Aluminium Window Sill Replacement: Substrates, Flashing and Drainage Continuity

Replacing an aluminium window sill is usually described as a small works item. In practice it is one of the few operations on a building that opens the entire water line of an opening at once: the base of the frame, the wall’s weather barrier, the flashing, the cladding termination, the internal lining and the floor edge all become visible in the same afternoon. What is found there decides whether the job is a repair with a new cover piece or a genuine reconstruction of the detail.

This article is written for the point at which replacement has been chosen or is being considered. It deals with the substrate that receives the new sill, the flashing and sill pan that must sit beneath it, the end dams and weeps that make the drainage path work, and the interfaces with masonry, render and timber that determine whether the finished line stays continuous. It deliberately contains no dimensions, no spans, no fall percentages, no commercial figures and no performance ratings, because those are properties of a specific tested system, a specific exposure category and a specific project. What follows is the reasoning that lets a competent installer or specifier arrive at those figures correctly.

Replacement Versus Repair: Making the Decision Honestly

Most sill problems present as a stain on an internal lining, a bubbling paint finish on a reveal, a damp patch on a carpet edge, or a bead of sealant that has opened at the external face. The temptation is to treat the visible symptom, because the symptom is cheap to reach and the cause is not. A replacement decision should be based on what the water is doing, not on how the damage looks from inside the room.

What a Repair Can Legitimately Achieve

A repair is appropriate when the water line itself is sound and only a component at its surface has failed. Typical legitimate repairs include re-forming a sealant joint that has debonded from a clean, intact substrate; clearing weep slots that have been blocked by paint, render crumbs or insect debris; replacing a perished gasket or a failed setting block; re-fixing a loose internal sill trim; and treating isolated corrosion on a fixing that can be replaced without disturbing the flashing.

In each of these cases the flashing is continuous, the pan falls outward, the end dams exist, and the frame drains to the exterior. The repair restores a component without altering the path that water takes. That is the test: if the water path is already correct, repair the component. If the path is wrong, repairing the component only changes where the water appears next.

When Replacement Is the Honest Answer

Replacement becomes the defensible choice in a number of recognisable conditions. The sill pan may be absent altogether, particularly in older buildings where the frame was bedded directly onto a masonry base or a timber plate with a bead of sealant as the only defence. The pan may be present but reversed, falling inward, or interrupted at a mullion or a coupling. The end dams may have been folded but never sealed, or cut short of the jamb line. The membrane may terminate above the upstand instead of lapping over it, leaving the rear edge of the pan open to the cavity. The frame may be bearing on packers that have crushed, rotted or been omitted, so that the base of the frame is deflected and the drainage slots no longer discharge where they were designed to.

There is also the case where previous repairs have accumulated. A sill that has been sealed, capped, rendered over and sealed again is difficult to assess and impossible to guarantee, because each layer hides the condition of the one beneath. Once the water path has been interrupted in several places, reinstating it in full is usually faster and more certain than tracing each interruption.

A Simple Decision Framework

Five questions are worth answering before any material is ordered. Is the entry point of the water known, or only the exit point? Is the flashing continuous beneath the full width of the frame, including behind mullions and at both ends? Can the ends of the sill be exposed and rebuilt without demolishing the jamb? Is the frame supported on sound, dry, correctly fixed packers? And will the cladding and internal finish allow the detail to be reinstated in the correct order, or will the last trade bury the work again?

If the answer to any of the middle three is no, the job is a replacement rather than a repair, regardless of how small the visible defect appears. If all five answers are favourable, a targeted repair is likely to be the better use of the opening.

What the Sill Has to Do

Before the substrate is examined, it helps to state the function plainly. A sill assembly has three jobs, and they are separate. It receives water that has passed the outer defences. It discharges that water to the exterior, under gravity, without allowing it to travel inward. And it separates the frame from materials that behave differently when they become wet, giving the wall’s weather barrier a defined place to terminate.

Those three jobs produce the concept that governs every detail in this article: drainage continuity. The water line must be continuous from the wall’s weather barrier, across the flashing and its upstand, along the pan to the front edge, and out past the cladding. The frame’s own drainage must discharge into that line rather than beside it. Where the line is broken, water does not stop at the break; it leaves the assembly at the break, and that exit is almost always inside the building fabric.

Replacement work is therefore not the installation of a new visible sill. It is the reconstruction of a continuous path, of which the visible sill is the last and least important part.

Surveying the Substrate Before Anything Is Ordered

An aluminium sill is dimensionally stable, impervious and relatively unforgiving. The substrate beneath it is none of those things. Replacement work inherits whatever the original construction left behind, including the consequences of every trade that followed. The survey exists to find out what the new detail will actually be fixed to, and it should be completed with the frame removed or at least with the sill line fully exposed.

