COP:flashings; window-flashings-metal-cladding

7.5 Window Flashings For Metal Cladding 

 

Profiled metal cladding has been used for many years as both vertical and horizontal cladding, particularly on garages and farm buildings where the aesthetic requirements and weathering may not have been the prime consideration.
As most of these buildings are open and unlined, the cavity is naturally vented, and it can tolerate minor water ingress, but there should be no ingress of moisture in lined buildings. For this reason cavity construction is required for lined and insulated buildings in higher risk areas.
Buildings in very high wind zones (more than 1.5 kPa) and buildings with a height of two floors or more are considered to be at higher risk. 

 

 

7.5.1 Unpainted Metal Cladding 

Designers should recognise that unpainted metallic coated steel may not last 50 years in corrosive environments or where the design allows the metal to remain wet. (See 4.11.1 Unwashed Areas  and 4.3 Metal Performance) When it is difficult to replace any of the cladding elements aluminium or other metal options that require similar detailing to steel, should meet the 50-year NZBC requirement.

The use of unpainted metal cladding as an architectural feature on domestic buildings requires a high standard of flashing and junction detailing by the designer and the installer to ensure acceptability for both durability and aesthetics.

 

7.5.2 Ventilation and Cavities 

It is particularly important that there should be no ingress of moisture where there is no air gap or cavity, and if the timber frame is untreated and kiln dried. Moisture from within the building can accumulate due to condensation, leakage or from the building use. Horizontal cladding, fixed directly to the structure without a cavity behind it, can inhibit natural venting.

When using cedar or treated timber with plain galvanised or AZ coated steel, designers should be aware that interaction between the timber and metal can occur if these materials remain wet.

Metal wall cladding should have some provision to remove condensation, which occurs when the humidity is high or when there is a significant diurnal temperature differential. Condensation form on metal more frequently when the walls are insulated and the building is heated.

All metal wall cladding must have a permeable and absorptive underlay, and ventilation must be provided when there is no air cavity behind the wall. See 9.3.2 Condensation  and 9.5 Underlay.
 
The minimum paved ground clearance of 100 mm must be provided to ensure that rain splash or moisture from the ground does not accumulate at the drip edge of the cladding and is not induced into the cavity.

Cavity construction is recommended for horizontal cladding in all areas that are exposed, or where ventilation is inhibited and the metal cladding may suffer from the effects of condensation.

7.5.3 Window and Door Flashings in Metal Cladding 

A window or door used in conjunction with profiled metal cladding is a vertical penetration, and the principles detailed in 8 Penetrations apply equally to window and door flashings.

Wall penetrations are similar to roof penetrations except that the nomenclature is different, as they require a head flashing (back curb), side flashing (side curb), and a sill flashing (apron), but the lapping and sealing requirements are the same.

Different requirements apply depending on whether the cladding is laid horizontally or vertically, and the module of the cladding dictates flashing shape and size needed to provide an acceptable and weathertight detail. Because the natural ventilation of horizontal cladding is inhibited by any closures, a hole should be punched through filler blocks to provide some air movement and ventilate the cladding.

Closed cell closures should not be used externally to stop water movement, as the accumulation of dust and dirt at the metal interface will eventually increase the time of wetness which can be a major cause of corrosion.

It is the role of the designer to coordinate the window penetrations and the cladding; it is not the responsibility of the cladding fixer unless he is specially requested to do so.

It is common practice not to use jamb or sill window flashings behind flat or shallow profiled cladding in garage buildings, although these profiles are usually plugged to prevent moisture or vermin entry at the window flange. It is not acceptable practice for domestic, commercial or industrial buildings, unless the cladding manufacturer provides specific cladding details.

Designers using different flashing designs from those described in this COP, or using metal cladding without any flashings should accept responsibility for this decision.

