The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
The weathertightness of a flashing is determined by the flashing cover, not the flashing dimension. Transverse flashings run across the roof slope at right angles to the ribs of the roof and longitudinal flashings run down the roof slope.
The minimum dimension for flashing cover (C) over profiled metal roof and wall cladding must be in accordance with 7.3A Minimum dimension 'C' flashing cover
There are two categories of exposure or pitch that determine the flashing cover width.
- Normal exposure.
- Where the pitch is more than 10°.
- Exposed (higher risk) where the design wind load exceeds 1.5 kPa.
- Where the pitch is less than 10°.
|TYPE||Direction||Category A||Category B||Drawing|
|RIDGE||Transverse over sheeting||130 mm||200 mm||5.2.A.|
|BARGE||Longitudinal trapezoidal & tray||one rib||one rib, two ribs (<20 mm)||5.2.C.|
|Longitudinal corrugate||2 corrugations||3 corrugations||5.2.C.|
|Vertically down face (smooth)||50 mm||75 mm||5.2.C.|
|Vertically down face (profiled)||75 mm||100 mm||5.2.C.|
|APRON||Transverse over sheeting||150 mm||200 mm||5.2.B.|
|Longitudinal trapezoidal & tray||one rib||two ribs (<20 mm)*|
|Longitudinal corrugate||2 corrugations||3 corrugations|
|Vertically up face (smooth)||50 mm+ hem or 75 mm||75 mm + hem or 100 mm||5.2.B.|
|Vertically up face (profiled)||75 mm + hem or 100 mm||100 mm + hem or 125 mm||5.2.B.|
|PARAPET||Vertically down face (smooth)||50 mm||75 mm||5.2.D.|
|Vertically down face (profiled)||75 mm||100 mm||5.2.D.|
The end of wall cladding should not extend down to any apron flashing but should have minimum clearance (CL) of 25 mm, and a maximum clearance of 50 mm. When describing a cover or a lap, the description should be clear and distinct, i.e., the designer and the roofing contractor should both be referencing the same item. (See 7.3.1 Flashing Laps)
The cover dimension for a flashing is the net width or length of the cladding covered by the flashing.
The factors that should be considered when determining the required cover width are:
- the pitch of the roof;
- the design wind load;
- the cladding profile;
- the type of metal scriber flashing;
- provision for fixing;
- the type of stopend;
- the use of plastic filler blocks; and
- the sealing and the ventilation of the cavity
For high design wind loads, where dynamic pressures fluctuate, a flat flashing over profile metal cladding cannot stop water ingress without baffles or special design provisions.
To equalize or minimise the pressure differential within the cavity and interrupt the passage of wind, baffles are placed in the path of driven rain; firstly by a metal scriber or close-fitting soft edging, then a profile closure, and finally the vertical stopend of the roof cladding.
When closures or filler blocks are used under ridge or apron flashings, they should be placed midway up the cavity and adhered to the side ribs of the profile or the top of the apron to ensure they remain in position. They should not be sealed to the pan as this may result in moisture retention in the cavity.
In all Category B areas where a full vertical dog-eared stopend cannot be made, a pull-up stopend must be provided in conjunction with profile closures. (see 13.1.3 Stopends and 7.10 Profile Closures)
- A soft edge, notched or scribed flashing must be provided to standard ridging, and the roof cladding carried up to the ridge to provide a minimum 130 mm cover.
- The positioning of the top purlin must be adjusted with the pitch to ensure that primary fastening is provided between 25 mm and 50 mm from the edge of the ridging.
- A custom made ridging must be used where 130 mm cover cannot be provided, in a Category B area, or where a vent ridge is used.
- Purlin spacings must be adjusted to suit.
- A hemmed apron flashing must have vertical cover height of 50 mm for flat cladding such as fibre cement or concrete panels . F or profiled claddings, the vertical cover must be 75 mm or increased as per table 5.2.
- Where the roof pitch is less than 10° or the wind design load is more than 1.5 kPa flashing cover must be increased as per Category B in table 5.2.
- The vertical upstand of an apron flashing must have a hem or weatherhook , unless this impairs the fixing of the cladding. (see drawing 5.2.2.B hems)
- If a hem or weatherhook is not provided the vertical upstand must be increased by 25 mm as shown in table 5.2.
- The minimum vertical cover height of a capping flashing covering flat cladding such as fibre cement or concrete panels must be 50 mm and for profiled claddings the vertical cover must be 75 mm or increased as per table 5.2.
- Where the roof pitch is less than 10° or the wind design load is more than 1.5 kPa , flashing cover must be increased as per Category B in table 5.2.
- Longitudinal barge flashings that run parallel to the ribs of the roof must have a minimum cover of one rib for trapezoidal profiles and one and a half ribs for corrugate or other profiles with a rib height of less than 20 mm. (See 7.3D Barge.)
- Where the roof pitch is less than 10° or the wind design load is more than 1.5 kPa flashing cover must be increased as per Category B in 7.3A Minimum dimension 'C' flashing cover.