Masonry and Brick Veneer

In a brick veneer wall the sill sits above the base course of the opening, and the survey should establish several things. The first is whether the masonry below the opening is sound and whether the cavity has been kept clear. Mortar droppings on the base of the cavity, or mortar that has bridged the cavity at the sill, will hold water against the flashing and can defeat a weep path that is otherwise correctly formed. The second is whether a damp-proof course or flashing was originally installed beneath the first course of the sill brickwork, and whether it is intact.

The third is the condition of any steel lintel above and of the ties around the opening. Efflorescence, rust staining and cracked mortar joints are evidence that water has been present for long enough to migrate, and they suggest the flashing has been compromised for some time rather than recently. Where the sill brickwork is to be rebuilt, the opportunity should be taken to core or form weep openings at the correct level and to confirm that the cavity remains open behind the new work.

Render, Stucco and Fibre-Cement Cladding

Rendered walls present a specific difficulty because the render often returns into the reveal and finishes hard against the frame, sealing the sill line visually and hydraulically at the same time. The survey should establish how thick the render is at the reveal, whether a movement or control joint exists at the sill line, and whether the base coat and mesh, in an external insulation system, terminate above the flashing or continue behind it.

Cracking at the sill line is common and is often read as a render defect when it is actually a symptom of frame movement or of a flashing that was never able to shed water clear of the wall face. If the render is to be cut back for the replacement, the new termination should be detailed so that the render stops above the flashing with a drip, rather than being brought down over the front edge. A render bead that sits directly on the flashing face will trap water and debris at exactly the point that needs to stay open.

Timber Framing

Timber is the substrate where the survey pays for itself most quickly. The sill trimmer, the bottom plate of the opening, the jack studs and the lower portion of the studs beside the opening should all be examined for decay, staining and softness, and the moisture condition should be measured with an appropriate instrument and recorded along with the method used. A frame that has been leaking for several seasons will often show decay below the sill line even where the visible timber appears sound.

The treatment class of the framing and the condition of the fixings matter as much as the timber itself. Replacement work introduces new penetrations, new packers and new sealant, and each of those must be compatible with the existing treatment and with the exposure the framing will see. Where decay is found, the repair of the framing precedes the sill work; installing a new flashing over softened timber simply moves the failure a season later.

Concrete, Slab Edges and Formed Openings

Concrete openings are usually the most dimensionally reliable substrate, but they bring their own conditions. Formed rebates may be out of level, honeycombed or interrupted by tie holes. The slab edge or upstand may contain a damp-proof membrane that has been damaged during earlier work, and the base of the opening may be the coldest part of the assembly, which makes it the place where interstitial condensation appears first.

On suspended slabs the sill is often close to the slab edge, and the survey should note how the external finish, the slab edge cover and the internal floor build-up meet. Replacement of the sill may require the removal of a floor finish or a skirting to allow the membrane and flashing to be turned up correctly at the internal face.

Recording the Survey

A useful survey record is photographic and dimensional. It should show the opening with the frame removed, the substrate at the base, the condition of the cavity or framing, the position and condition of the existing flashing and membrane, the location of the original fixings, and the level of the substrate relative to the internal floor and the external paving or cladding line. Levels should be recorded with the instrument used noted, and defects should be described rather than simply photographed, so that the person detailing the replacement can see what the installer saw.

Sill Pans and Flashing Concepts

The terms sill pan and sill flashing are sometimes used interchangeably, and in many systems they describe the same component. The distinction worth keeping is functional. A flashing is the water-shedding element; a pan is the flashing formed as a tray, with a rear upstand and turned ends, so that water collected beneath the frame cannot escape sideways or inward.

The Pan as a Collection Tray

A pan works because it is continuous and because it is below the frame rather than behind it. Continuity means the full width of the opening, plus the depth of the jamb returns, with no interruption at mullions, couplings or structural posts. A pan that stops at each side of a mullion, or that is cut to allow a fixing bracket through, is no longer a tray; it is a set of trays, each with an open end.

Upstands and Rear Legs

The rear upstand is the part of the pan that turns up against the wall’s weather barrier. Its function is to prevent water that has collected in the pan from continuing inward, and to provide a lap for the membrane. The upstand must be high enough to sit above the expected water level in the pan and to allow the membrane to overlap it properly, and it must be fixed in a way that does not puncture it at low level.

