Why Modern Windows Leak – A Pascoe Vale Case Study

Most people assume that when water appears around a window, the window itself is leaking.

In reality, modern windows are only one component of a much larger water management system. In many cases the actual cause of the leak lies in the surrounding wall construction, flashing details or drainage paths rather than the window itself.

This case study from Pascoe Vale demonstrates how water ingress can occur even when the visible symptoms suggest a faulty window.

How Modern Aluminium Windows Work

Contrary to popular belief, aluminium windows are not designed to be completely waterproof.

Modern aluminium windows are designed as water management systems. Water is expected to enter parts of the frame during heavy weather events. The window relies on internal drainage chambers, subsills and weep holes to collect this water and direct it safely back outside.

Unlike uPVC windows, which are typically manufactured from welded sections, aluminium windows are assembled from multiple extrusions which are mechanically joined together. These joints create additional pathways for water movement and increase the importance of correct flashing and drainage details.

uPVC Windows

• Extrusions are welded together.

• Fewer frame joints.

• Reduced opportunity for water migration through frame corners.

• Good thermal performance.

Aluminium Windows

• Multiple mechanically assembled extrusions.

• Greater reliance on drainage paths and sealants.

• More potential leakage points if installation or flashing details are inadequate.

Neither system is inherently leak-proof. Both depend heavily on correct installation and water management.

Flashings Are Not Waterproofing

One of the biggest misconceptions in construction is that flashings stop water.

They do not.

Flashings are designed to manage water by collecting it and redirecting it back to the exterior of the building.

Modern wall systems assume that some moisture will bypass the external cladding. The role of the building wrap, drainage cavity and flashings is to safely collect this moisture and discharge it externally before it can damage the building.

The Building

This Pascoe Vale townhouse incorporated:

• Lightweight rendered cladding over polystyrene.

• Aluminium windows.

• Upper-floor windows located directly above lower-level windows and doors.

• An alfresco roof beneath some upper windows.

• Internal living areas beneath others.

Previous attempts had been made to stop the leaks by applying sealant around the perimeter of the windows.

Unfortunately, this did not address the underlying issue.

What Should Have Happened?

Where upper-level windows are positioned above roofs, doors or other vulnerable building elements, the building wrap and flashing systems should be designed to direct any water that enters the wall cavity back to the exterior.

In this case there were two critical areas.

Window Above Alfresco Roof

The building wrap should ideally have terminated onto the alfresco roof via an appropriate flashing arrangement.

This would have allowed water travelling down the drainage plane to discharge harmlessly onto the roof surface before reaching the door below.

Window Above Internal Ceiling

Other upper-floor windows were located above the lounge and dining areas.

The wall wrap and flashing systems should have directed any moisture safely onto the flat roof below and then back to the exterior.

Instead, water appeared to be bypassing these drainage pathways and entering the building structure.

Why These Problems Are Difficult To Fix

Once the following have been completed:

• Cladding installation

• Rendering

• Roofing

• Internal plastering

Retrofitting compliant flashing systems can become extremely invasive and expensive.

In many cases rectification may require:

• Removal of windows

• Removal of cladding

• Roofing modifications

• Internal repairs

This can result in significant disruption and cost.

Practical Remedial Solution

A full reconstruction of the wall system was not considered practical.

Instead, a targeted remedial solution was adopted.

The aluminium extrusion junctions were carefully cleaned, dried and sealed. Particular attention was paid to frame joints and interfaces where water migration was most likely occurring.

This process was far more involved than simply applying a bead of sealant around the perimeter of the window. Successful remediation required:

• Clean surfaces

• Dry conditions

• Careful preparation

• Attention to detail

• Identification of likely water pathways

Outcome

Following completion of the works, the windows were monitored through subsequent heavy rainfall events.

No further water ingress was reported.

Most importantly, the client was able to enjoy their home without the ongoing concern of recurring leaks.

Key Lessons

This case highlights several important principles:

• Windows rarely leak in isolation.

• Building wraps and flashings must provide a continuous drainage path to the exterior.

• Water management is more important than sealants.

• Perimeter caulking is often not the solution.

• Design decisions made during construction can have significant long-term consequences.

At Leak Masters we regularly investigate leaks that are ultimately caused by failures in water management rather than the window itself. Understanding how water moves through a building is often the key to finding a practical and lasting solution.