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 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.


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