COP v3.0:penetrations;

External Moisture Penetrations 

This section should be read in conjunction with 6 External Moisture Overview and 8 External Moisture Flashings.

A penetration is any hole cut in a roof or wall cladding to accommodate projections such as pipes, ducts, chimneys, roof lights doors and windows.

This section focusses on roof penetrations only. The type of penetration design is determined by:

  • the size of the hole,
  • shape,
  • the roof pitch,
  • the type of roof,
  • the catchment area,
  • placement on the roof, and
  • aesthetic requirements.

Designers are urged to consider what type of penetration design matches the building application and their customer’s needs, and detail accordingly, rather than allow the installer to make an on-site decision.

Many of the penetration details drawn in the Code of Practice (COP) are included in a step by step How -To  Guide published by the Roofing Association of New Zealand.  A copy of this may be obtained by contacting

9.1 Penetrations Minimum Pitch 

Penetrations may be executed in roofs of any pitch down to the minimum pitch allowed for that profile. Penetrations are allowed in the portion of curved roofs where the pitch falls below these limits, providing the penetration flashing bridges the apex and terminates where the pitch is a minimum of 3°.

9.2 Additional Support 

Terminating a sheet above and below a penetration creates an end span situation and sheet support and fastener patterns should be checked accordingly, or additional support must be provided. (See 3.5.8 Continuity.)

Penetrations requiring removal of a roof section greater than 300 mm x 300 mm require additional supporting framework. Ideally this should be in position before the roof cladding is fixed; alternatively, the supporting framework should be in position before cutting a hole in the cladding.

The additional support for larger penetrations must have the same strength as the adjacent purlins. Purlins and the support structure must be designed to take the additional weight of any plant exceeding 100 kg. Structural members must not be removed without engineering calculations.

9.2.1 Penetrations Safety 

The person who cuts a hole greater than 600  mm x 600 mm in the roof is responsible for safety precautions, preventing workers from falling through the gap. A hole of this size is regarded as a hazard under the Health and Safety in Employment Act.

9.3 Penetration Durability and Compatibility 

All fittings and materials above a penetration must be made from compatible materials and there must be no runoff onto the roof from incompatible materials or corrosive discharge.
Condensate and outflow from any sources such as air conditioning units, solar units or hot water pipes must not be discharged onto metal roof cladding but must be separately drained to an inert gutter or downpipe.

Level back curbs will not have the same durability as arrowhead or cricket designs and may require maintenance of the coating to match the durability of the roof cladding.

9.4 Penetration Design 

It is the designer’s responsibility to select the type of penetration flashing appropriate to the design requirements and the client’s expectations. Penetrations can be broadly put into two categories: Sheetmetal flashings and Boot flashings.

The positioning of the penetration in relation to the apex, eaves and other architectural features must be taken into consideration when selecting the type of flashing to be employed.

9.4.1 Sheetmetal Penetration Back Flashings 

The first decision should be the back flashing, should it be over the profile Over-Flashed (Watershed) Back Flashings or under the profile Under-Soaker Back Flashings. Over-Flashed (Watershed) Back Flashings 

Watershed back flashings are easy to install and to weatherproof, particularly if the roof is already in place. The drawbacks are their limits in width and, sometimes, noise or condensation issues. Long lengths of watershed flashings may require multiple end laps which are vulnerable to leakage. Where there are end laps or foot traffic is expected on the watershed flashing, the flashing must be supported in the pan or the profile by rigid closed cell foam or similar.

In many residential cases where the flashing is visible, the aesthetic values of watershed flashings may render them inappropriate for this application, unless the penetration is situated close to the apex.

The maximum width of a watershed flashing is controlled by the coil width of 1.2 m The practice of making wider watershed flashings by running flashings horizontally with laps at 1.1 m is not acceptable, as the numerous joins are prone to leakage. Wider watershed flashings can be fabricated using longitudinal standing-seam techniques on suitable support. Under-Soaker Back Flashings 

Soaker back flashings are visually attractive and are less prone to noise or condensation issues. They are relatively easy and economical to install at the time of roof laying, but more difficult and costlier if post installation is required.

