COP:penetrations; penetration-design

8.3 Penetration Design 

8.3.1 Nomenclature 


The nomenclature used in describing penetration flashings may differ in different parts of New Zealand (See 2.1 Definitions):


Level back curbs are only acceptable as penetration flashings when they are lapped and installed without ponding.
In practice, this is very difficult to achieve; diverter and cricket designs should be used where the width of the penetration is more than 600 mm or where the pitch is less than 10°.
The intention of cricket and diverter flashing designs is to provide a minimum fall of 1½° to provide a self-cleaning action of rainwater at the back curb of the penetration. This action more effective as the pitch of the roof increases, but the designs are suitable for minimal fall roofs as described.
The back curb and the cladding can be cut at an angle, but roof fasteners should not penetrate the back curb flashing.

Roof underlay should lap the back curb by a minimum of 50 mm.

Penetration flashings must:
  • have back and side curbs 50 mm higher than the cladding profile;
  • have side and back curbs with a minimum height of 100 mm—except where the penetration is wider than 600 mm or the catchment area exceeds 20 m², when the height must be increased by the height of the profile rib;
  • not restrict maximum likely water flow;
  • be at least as durable as the roof cladding;
  • not rely solely on sealant for the prevention of water ingress;
  • have enough fall to avoid ponding and debris build-up;
  • have a back-curb minimum lap of 150 mm under the cladding unless it is sealed;
  • have a minimum 100 mm clearance between the end of the sheeting upside of the penetration and the back-curb upstand;
  • have no fasteners that penetrate the back curb; and
  • have primary fastening to the structure.

Where a penetration is used on a trapezoidal ribbed roof, the ribs upside of the penetration should be closed by folding, cutting, and fastening, or with end caps or plastic closed cell filler strips formed to match the profile.

The ends of the ribs should be closed, but not sealed, to allow ventilation and to disperse any condensation draining into this space. Building underlay should overlay the flashing to drain condensation, because water ingress at the penetration caused by condensation is often attributed to a leaking flashing.

The end of the sheets on the downside of the penetration should be stopended vertically to the full height of the rib using a pull-up stop-end. If the position is exposed or in a high wind area, the ribs should be cut and the pans dog-eared or foam closures used. See 13.1.3 Stopends.

In an exposed or high wind area, it is necessary to provide a further wind barrier by using closed cell filler blocks to reduce the pressure differential in this cavity.

The design of a penetration flashing must not rely on sealant alone and must ensure that the lap is sufficient to shed water naturally.

The over-flashing skirt or apron should overlap the curb by a minimum of 50 mm and have a stiffened edge; it is not normally fixed to the side-curb. If the site exposure or the size of the skirt necessitates fixing to the side curb, the fixings should provide for expansion and for the vibration that occurs on penetrations for venting fans or other dynamic equipment.

All side curb flashings should be made from one piece, having an upstand the same height as the back-curb, and a side-lap that overlaps the sheets.

The side-curb can be cranked or splayed to preserve the penetration hole size, or the skirt can be cranked or splayed to match the profile module. See 8.3.7F Splayed Skirt.


8.3.2 Diverter Or Cricket Designs 

A cricket metal roof penetration flashing is designed and constructed to divert water around a penetration and to avoid ponding.
Either a diverter or a cricket flashing design is required to avoid the build-up of debris behind the back-curb of a square or rectangular penetration flashing. It has to comply with the manufacturer's warranty, and a 1.5° fall is the minimum considered to be self-cleaning.
If the roof pitch is at the minimum of 3°, it is necessary to increase it to provide the 1.5° fall to the back curb. The amount of increase will depend on which angle of cricket or diverter flashing is used. See 17.7 Cricket Penetration Patterns
The pitch of a valley, diverter or cricket flashing will always be less than the roof pitch, but the valley pitch depends on the angle of the valley to the roof. See  Valley Gutters.


8.3.3 Penetration Flashing Installation 

All penetration flashings should have all-around clearance so that vibration, wind or expansion does not induce any stress or noise at the juncture with the roof. If the penetration flashings are fixed positively to the roof, they should be considered a part of it and expansion allowance should be provided. A 10 mm minimum clearance allowance should be made for movement between the flashing upstand and the penetration structure.

A minimum of 100 mm clearance must be provided between the end of the sheeting upside of the penetration and the back-curb upstand to provide for any temporary obstruction or abnormal runoff. The back-curb must be deep enough to provide 150 mm roofing cover and roofing fasteners must not penetrate this flashing.