  • Sealant must not be relied on as first line external weathering of penetrations through metal cladding.
  • Using a pressure equalisation system which requires a seal on the inside around the window frame can minimise the entry of wind-driven rain.
  • The sill flashing should never be sealed to the window. It prevents the discharge of water accumulated through ingress or condensation and will void any pressure equalisation system.
  • Some options may be dictated by aesthetics rather than weathering, and the integration of these details should be established before starting any cladding work.

The limitations of module creep should provide the cladding contractor with reasonable building tolerances.

Window flashings are divided into three types:
  • flush mounted;
  • recessed or reveal; and
  • butt.
Windows and doors can be flush in line with the cladding or recessed to the front of the frame. If the window is set inside the depth of the wall, the flashings are called reveal or recessed flashings. Recessed and flush flashings have the same over-flashing design. A recessed design has the advantage of a better weathering detail but can result in areas of unwashed metal, which need maintenance for durability.
Butt flashing details, commonly used on horizontal cladding of industrial or commercial buildings, are not suitable for residential closed cavity construction, because they do not over-flash the metal cladding.
 
The height of the window should determine the module set-out of horizontal cladding,

The sequence of flashing around penetrations such as windows can determine the design, as good weathering details can only be achieved if the windows are installed after the metal cladding and flashings. The installation of a head flashing requires that it is behind the cladding and weathers the window by exiting over the cladding, with the thickness or depth of the cladding determining the offset, which should have a minimum fall to the front of 10°.

It is, therefore, logical to lap horizontal cladding at the window or door head height.

The designer should recognise that the metal cladding is a modular material and deciding to use a particular profile, the size of the window can be made to suit the cladding module. Window heights should be the same to avoid different details at adjacent windows.

 

7.5.4 Horizontal Metal Cladding 

The weathertightness of horizontal cladding is entirely dependent on the flashings of cladding and penetrations, and because it is susceptible to the accumulation of condensation a cavity is required for all lined buildings.

It is recommended to seal horizontal laps with lap tape when they are enclosed in a cavity that is subject to condensation. This is needed to avoid the build up of moisture at the lap that can occur when dirt and condensation collect, which can increase the time of wetness and cause corrosion of the coated steel cladding.

All exposed horizontal metal flashings including the head or sill flashings must have a minimum pitch of 10° to avoid ponding and the build-up of dirt and debris. All hidden flashings must be durable for 50 years.
 
It is advisable to start the horizontal cladding at the window head level to provide a weathering the junction at this point. For this reason, it is advisable to make a horizontal break in multi-story construction to adjust the vertical module.

When the lap of the horizontal cladding does not coincide with the height of the penetration, it is necessary to slit the cladding to enable the fitting of 'under/over' flashings to allow water egress at head or sill flashings.

This method requires accuracy and skill to make a neat joint, at what is a vulnerable point, and which should be sealed to prevent water ingress. Because this joint relies solely on sealant and cannot be mechanically fixed, it is not a recommended option.

To avoid slitting the sheets where in a single storey building, a double hemmed jamb soaker flashing can be continued to ground level behind the cladding—providing there is provision to enable water to drain clear of the building. This is not a recommended option unless there is a cavity behind the cladding. Multi-storied buildings should have a sill at each floor level so that soaker flashings do not need to drain more than one level.

 

7.5.5 General Principles 

The details shown are not the only options; however, they represent a standard of weathering principles outlined elsewhere in this COP and are consistent with good trade practice. Deviation from the details provided would require specific approval.

Underlay must be used under metal wall cladding on lined buildings in the same manner and for the same purposes as for roof cladding.

Underlay controls condensation in an insulated cavity and offers a secondary provision to drain any ingress of moisture. For this reason, the underlay should exit at the penetration (window or door), similarly to the requirements for roof penetrations. Failure to do so can lead to the retention of moisture in the wall, with consequential corrosion of metal and the degradation of timber.

All sills should be dog-eared, or riveted, and sealed to provide a weathertight joint.

The cross section details shown can be used for vertical cladding. For horizontal cladding refer to 7.5.7A Window Flashings (Recessed)

N.B. Head flashings are not taken to the back of the cavity as they are meant to flash the window, not the cavity. These details are similar to those supplied by WANZ WIS. 