9.4.2 Curb Design 

Curb design (i.e., level, arrowhead, or cricket) depends largely on the penetration width and the expected amount of debris, e.g., tree leaves. Proximity to the apex determines penetration flashing design (i.e., over flashing, under-soaker, or hidden gutter). Level Back Curbs 

Level back curbs are the most common solution for flashing penetrations and are the easiest to fabricate and install.

They may tend to collect debris as they have little or no transverse fall, which can limit durability. However, with normal maintenance when manufactured from the same material as the roof they should achieve the durability requirements of the NZBC.

For penetrations wider than 600 mm, or those in aggressive environments or in situations where maintenance is difficult, a freer draining design such as an arrowhead or cricket is preferable. Arrowhead Back Curbs 

Arrowhead back curbs have a diverter that provides transverse fall for diverting rainwater, enabling them to accommodate bigger catchment areas and self-cleanse. They have a small flat area at the base of the arrowhead that may require maintenance. Cricket Back Curbs 

Cricket back curbs divert water with less turbulence than either arrowhead or flat back curbs and have no flat areas to catch debris. They may be fabricated from the same material as the roof or welded from 1.6 mm aluminium and powder-coated to match the roof colour, to give a durable and matching solution. They offer the most durable and weathertight solution to penetration back curb. Sheetmetal Penetration-Flashings Reference Sheetmetal Penetration-Flashings Quick Reference

 Proximity to the apex determines back flashing design

Close to apex

  • Aka Dry pan or Watershed
  • Suitable for retrofitting

Distant from the ridge

  • AKA Tray
  • Best solution for mid-roof penetrations

Adjacent to the Eave
Hidden Gutter

Only practical for penetrations located neat the eaves
Penetration width and debris determine curb design   

Narrow/Little Debris

  • Only suitable for small catchments not prone to leaf debris.
  • Relatively easy to manufacture on site Level Over-Flashing: Corrugate Level Over-Flashing: Trapezoidal Level Under-Soaker: Corrugate Level Under-Soaker: Trapezoidal Level Hidden Gutter: Corrugate

Medium/Moderate Debris

  • A diverter is formed by joining two trays into the shape of an arrowhead on site.
  • Suitable only for small catchments not prone to leaf debris.
  • Parts can be pre-ordered and final fitting done on site Arrowhead Over-Flashing: Corrugate Arrowhead Over-Flashing: Trapezoidal Arrowhead Soaker: Corrugate Arrowhead Under-Soaker: Trapezoidal


Wide/Much Debris

  • Most suitable for larger catchment areas.
  • Requires careful calculation and off-site fabrication. Cricket Over Flashing: Corrugate Cricket Over-Flashing: Trapezoidal Cricket Under-Soaker: Corrugate Cricket Under-Soaker: Trapezoidal


9.4.4 Maximum Area Above Penetration Calculator 

A responsive online tool for calculating Maximum Area Above Penetrations is available at

Before using this calculator, please read 5.2 Roof Drainage Design.

Penetrations concentrate runoff from above into a single trough. Use this calculator to get the maximum allowable area above penetrations by entering the values in the designated fields.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
1: =  mm per metre
Select whether runoff will drain on both sides of penetration or just 1;

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only and is not to shape or scale.
Wetted Perimeter=49Pitch=76Cross Section Area=272Free Surface Width=42Depth=17Wetted Perimeter=85Pitch=76Cross Section Area=585Free Surface Width=76Depth=17
Illustration is for explanatory purposes only and is not to shape or scale.
DepthPitchCrestCapillary DepthPanDepthPitchCrestPan
Illustration is for explanatory purposes only and is not to shape or scale.
Wetted PerimeterPitchCross Section AreaFree Surface WidthWetted PerimeterPitchCross Section AreaFree Surface Width
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
Must be less than the upstand, D
 x   mm
Select Manufacturer (if applicable) and Profile
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
Width of the pan.
Calculated result from (Pitch) - (Crest).
Width of the crest (top of rib).
Total depth of profile.
Depth of profile from the pan to the height of the capillary tube.
Data provided by a manufacturer, especially for irregular profiles.
Data provided by a manufacturer, especially for irregular profiles.
Data provided by a manufacturer, especially for irregular profiles.
Data provided by a manufacturer, especially for irregular profiles.