The front edge is equally important. It should project past the face of the cladding or the reveal finish and terminate with a drip, so that water leaves the assembly clear of the wall rather than running back across the underside. A front edge that is flush with the render, or buried behind it, will stain the wall and eventually draw water back into the junction by capillary action.

Laps and Their Direction

Every lap in a water-shedding assembly must be arranged so that the upper element overlaps the lower one in the direction of flow. That rule is simple, and it is broken constantly, particularly where a pan meets a jamb flashing, where a membrane meets a pan, and where a pan is extended to suit a wide opening. Horizontal laps in a pan should not occur at all if a single pressing can be used, and where they are unavoidable they must be lapped in the direction of fall over a solid backing, sealed, and kept clear of the drainage path.

Sealant in a lap is a secondary measure. It should not be the reason the lap works. Laps are held by geometry, by the stiffness of the material and by fixing, and sealant is applied to the lap to prevent capillary travel, not to replace the overlap.

Materials and Compatibility

Aluminium, coated steel, stainless steel and formed plastic pans are all used, and each brings different behaviour. Aluminium is light and easily formed but must be isolated from mortar, from alkaline run-off and from dissimilar metals. Stainless steel performs well in aggressive environments but requires the correct grade and compatible fixings. Coated steel depends entirely on the integrity of its coating at cut edges and at fixings. Plastics avoid corrosion but must be compatible with the bituminous or butyl materials they may contact and must be resistant to the movement the detail requires.

Whatever the material, the survey should identify what the pan will touch: mortar, timber treatment, sealant, membrane, insulation, and the frame’s own finish. Dissimilar metal contact, plasticiser migration between a membrane and a sealant, and bitumen against an unpainted aluminium surface are the common incompatibilities, and each is cheap to avoid at the detailing stage and expensive to discover after the cladding is closed.

End Dams

End dams are the least visible and most frequently omitted part of a sill detail. They are the upturns at each end of the pan that turn the tray into a container rather than a chute. Without them, water collected beneath the frame runs sideways into the jamb, into the cavity, or into the wall lining, and it does so at the point where the frame and the wall meet, which is precisely where there is least opportunity to dry.

Why the Ends Fail

Ends fail for predictable reasons. The dam may be folded but not sealed at the corner, leaving a pinhole that passes water steadily. It may be formed short of the jamb line, so that the last part of the pan is open. It may be lower than the front edge, so that the pan overflows at the end before it overflows at the front. It may be interrupted by the frame fixing, or by a packer that was placed after the pan was installed. And it may be defeated by the membrane, which is often lapped over the rear upstand but left short at the ends, leaving the corner where the two meet unprotected.

Formed Versus Applied

Formed ends, produced as part of a single pressing or a welded fabrication, are inherently more reliable than ends created on site by folding a flat sheet and sealing the corner. Site-formed ends can perform well, but they depend entirely on the skill of the folder and on the preparation of the corner before sealant is applied. Applied end pieces, screwed or adhered into position after the pan is laid, introduce two new joints at the point of highest water concentration and should be treated with suspicion unless the system is designed that way and the installation follows the manufacturer’s sequence exactly.

Whichever approach is used, the test is the same: fill the pan with water and watch the ends. If water appears at the corner within a short period, the dam is not a dam.

Weeps and the Drainage Path

Weeps are the openings that let water leave. Their performance depends less on their size than on their position, their continuity and whether anything is allowed to block them after the sill is installed.

Position and Continuity

Weeps must be at the lowest point of the drainage path they serve. If the pan falls toward a point that is not wepped, the water will stand until it finds a lap. If the weep is above the lowest point, the pan will hold a permanent shallow pool. If the weep is blocked by a baffle that was installed the wrong way round, or by a render bead that was run across the opening, the path is closed while appearing to exist.

The frame’s own drainage outlets must align with the pan and with the exterior, not with the cavity. It is common to find frame slots that discharge directly into the wall cavity because the frame was set too far back, or because the pan was detailed without reference to the frame’s drainage positions. In that condition the frame drains correctly and the wall receives the water.

Pressure Equalisation and the Drained Cavity

Modern window detailing relies on the principle that a drained and ventilated cavity behind the cladding, and a compartmentalised drainage chamber within the frame, reduce the pressure difference across the outer seal. That principle only works if the cavity is continuous, if the compartments are actually separated, and if the weeps are open. When a cavity is filled with mortar or blocked by insulation installed without a drainage path, the assembly reverts to a face-sealed detail without the seals to support it.

It is worth stating clearly that the cavity is not a drain. Water that enters the cavity should leave it at a defined point, at the base of the wall or at a flashing, and the sill flashing is one of those points. A cavity that has become the primary drainage path is a defect, not a system.

