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.
Many of the penetration details in the Code of Practice are expanded on with step-by-step, interactive 3D instructions in the RANZ Roofing Guide, developed by the Roofing Association of New Zealand in association with the NZMRM.
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.
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.
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 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 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 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. One-piece welded flashings offer the most durable and weathertight solution to penetration back curb.
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.
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 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
Minimum 1°, Maximum 60°
°
rads
Secondary pitch only needs to be entered manually if it is different to the main Roof Pitch
°
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.
Illustration is for explanatory purposes only and is not to shape or scale.
Illustration is for explanatory purposes only and is not to shape or scale.
Describe the product: this does not control the calculation which relies on you entering accurate data
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
=IF ( ( h3 > 0) , ( W * cos ( C5; ) - 0.5 * h3 * tan ( C5; ) ) * h3 , 0 )
=( W * cos ( C5' ) - 0.5 * h4 * tan ( C5' ) ) * h4
=A1 + A2 + A3 + A4
=h1 / sin ( C5 )
=h2 / sin ( C5' )
=IF ( ( h3 > 0 ) , h3 / cos ( C5 ) , 0 )
=h4 / cos ( C5' )
=WP1 + WP2 + WP3 + WP4
=h2 * tan ( PI()/2 - C5 ) - IF ( ( h3 > 0 ), h3 * tan ( C5 ) , 0 )
=h2 * tan ( Beta - PI()/2 + C5 ) - h4 * tan ( C5')
=FWSW13 + FWSW24
mm
x mm
mm
Select Manufacturer (if applicable) and Profile
Describe the product: this does not control the calculation which relies on you entering accurate data
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
m²
m²
m²
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.
m²
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.
m²
Conditions and assumptions for flat gutters:
Mannings n assumed to be 0.014 to represent long term friction conditions.
Equations valid for gutters with min gradient 1:500, max gradient 1:100.
Bends are accounted for by local loss coefficients (0.5 for each 90° bend).
Conditions and assumptions for downpipes:
Mannings n assumed to be 0.014 to represent long term friction conditions
Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
Gutters must have fall for downpipe sizing to be valid
Calculations consider weir, orifice and friction effects
Orifice discharge coefficient of 0.61 assumed
Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
Minimum pipe gradient of 20% assumed for friction conditions
Conditions and assumptions for valleys:
Mannings n assumed to be 0.014 to represent long term friction conditions
Minimum height of Type A valley returns to be 16 mm
Minimum freeboard of 20mm mm for valleys below 8°
Minimum freeboard of 15mm for valleys 8° and steeper
Conditions and assumptions for maximum run:
Mannings n assumed to be 0.014 to represent long term friction conditions
Only valid for supercritical flow (most roofs)
Conditions and assumptions for penetrations:
Mannings n assumed to be 0.014 to represent long term friction conditions
Only valid for supercritical flow (most roofs)
Where Both Sides selected, assumes an even split of flow to either side of penetration
Conditions and assumptions for level spreaders:
Mannings n assumed to be 0.014 to represent long term friction conditions