All back curbs should pass the 'tennis ball test', which requires the free passage of a tennis ball down the roof, past the penetration. N.B. Back curbs without fall will fail this test.

Roof underlay should lap the back curb by 50 mm.

When several penetrations are in tandem or close proximity and an alternative design is considered appropriate, another material such as membrane roofing can be used. Such materials should be compatible with the metal cladding and be designed with the same parameters as metal penetrations as described in this COP. See 8.2.8 Alternative Materials
It is usual with low pitch roofs to fold the back and side curbs and the front apron at 90° to the roof, which means a transition is needed if the unit within the penetration is required to be vertical, or the front and back curbs should be folded to suit the roof pitch. See 8.3.3A Penetration Upstand.
There are two conditions under which a different design of penetration flashing may have to be considered.
  • When it is provided in conjunction with the laying of the roof.
  • When it is provided as a retrofit.

It is accepted practice when roofing large commercial or industrial buildings that the roof cladding will be laid over the openings for any intended penetrations, which means that timber or other support should not extend above the purlin line at the time of laying.

The penetration design must be determined before the roof cladding is installed temporarily over the opening, and the extra purlin support for the cladding must be in position before roof cladding work commences.
This technique has the advantage that it closes the building in quickly and, also, provides safety for workers since penetration openings are regarded as a safety hazard. The preferred method is to order longer sheets so that they may be lapped at the penetration.
There are two positional variations that can affect the design of the flashings used to weather any penetration.
  1. When the opening falls between the ridge purlin and the penultimate or next purlin down the roof, a back flashing, known as a watershed, can be run up to the ridge for weathering, as shown in 8.3.8A Watershed (a)
  2. When the opening falls between the gutter purlin and the penultimate or next purlin up the roof, a flat tray under-flashing known as a soaker can be used to drain directly into the gutter.
All other positions on the roof require soaker flashings Type A. See 8.3.7 Type A Soaker

8.3.4 Laps 

There are two main types of penetration laps described.

Type A:

Type A  is the normal type of roof cladding flashing overlap, which is not necessarily mating or sealed and has a minimum cover of 150 mm. It can be an under or overlap. An under-lap on a flat sheet, however, requires a flattened hem or hook. Sealing is required if the lap is exposed to the wind, if the pitch is below 10°, or in a very high design wind zone.

Level back-curbs without fall can be sealed if snow or ice is to be expected, but 45° diverter or cricket flashings do not require sealing. Snow and hail do not cause leaks, as it is only when they melt and water discharge is blocked that leakage can occur.

Type B:

Type B laps are completely filled with sealant, are a minimum of 25 mm wide, used with mating surfaces, and is mechanically fastened using 4 mm sealed rivets at a minimum of 50 mm centres. As all laps are required to be self-cleaning, they should be made in the horizontal plane, i.e., across the roof. If sheets or flashings require being lapped vertically ( i.e., down the roof) they should also be angled to be self-cleaning. See 7.3.1 Flashing Laps.

Because of the many permutations of roof pitch and penetration width, it is not possible to provide a universal design or pattern for penetrations. It is, however, possible to provide a pattern for cricket flashings and acceptable angles for diverter flashings using the same method described in 17.7 Cricket Penetration Patterns


8.3.5 Details 

The penetration details drawn in this COP have been endorsed by the New Zealand Metal Roofing Manufacturers Incorporated (NZMRM Inc) and the Roofing Association of New Zealand (RANZ), as being acceptable trade practice. The designs are conservative because the most unfavourable circumstances have been assumed, i.e., minimum pitch, ribs not aligning with the penetration, and maximum rainfall.

The details and techniques can be used in most circumstances and can be combined in various ways. There are many types of acceptable penetration flashing designs, but some are only suitable where the roof pitch is greater than the minimum recommended pitch for that particular profile.

Some are acceptable practice only, and the preferred method is recommended.

All penetration flashing details should be planned; on-site improvised solutions are not acceptable. Where the skill of the roofing contractor does not extend to sheet metal work, other people possessing those skills should be used for the design and manufacture penetration flashings.

All penetrations over 300 mm wide require the additional structural support of either extra purlins or plywood with a minimum thickness of 12 mm. H3 treated plywood should be securely fastened to the structure and provide not only support for the flashings and sheeting, but is also required for additional fixing.

The penetration flashing must be separated from treated plywood support by a water-resistant underlay.





8.3.6 Types Of Penetration Flashing Design 


8.3.6A Four Types of Penetration Flashings

Type A
Under-flashings are those that drain at the plane of the roof pan.