All details should be made to allow for some air movement to dissipate condensation and without the use of sealant as the first defence against water ingress.

In high wind design load areas, all the details shown should be used in conjunction with vented closed-cell non-absorbent filler blocks fixed to the backside of the profile to baffle the wind pressure.

Filler blocks can be fixed and positioned to metal cladding by spray adhesive, in sheltered areas, however, they are not a requirement.

The design and installation of metal flashings is a crucial part of the cladding weathering process, and without detailed requirements, definitive objectives, quality checks, and known responsibility the results can be unsatisfactory and expensive to repair or replace.
Ingress of water can be insidious, and the resulting degradation of insulation, framing, and internal linings poses a threat to the health and safety of the occupants.

7.5.6 Flush Window Flashings 

 

 

 

 

 

 

 

 

The height of the sill flashing may have to be altered to suit the position of the profile.

Flush mounted windows or doors may have picture frame flashings, which means that the window frame is mounted externally to the cladding or can have timber facings.

All types should be designed without total reliance on sealant, and improvised solutions are not acceptable.

Both the flush and recess flashings have an under jamb and a front or face jamb.

Trim sizes for aluminium windows are usually 10 mm over the jamb liner size, however, to obtain a vertical upstand, the trim size for flush flashing design requires plus 15 mm vertically to allow for a 10 mm packer.

An option for the head flashing profile can be provided if the corrugations on the underlap of the sheet are flattened, which enables the head flashing to be folded vertically and not shaped to the profile.

 

 

7.5.7 Recessed Window Flashings 

 

 

 

 

 

 

A recessed window flashing design is considered the best option for horizontal cladding but accurate flashing measurement and installation are required to provide weathertightness.

N.B. For lined buildings a cavity is required for horizontal metal cladding.

Similar to the flush detail, the head flashing can be folded vertically and not shaped to the profile if the corrugations on the underlap of the sheet are flattened .

7.5.8 Butt Window Flashings 

Alternative details that allow the cladding to terminate up to a flashing without over-flashing it, require accurate fitting to be weathertight. See 7.5.8 Butt Window Flashings
These details are known as butt flashings and can be used on flush or recess designs with a one-piece jamb. They are only suitable for areas with low or medium wind design loads, or in protected areas and require precise design, measurement, and manufacture to provide a weathertight solution.
Butt window flashings also require accurate installation to provide an aesthetically acceptable solution.
Butt flashings are not a preferred detail because they need extra timber for fixing and they should allow for drainage of a large catchment area.

The total amount of water draining from the catchment area of a multi-storey construction can be considerable, and butt flashings should dispose of it as a penetration. The catchment area is calculated by multiplying the width of the window opening by half the height.

Rain-water should discharge at a lap of the cladding, or if the building is not lined or has a cavity, it can be tracked to the outside via a double hemmed soaker flashing to discharge at the base of the building.

To slit the cladding and discharge a large volume of rainwater from butt flashings is not acceptable and, as it should be assumed that butt flashings will leak, provision should be made to drain the water to the outside. See 7.5.8 Butt Window Flashings.

 

 

 

 

 

 

7.5.9 Flashings For Vertical Cladding 

 

Window and door flashings for vertical cladding are similar to those used for horizontal cladding, a cavity is, however, not required for vertical cladding.

The main difference is that the module is running horizontally and therefore some flexibility is required if the flashings are to be equal on both sides. To provide for this the trimming size of the window opening should be plus half the module dimension. The under lap of the profile should be flattened as shown in drawing 6.4.4.B.

 

 

 

 

7.5.10 Alternative Flashing Designs 

 

Designers should consider more alternatives for butt designs to provide weatherproof window flashings.
For example: If the opening is "panelised" the vertical jamb flashing can be extended from soffit to the ground for single storey buildings or to a module break in a multi-storey building as shown in drawing 6.4.5.

This alternative provides the opportunity to use the same or a different material at the head and sill areas, and a top hat flashing is used to obtain a module break.