A window leak rarely starts as a dramatic stream of water. More often, it appears as a damp patch below the sill after a Melbourne downpour, peeling paint around the reveal, or a musty smell in a room that should be dry. If you are asking, “why do windows leak?”, the answer can sit in the window itself, the installation around it, or the wall system directing water towards the opening.
The key is not to cover the symptom with silicone and hope for the best. Finding where water enters helps you decide whether a practical repair is worthwhile or whether replacement windows will deliver the better long-term result.
Why do windows leak during rain?
Windows are designed to manage water, not necessarily to keep every drop out of the frame. Aluminium windows, particularly sliding systems, use drainage paths known as weep holes to let water that reaches the track drain safely outside. Water in an external track can be normal. Water travelling past the internal sill, staining plasterboard or pooling on the floor is not.
The most common reason for rain-related leaks is failed sealing around the perimeter of the frame. Sealant can shrink, crack, lose adhesion or separate from brickwork, cladding and render over time. This is especially common where older windows have been exposed to strong sun, movement in the building, or repeated wet and dry cycles.
Installation details matter just as much. A window must be correctly packed, fixed, flashed and sealed so water is directed back to the outside. If flashing is missing, poorly lapped or damaged, rain can enter above or beside the frame and show up indoors well away from the actual entry point. A leak at the top corner of a window may be coming from the wall, roofline or cladding above it rather than from the glass or sash.
Wind-driven rain makes faults easier to expose. A window may look fine in light rain but leak when southerly weather pushes water against the seals. High-exposure elevations, upper-storey openings and homes near open parkland, bayside areas or elevated sites often need particular attention to system selection and installation quality.
Blocked drainage paths and weep holes
Sliding windows and doors need clear drainage channels. Dust, leaves, insects, paint, construction debris and old sealant can block weep holes, allowing water to build up in the track. Once the water level rises, it may overflow to the room side.
Check the outside face of the frame for small drainage slots. Clear loose debris carefully with a soft brush or vacuum rather than forcing sharp tools into the openings, which can damage internal components. Also inspect the track after heavy rain. If it holds water for hours after the weather clears, drainage is likely restricted.
It is tempting to seal every visible gap in a window track, but this can make the problem worse. Drainage holes are there for a reason. Sealing them shut traps water inside the frame and changes where it eventually escapes.
Worn seals, damaged hardware and poor window adjustment
Opening sashes rely on compression seals, pile seals, rollers and locking hardware to close firmly against the frame. When seals flatten or become brittle, small gaps allow wind and water through. On sliding windows, worn rollers can cause the sash to sit unevenly, leaving a gap at the top or side. Awning windows may leak if their chain winders, hinges or catches no longer pull the sash tightly into its seals.
A simple check is to look for daylight around a closed sash from inside the room. You can also inspect rubber seals for cracking, gaps at joins and sections that have pulled away from their channels. If a window rattles in wind or requires force to lock, adjustment or hardware repair may be needed.
For a relatively new window, a targeted adjustment, new seals or replacement rollers can be cost-effective. For an older unit with corroded hardware, worn tracks and failing glazing seals, repairs can become a series of short-term fixes. Replacement may offer better value, particularly if the existing window is single glazed or poorly suited to the opening.
Condensation can look like a leak
Not every wet window is leaking rainwater. Condensation forms when warm, moisture-laden indoor air meets a cold glass surface or aluminium frame. It is common in bedrooms, bathrooms, laundries and kitchens during winter, especially in homes with single glazing, limited ventilation or no extraction fan.
Condensation usually appears on the inside face of the glass, often across a broad area rather than at one isolated corner. It may be worse first thing in the morning and present even when there has been no rain. A true leak generally follows wet weather, shows around the frame, or becomes worse on one particular side of the house.
The distinction matters. Exterior sealant will not fix condensation. Improving ventilation, using bathroom and kitchen exhaust fans, reducing indoor moisture sources and maintaining consistent heating can help. Double-glazed aluminium windows also reduce the temperature difference at the internal glass surface, improving comfort and lowering the likelihood of condensation in many homes.
Glass and glazing faults
Water can sometimes enter through failed glazing seals, particularly in older windows where the external glazing bead, gasket or putty has deteriorated. Look for loose glass, gaps around the pane, perished rubber or visible movement when the sash is gently pressed.
A double-glazed unit can also fail at its sealed edge. The usual signs are fogging, haze or moisture trapped between the two panes. This does not normally cause water to run into the room, but it does reduce insulation performance and affects the clarity of the glass. The insulated glass unit generally needs replacement rather than a surface repair.
Cracked glass should be addressed promptly. Even a small crack can expand with temperature changes, compromise weather resistance and create a safety issue.
Problems around the wall, sill or roofline
Water follows gravity, framing and wall cavities. That means the stain you see may not identify the source. Leaks above a window are often linked to cracked render, failed cladding joints, deteriorated brickwork, missing head flashing, blocked gutters or overflowing downpipes.
Timber reveals and internal plaster can absorb moisture before obvious damage appears. By the time paint bubbles or mould develops, water may have been entering for some time. If you see staining above the window rather than beside it, inspect the wall and roof drainage before assuming the window needs replacement.
This is also why a full perimeter bead of silicone is not always the right answer. Applied in the wrong location, silicone can trap water behind the frame or interfere with drainage. External remedial work should preserve the intended flashing and weep paths.
How to find the source before arranging repairs
Start by recording when and where the moisture appears. Note whether it occurs only in heavy rain, when wind comes from a particular direction, or on cold mornings without rain. Take photos of the internal damage and the external frame before cleaning anything away.
Then inspect the outside perimeter for cracked sealant, open gaps, damaged render and blocked weep holes. From inside, check the sill, corners and plasterboard around the opening. If it is safe to do so, a controlled hose test can help isolate the area: wet one section at a time, starting low and moving upward, while another person watches inside. Do not use a high-pressure spray, as this can force water into places that normal rain would not reach.
For persistent leaks, a qualified window installer or building professional can assess both the frame and the surrounding wall details. This is particularly worthwhile before spending money on cosmetic repairs, repainting or replacing flooring.
When repair makes sense and when replacement is smarter
Repair is often suitable when the window frame is sound and the issue is limited to blocked drainage, worn seals, loose hardware or an isolated perimeter seal failure. It can be a practical option for newer aluminium windows that otherwise operate well.
Replacement becomes more compelling when several issues occur together: recurring leaks, difficult operation, corroded frames, warped sashes, failed glazing, poor security or substantial heat loss. A new aluminium system can address weather sealing, secure locking, smoother operation and energy performance in one project.
For Melbourne renovations and extensions, choosing the right configuration is as important as choosing a new frame. Awning windows can provide ventilation while shedding light rain when partly open. Sliding windows suit areas where outward-opening sashes would interfere with paths or outdoor spaces. Double glazing can improve thermal comfort and reduce outside noise, while the correct water rating and installation details help the window perform in exposed conditions.
At WINDOWS DOORS INSTOCK & REPLACEMENT, standard-size options can help keep straightforward replacements moving, while custom-sized aluminium windows allow the new unit to suit the existing opening and project requirements. Accurate measurements, glazing selection and site conditions should be confirmed before ordering.
A dry window opening protects far more than the frame. Address the cause early, and you can avoid damaged linings, mould growth and repair work that costs far more than fixing the original fault.