This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile

9.5 Boot Flashings 

A boot flashing is a proprietary EPDM flashing designed to weatherproof cylindrical penetrations protruding from a roof or wall. The top is trimmed to form a tight weatherproof collar around the penetration, and the base is formed with a series of concentric rings to the underside and a malleable stiffener of aluminium which is dressed to conform to the shape of the roofing profile. It is generally top-fixed to the roof surface with screws or rivets, and sealant.

The Profiled Metal Roofing COP allows pipe penetration flashings to be fitted directly to the profile or on to an over flashing. Pitch limitations depend on the method used and the cladding profile.

Direct-fixed options are pitch sensitive. When laid directly on to the profile at too low a pitch, they will entrap water rather than allow it to discharge over the profile crests that they traverse. The practical limits of direct-fixed boot flashings that cross an entire pan are 8° for standard corrugated and 10° for low rib trapezoidal products. Where the base of a boot does not obstruct a pan it can be direct-fixed to the minimum pitch for that profile.

Direct fixed applications for high rib trapezoidal profiles and trough sections vary according to the profile, and the size and position of the penetration. For these applications, the manufacturer should be consulted or the flashing can be attached to an over flashing, or a top fixed soaker type can be used.

Where the penetration is wide such as a chimney flue casing, and the penetration is far from the apex, soaker flashings may be used where the profile ribs are cut back so water can divert into the adjacent pan.

Where overall width is not a constraint, directly fixed boot flashings should be installed with their edges diagonal to the fall of water. Where this is not practical, they may be laid square at pitches of 10° or more.

Where boot flashings traverse a lap, the lap must be fully sealed or other actions must be taken to avoid leaks through capillary action. Where possible the fixing of a boot flashing over a lap should be avoided

The vertical sections of a boot flashing must not constrict the free flow of water. Where more than 50% blockage of the pan occurs other penetrations must be considered, or catchment calculations of the capacity of the remaining pan area should be made. (See 5.3.7 Gutter Capacity Calculator)



9.5.1 Boot Flashings to an Over Flashing 

Boot flashings fitted to an over flashing are acceptable at pitches down to the minimum of that allowed for the profile. Typically, this is 8° for standard corrugated, and 3° for trapezoidal and trough sections. These boot flashings must be fixed diagonally to the fall of the roof at pitches below 10°.

Over flashings can be continuous to the apex, or terminate with a soaker at the upper edge.

9.5.2 Other Applications for Boot Flashings Plant Room And Conduit Penetrations 

Where flexible power conduits or telecommunication cables are required to penetrate the roof cladding, accessibility can be improved by using P.V.C pipe fittings and an E.P.D.M. flashing to weather a number of conduits.

Cable penetration flashings must be goose-necked. It is not acceptable to exit cables through a vertical flashing such as a boot flashing where sealant is the only barrier to water leakage. PVC and E.P.D.M Flashing

This flashing should be fixed next to the purlin for support.


Safety bollards for fall arrest anchorages are required where regular maintenance is required, and these can also be weathered by E.P.D.M. flashings. Mechanical Services 

Where plant room supports are required to penetrate the roof cladding, the designer should provide the support framing from Circular Hollow Sections (CHS) in preference to Rectangular Hollow Sections (RHS) or other hot rolled steel sections, because it is easy to flash the CRS with E.P.D.M. flashings. This procedure allows the E.P.D.M. flashings to be slid over the pipe framing during erection, and avoid the necessity of using retrofitting types.

The support framing should be in place, but below the top of the purlin, before installing the roof cladding. That allows the cladding installation to proceed without having to weatherproof multiple penetrations at the same time.


(optional caption)

Proprietary support systems are available for lightweight support through to the purlins.


These types of supports provide clearance for cleaning but should not create an unwashed area underneath them.




9.5.3 Flush Penetrations 

Penetrations such as roof window may be mounted flush with the crests of the roofing profile.  In such cases, the side flashing onto the roof shall be the same as required for a barge cover. The flashing termination onto the roof window shall be as per window manufacturers requirements