Membranes and the Wall’s Weather Barrier

The membrane, or weather barrier, is the wall’s own defence, and the sill is where it must hand over to the window’s defence. The handover is a lap, and the lap has a direction. The membrane should pass over the rear upstand of the pan, not behind it, and it should be terminated and sealed so that water running down the wall face arrives on the pan rather than behind it.

Termination and Lap

A membrane that stops above the upstand leaves a gap that is invisible once the frame and lining are installed and that admits water directly to the framing. A membrane that is dressed behind the upstand creates a path for water to pass between the two layers by capillary action. The correct arrangement is a shingle lap: wall barrier over the upstand, with the lower edge of the barrier at or below the top of the upstand, and the flashing’s own material continuing outward beneath the frame.

Self-Adhered and Fluid-Applied Membranes

Self-adhered sheet membranes depend on substrate preparation and primer for their bond, and they do not adhere reliably to dusty, damp or frozen surfaces. Fluid-applied membranes depend on film thickness and cure, and they are difficult to inspect once covered. Both must be detailed at the corners, where the membrane turns from the jamb into the sill, with a reinforcement or a pre-formed corner piece rather than a simple fold.

Neither type should be used to bridge a movement joint. Where movement is expected between the frame and the wall, the sealant joint and the membrane termination must be detailed to accommodate it separately, or the membrane will be the first element to tear.

Interfaces: Masonry, Render and Timber

The same functional detail produces very different construction details depending on the wall type, and the differences are where most failures originate.

Masonry

In masonry the flashing must be built in, not laid on top. The rear upstand is typically turned up against the inner leaf or the frame, and the flashing is carried through the cavity to the outer leaf, where it terminates with a drip clear of the wall face. Weep openings must be left in the outer leaf at the flashing level, and the cavity must be kept clear behind them. Where the sill brickwork is supported on a steel angle, the flashing must pass over the angle, not under it, and the angle itself must be protected.

Render

Render changes the geometry because it adds thickness to the wall face and because it is applied after the flashing. The flashing must project beyond the finished render line, which means its projection must be set before the render thickness is known with certainty. This is a common source of failure: the flashing is set to the substrate, the render is applied thicker than expected, and the drip is buried. The render should terminate in a bead or a groove above the flashing, with a clear gap maintained, and the joint between render and frame should be treated as a movement joint rather than a rigid seal.

Timber and Fibre-Cement Cladding

With timber weatherboards the sill flashing is usually a combination of a pan and a scribed or formed sill trim, with the cladding terminating above the flashing and a drip provided at the board edge. The critical detail is the junction between the bottom board and the flashing, where wind-driven water can be forced upward. A flashing that relies on the board sitting tightly on it will leak; the board should be held clear and the flashing should extend behind the cladding line.

Fibre-cement cladding behaves similarly but is less forgiving of point load and more dependent on the correctness of the jointing system. Sheet edges at the sill should be supported, sealed and drained, and the flashing should be continuous behind the sheet line rather than fitted between sheets.

Frame Support, Packing and Fixing

A sill replacement is also an opportunity to correct the support of the frame, and it is a wasted opportunity if the frame is simply re-fixed as it was found. Aluminium frames are stiff but not rigid over an opening, and the base of the frame carries the weight of the sashes, the glass and the operating hardware.

Packing and Load Path

Packers should be at the fixing points, should be of a material that will not crush or rot, and should be sized so that the frame bears on them rather than on the sill trim or the internal lining. Timber offcuts used as packers are a common defect: they compress, they hold moisture against the frame, and they decay. The load path from the frame to the structure should be continuous and should not pass through the flashing in a way that punctures it.

Fixings and Corrosion

Fixings must be of a material and coating appropriate to the exposure, and they must be isolated from the aluminium where dissimilar metal contact is possible. Screws should not penetrate the horizontal surface of the pan at low level, because every penetration is a potential leak and a potential collection point for debris. Where a fixing must pass through the flashing, it should be sealed with a compatible washer and sealant, and the detail should be treated as a known weakness to be inspected.

Movement

Frames move with temperature, and the building moves independently. The fixing detail must allow the frame to move without transferring that movement into the flashing or the render. Slotted connections, correctly positioned packers and a sealant joint designed for movement are the standard means, and they must be consistent with the frame system’s own installation instructions and with AS 2047.

Glazing, Gaskets and the Frame’s Own Drainage

The sill is the bottom of the frame’s drainage system, and the glazing is where most of the water enters. Water passes the external glazing gasket, runs down the rebate, and should reach the frame’s drainage chamber and its outlets. If the gasket is perished, if the setting blocks have moved, or if the rebate is blocked with debris, the water arrives at the sill faster than the sill was designed to handle.