Soaker (under) flashings :
  • are suitable for all pitches;
  • is the preferred detail;
  • bring the side curb in line with the rib;
Installation of under-flashings.
  • Leave minimum all-around clearance of 10 mm.
  • All upstands need a minimum height of 100 mm.
  • Drill for rivets before sealing.
  • Sealed laps must have a minimum width of 25 mm.
  • Order longer cladding sheets to allow for lapping.
width=230 height=193 /> </td> </tr> <tr> <td> <p> <strong style=color: #000;> Type B</strong> <br /> Overflashings are those that drain at the plane of the rib of the roof.<br /> These are also known as Watershed or back flashings.</p> <p> <span style=color: #000;> Watershed flashings (over or back) : </span> </p> <ul> <li> are simple; </li> <li> are recommended when the penetration is more than 300 mm wide and within the first purlin from the ridge. </li> <li> are not suitable for penetrations of over 1.1 m wide ; </li> <li> are not universally accepted aesthetically pleasing; and </li> <li> can be used in conjunction with a sprung or over-roof design.</li> </ul> </td> <td> <img  data-cke-saved-src=/sites/default/files/cop_22images/diag_6.2.6b.gif src=/sites/default/files/cop_22images/diag_6.2.6b.gif alt=

Type B
Overflashings are those that drain at the plane of the rib of the roof.
These are also known as watershed or back-flashings.

Watershed flashings (over or back):
  • are simple;
  • are recommended when the penetration is more than 300 mm wide and within the first purlin from the ridge.
  • are not suitable for penetrations of over 1.17 m wide;
  • are not universally accepted as aesthetically pleasing; and
  • can be used in conjunction with a sprung or over-roof design.

Type C

Tapered flashings that drain at the plane of the roof pan at the top, and
over the ribs at the bottom.

These are also known as 'under/over' or transition flashings.

Tapered or transition flashings:
  • provide greater runoff capacity;
  • are suitable for all roofs with a pitch of more than 5°;
  • are used for a retrofit, and
  • requires support.

Type D
Soaker flashings that drain at the plane of the roof pan at the gutter:

  • are simple;
  • provide greater run-off capacity
  • are suitable for all roofs with a pitch great than 5°;
  • are used for a retrofit;
  • requires support; and
  • are limited to 1.1 m wide and 2.4 m long.


8.3.7 Type A Soaker 

Soaker or under-flashing penetration designs are those that drain at the plane of the roof pan.

They are the preferred design and will produce a weathertight flashing in any circumstance where a hole is cut in metal roof cladding.

For new installations, the location of penetrations should be designed and the position known before the roof is installed.

For safety and simplicity the safety mesh, underlay and roof cladding should be laid to cover the openings, provided the sheets lengths covering the openings are increased by 150 mm or to the same measurement as the lap required to break the sheet at the head or back- curb of the penetration. See 8.3.7B Non-Aligned Ribs Flashing.

All soaker flashings have an overlap to mate with the side overlap of the profile, which ensures that even in an overflow situation, water ingress is avoided.

It is highly unlikely that a penetration will be the exact width of the roof sheet, so the preferred method of weathering is to cut full cladding
sheets above the penetration and flash at the transverse lap.
When penetrations line up with the ribs of the sheeting, the
module of the roof cladding determines the width of
the penetration.


Sheet 1 is full length.
Sheet 2 is cut to the bottom of the
penetration dimension.
Flashing 3 is the side-curb.
Flashing 4 is the back-curb.
Sheet 5 is cut to the top of the penetration
cricket + 75mm .

When the ribs do not align with the penetration, weathering can be achieved in several ways, but the simplest and preferred method is to order longer sheets at the time of installation.

Sheet 1 & 2 are cut to the bottom of
the penetration and 150mm minimum
above the cricket.
Flashing 3 is the side curb.
Flashing 4 is the back curb.
Sheet 5 & 6 are cut to the top of the
penetration hole and shaped to the

The roof cladding sheets can be cut behind the penetration and overlap the longer sheets if the position of the penetration is known before the roofing sheets are ordered. When this is not the case it is either necessary to order longer new sheets, or install 'filler sheets'.



Where a wide penetration is bigger than 2 m, has a catchment area of more than 30 m² behind it, or where the pan capacity is insufficient, special draining provisions should be made.

This design may require the removal of the purlins adjacent to the penetration to allow a gutter to surround the penetration and discharge at the eave or internal gutter line. The gutter should be sized according to 5.3.2 Capacity Calculations.

