COP v3.0:penetrations; penetration-design

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 9.4.1.1 Over-Flashed (Watershed) Back Flashings or under the profile 9.4.1.2 Under-Soaker Back Flashings.

9.4.1.1 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.

9.4.1.2 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).

 

9.4.2.1 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.

9.4.2.2 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.

9.4.2.3 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.

9.4.2.4 Sheetmetal Penetration-Flashings Reference 

9.4.2.4A Sheetmetal Penetration-Flashings Quick Reference

 Proximity to the apex determines back flashing design
 

Close to apex
Over-Flashing

  • Aka Dry pan or Watershed
  • Suitable for retrofitting

Distant from the ridge
Under-Soaker

  • 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
Level

  • Only suitable for small catchments not prone to leaf debris.
  • Relatively easy to manufacture on site

9.4.2.5A Level Over-Flashing: Corrugate

9.4.2.6A Level Over-Flashing: Trapezoidal

9.4.2.5B Level Under-Soaker: Corrugate

9.4.2.6B Level Under-Soaker: Trapezoidal

9.4.2.5G Level Hidden Gutter: Corrugate

Medium/Moderate Debris
Arrowhead

  • 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

9.4.2.5C Arrowhead Over-Flashing: Corrugate

9.4.2.6C Arrowhead Over-Flashing: Trapezoidal

9.4.2.5D Arrowhead Soaker: Corrugate

9.4.2.6D Arrowhead Under-Soaker: Trapezoidal

 

Wide/Much Debris
Cricket

  • Most suitable for larger catchment areas.
  • Requires careful calculation and off-site fabrication.

9.4.2.5E Cricket Over Flashing: Corrugate

9.4.2.6E Cricket Over-Flashing: Trapezoidal

9.4.2.5F Cricket Under-Soaker: Corrugate

9.4.2.6F 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 https://www.metalroofing.org.nz/maximum-area-above-penetration-calculator.

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.
 mm/hr
 
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
  rads
 bends
 
 m
 
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

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
 mm
 
 mm
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
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.
 m
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