to drain the catchment without making the calculations necessary to know if this assumption is correct. The discharge capacity of the profile should be calculated as shown in 8.2.6 Discharge Capacity.

8.2.6 Discharge Capacity 

The back and side curbs of roof penetrations must be regarded as internal gutters and, therefore, must be designed with the same parameters as gutters.
As the catchment area above any penetration concentrates the runoff, the design should allow for turbulence at the discharge exit and the extra capacity required. The extra capacity required depends on the catchment area, the profile, and rainfall and can require the discharge to be spread over two or more pans.
Corrugate and symmetrical trapezoidal profiles do not have the rainfall carrying capacity of those with wider pans or deeper profiles. To determine if one pan or corrugate valley discharge is adequate for the rainfall, the catchment area behind the penetration should be calculated as per table 6.1.6.

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.

Maximum discharge capacity for penetrations discharging equally per side in a 100 mm/hr Design Rainfall Intensity (DRI) area.
Max. catchment
Symmetrical trapezoidal
5 & 6 rib trapezoidal or trough

5 m²
10 m²
20 m²
This table is based on equal discharge to each side of the penetration.If the catchment area per side is unequal or exceeds that shown in the table the discharge must be spread over two or more crests each side. The maximum catchment area shown in the table must also be adjusted proportionately if the design rainfall intensity is greater than 100 mm/hr.

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.

8.2.6B Example 1

Five-rib trapezoidal with a 1 m wide penetration 15 m below the apex in a 100 mm/hr DRI area.

WidthLengthDRICatchment per side
1151007.5 m²

Result—OK to Use


8.2.6C Example 2

Corrugate with a 1.2 m penetration 10 m below the apex in a 140 mm/hr DRI.

WidthLengthDRICatchment per side
1.2101408.4 m²

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


8.2.6D Penetration Discharge

The pitch of the side curb of the penetration flashing shown in 8.2.6D Penetration Discharge depends on the pitch of the roof and the height of the rib. However, it can be reduced to 1.5° as allowed for a cricket back curb. 


8.2.7 Sealing 

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.

8.2.8 Alternative Materials 


Where the use of sheet metal is not the most practical method to flash a penetration, alternative materials can be used, provided they are compatible and comply with the design requirements and conditions outlined in this COP.