8.3.7D Penetration Gutter


8.3.7C Filler Sheetsis the preferred detail for all penetration designs and has a separate side, back curb and apron flashing. The vertical corner intersection of the side and back curb can be 'double lapped' with the side curb under horizontally, and the vertical curb over. This technique makes a strong joint before fastening and sealing, and provides a double seal.

The module of the roof cladding determines the penetration flashing width Although it can be wider than the penetration hole required, the original dimension can be maintained by using a 8.3.7F Splayed Skirt or by splaying the side curbs.




8.3.8 Type B Watershed 

Watershed flashings are not the preferred method of roof penetration flashing, and they are not aesthetically acceptable in many installations. They are an acceptable method of penetration flashing within the restrictions stated.

A 'watershed' or back flashing is used in the end span and is best restricted to within 1.5 m of the ridge so that it will not restrict roof movement, or cause expansion or condensation problems. If it is no wider than 1100 mm (1200 mm – 50 mm downturn at each edge) the back tray can be made in one piece, but a deeper or longer penetration is subject to a different design detail. It is not acceptable to seam or join flat sheet as a back flashing. See 8.3.8A Watershed (a).





When a watershed flashing extending further down the roof than the first purlin is not acceptable, alternative designs using a Type A soaker, a Type C tapered flashing, or a sprung roof flashing should be used. See 8.3.9 Type C Tapered

If the width of the watershed flashing does not exceed 300 mm it has the length restriction as other flashings. See 6.2.2 Roof Cladding Expansion Provisions.

The over-flashing should be made in maximum lengths of 6 m, using the same fixings and the same expansion provisions as the main roof cladding. (e.g. oversize holes). It is acceptable to use a boot flashing in conjunction with a watershed flashing without restriction on the pitch.

Watershed design is used where the penetration would obstruct the water flow in the pan, and is used for a retrofit or where the pipe cannot be shifted.

Washered screws should fasten the back flashing to the underneath roof cladding as rivets are not acceptable fasteners for this situation.

Another method of using a watershed type flashing can be achieved by a 'sprung' roof. Using this method the sheet is sprung by lifting the sheet from the existing fastening by the rib height.

For deeper ribs, this distance would be increased automatically because the purlin spacing is presumed to be greater. Additional support is required at the side lap of the sheeting. A tapered flashing is used at the sheet edges to provide the weathering as shown in 8.3.8C Tapered Flashing and 8.3.8D Sprung Roof.








8.3.9 Type C Tapered 

Tapered flashings are also known as transition flashings because they provide a method of changing from one level to another.

A tapered flashing can be made by tapering the flashing from the pan to the rib height in the depth distance of the penetration. However, as this design lowers the pitch, it is only suitable where the pitch is not reduced below 1.5°. If the roof pitch has to be increased, it is necessary to add the tangent angle to the minimum permissible pitch for the particular profile being considered.

This amount is determined by the height of the rib and horizontal distance through which it is raised. It is necessary to determine the angle which can be calculated using the formula tan  = h/d.

The calculation is derived from the tangent of the difference in the roof pitch and 1.5°. Tangents are in section 14.

8.3.9A Example: Taper Calculation

With a rib height of 29 mm and a taper length of 664 mm, the flashing pitch is decreased by 2.5°; it means that at minimum fall of 1.5 °, the minimum roof pitch would have to be 4°. When the minimum roof pitch is 3°, the taper length would have to be increased to 1107 mm for this rib height.

tan 2.5°0.043729÷0.0437=663.6
tan 1.5°0.26229÷0.0262=1106.8

A tapered design can be used as a retrofit for any profile by slitting the ribs to half their height and sliding the tapered flashing into position. When it is used in a new installation, extra sheets can be provided to weather the lap without slitting the sheets or the use of sealant.


8.3.10 Type D Tray 

If the penetration opening falls between the gutter purlin and the penultimate or next purlin up the roof, a flat tray under-flashing can be used to drain directly into the gutter. The tray can be weathered under the ribs or over the ribs if the sheet is cut, but requires additional structural support.

8.3.10B Enclosing Flashing shows a simple method of providing a flashing for a penetration such as a solar collector. By enclosing the collector, all the pipes are protected from the weather, which eliminates the dissimilar metal contact with or runoff onto the metal cladding. Because this flashing is made from separate curb pieces
there is no restriction on the width of the flashing. See 8.3.1 Nomenclature

The detail in 8.3.10C Penetration at the Gutter Line is required where a large air conditioning unit or a large solar collector is placed at the gutter line and the catchment area behind the penetration is also large.

N.B. Both 8.3.10B Enclosing Flashing and 8.3.10C Penetration at the Gutter Line require a cricket design.