Reducing the risk of leakage through cladding penetrations

Designers should locate services to minimise the number of cladding penetrations.

To reduce the risk of water leakage through cladding penetrations, the pressure difference between the cladding surface and the wall assembly needs to be moderated – see air movement and pressure moderation.

This is done by providing gaps in the cladding that allow higher pressure air into the wall assembly so that the pressure in the assembly is increased to a similar level as the external pressure.

Designers should locate services to minimise the number of cladding penetrations.

Designers will incorporate openings in a direct-fixed cladding at:

• window and door penetrations
• inter-storey junctions 
• at the bottom edges of the cladding. 

These openings incorporate deflection devices (such as window head and sill flashings) that allow air to enter and water to drain out but deflect rainwater away and protect the junction between the penetration and the cladding. 

Some builders may consider gaps in the exterior cladding to be a source of leaks, but when they are correctly designed and built with deflection devices, they will restrict water entry.

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Window and door penetrations in direct-fixed cladding

Flashing tape

Windows and door penetrations in direct-fixed claddings need to incorporate flexible flashing tape flashings at the head and sill.

The tape is installed over the wall underlay, which must be turned into the framed opening around all sides. This tape will protect the vulnerable timber frame from any water that may get into the wall assembly. Aluminium window and door units can leak (through the corner mitres or glass seals), and the flexible flashing tape protects the framing at the corners and across the sill.

Head flashing

A metal head flashing with a 15° slope is installed against the wall underlay and over the window frame to provide 10 mm cover to the face of the window frame. A drainage and ventilation gap of 5 mm must be left between the bottom of the cladding above the flashing and the top of the sloped flashing.

This opening will allow water to drain from the assembly and air to enter. The head flashing upstand must be sealed to the face of the wall underlay with flexible flashing tape, or a layer of wall underlay must be dropped from above to ensure a gravity drainage path out over the flashing is created.

Window penetration preparation.

Window head – typical direct-fixed cladding.

Sill flashing

Under E2/AS1, window sills within direct-fixed claddings must incorporate a metal sill flashing that:

• extends back into the framed opening for the full extent of the window extrusion 
• overflashes the lower cladding by 35 mm minimum. 

The bottom edge of the flashing should be formed to create a drip edge. The ends of the flashing must be upturned to form a stop-end that will prevent the wind from driving water over the ends of the flashing. Additional packers are fitted to the trimming studs to allow the flashing to fit the full width of the framed opening.

The sill flashing must be located to allow a gap of 5 mm between the face of the flashing downturn and the back of the bottom window flange – this gap will allow water to drain and air to enter. To maintain this 5 mm opening, check that windows are not pulled back tight to the cladding at the sill.

The window flange at the jambs must be sealed (or protected with a facing or scriber) to restrict water entry at these points.

Install a window with direct-fix cladding, steps 1–2.

Install a window with direct-fix cladding, step 3.

Install a window with direct-fix cladding, steps 4–8.

Install a window with direct-fix cladding, steps 9–14.

Install a window with direct-fix cladding, steps 11–15.

Air seals

The opening at the sill allows air to enter the trim cavity around the window. This air must be prevented from entering (and carrying moisture into) the interior of the building. See Air barriers/air seals for details.

Cladding installation

Accurate installation of direct-fixed claddings will minimise the potential for water to reach vulnerable components of the wall assembly.

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Window and door penetrations in drained and vented cavity construction

In most respects, details for window and door penetrations in drained and vented cavity construction are the same as for direct-fixed cladding.

However, window sills in drained and vented cavity claddings do not need a sill flashing. As the window extrusion is located out over the cavity, any water leaking through the window sill extrusion will be picked up by the cavity. The timber sill trimmer will be protected by the flexible flashing tape.

The window flange at the jambs must be sealed (or protected with a facing or scriber) to restrict water entry at these points.

Windows and doors can be fitted back tight to the cladding, as air can enter into the drainage cavity from other openings, and this air moves into the trim cavity. Gaps at the sill are not required for air entry or drainage. Air within the drainage cavity will enter the trim cavity around the window.

Install a window into cladding with cavity, steps 1–2.

Install a window into cladding with cavity, steps 3–4.

Install a window into cladding with cavity, steps 5–6.

Install a window into cladding with cavity, steps 7, 12 and 13.

Install a window into cladding with cavity, steps 8–12.

Cavity closures

Cavity closure devices that allow drainage and ventilation to occur through small slots or holes, while blocking out vermin and wind-driven rain, must be installed at the bottom of the cavity and in any location where the cavity opens to the exterior, such as at window heads and inter-storey junctions. These need to be formed from a durable metal or plastic. 

It is important that closure devices are installed accurately and fit tightly to the back of the cladding and the face of the wall underlay, completely closing off the cavity to vermin. 

Water will drain from the cavity through the slots or holes in the closure device, and these will also allow air to enter the cavity.

It is also important that ground levels are kept well clear of the bottom of the cladding to ensure that cavity drainage and ventilation is not restricted.

Accurate installation of drained and vented cavity-fixed claddings will minimise the potential for water to reach vulnerable components of the wall assembly.

Drainage provided at the base of a wall cavity.