Glass selection and installation are governed by AS 1288, insulating glass units by AS 4666, and safety glazing by AS 2208. The relevance to sill replacement is practical rather than theoretical. Units that have lost their edge seal will show condensation and will shed water into the rebate. Gaskets that have hardened will not reseat after the frame is disturbed. And any glazing that must be removed to allow the sill to be replaced should be reinstalled with new gaskets and setting blocks rather than reused, because the old ones will have taken a set.

The frame’s drainage slots should be checked against the pan before the frame is re-fixed. If the slots sit behind the pan’s upstand, or over a packer, the frame will drain into the wall no matter how well the pan is made.

Sealants: Where They Belong and Where They Do Not

Sealant is the most over-used material in sill work and the most frequently blamed for failures it did not cause. The governing rule is that sealant closes a joint; it does not create a water line.

The Sealed and Unsealed Rule

Joints that should be sealed are those that must exclude water and air at the face: the junction between the frame and the wall finish, the junction between the sill trim and the frame, and the perimeter of the opening where the cladding meets the frame. Joints that should not be sealed are the weeps, the drainage slots, the lap between the pan and the membrane where water must pass, and any gap that forms part of the pressure equalisation path. Sealing a weep because water is coming out of it is the single most common error in sill repair; the water is coming out because the weep is working.

Joint Design and Application

A sealant joint needs a backing material and a bond breaker so that it adheres to two faces rather than three. Three-sided adhesion prevents the sealant from stretching and causes it to tear from the substrate when the joint moves. The joint should be clean, dry and primed where the manufacturer requires it, and the sealant should be tooled to a profile that sheds water rather than one that forms a shelf.

Compatibility must be checked between the sealant, the membrane, the frame finish and the pan material. A neutral-cure silicone that is suitable for aluminium may not be suitable over a bituminous membrane, and a urethane that bonds well to concrete may stain a porous stone sill. A small test application, left to cure and then examined, is a reasonable precaution on any unfamiliar combination.

Sequencing and Hold Points

Most sill failures are sequencing failures. The detail is correct on paper and is destroyed by the order in which it is built. A workable sequence for replacement is as follows.

Remove the frame, the glazing and the existing sill finishes, and protect the opening from weather.
Survey and record the substrate, the cavity, the framing and the existing membrane.
Repair the substrate and the framing, and confirm the support and fixing positions.
Install the pan or flashing as a continuous element, with formed ends and a front drip clear of the finished wall face.
Seal the laps and corners, and test the pan before it is covered.
Install the membrane over the rear upstand, with reinforced corners and a shingle lap.
Re-fix the frame on correct packers, with isolated fixings that do not puncture the pan.
Reinstall the glazing with new gaskets and setting blocks, and check the frame drainage alignment.
Terminate the cladding or render above the flashing, keeping weeps and the drip clear.
Install the internal lining, trim and finishes without disturbing the membrane.
Apply sealant last, to the joints that should be sealed, and leave the drainage path open.
Test, inspect and record before handover.

The hold points in that sequence are the pan test, the membrane inspection before the frame is re-fixed, and the inspection of the cladding termination before the internal lining closes the work. Each is inexpensive to inspect and expensive to revisit.

Water Testing and Verification

Testing a replaced sill is not a formality. It is the only way to confirm that the water line is continuous before the interior is finished. Two methods are in common use, and they answer different questions.

Hose Testing

A hose test applies water to the exterior face at a controlled rate and direction for a defined period, and it is useful for identifying gross defects at the frame-to-wall junction and at the sill ends. It is quick, it can be done at each stage of the sequence, and it is well suited to small replacement work. Its limitation is that it does not reproduce the pressure difference that drives water through a defect during a storm, so a passing hose test is evidence of sound workmanship rather than proof of performance.

Chamber Testing

Chamber or spray-rack testing applies water together with a controlled pressure difference across the assembly, and it is the method described in AS 4284 for the testing of building facades. It is more representative of service conditions and is the appropriate method where the assembly is part of a tested system or where the consequences of failure are significant. It requires equipment, calibration and a documented procedure, and it should be carried out by a competent testing provider before the internal lining is installed.

Interpreting Results

Water appearing at an internal lining during a test is a failure, but the absence of visible water is not necessarily a pass. Water can travel within the cavity and appear elsewhere, and it can be absorbed by materials that will later stain or decay. The test record should note the method, the duration, the pressure where applicable, the points observed and the condition of the weeps and drainage outlets before and after. Where a defect is found, the repair should be made and the test repeated rather than assumed to be effective.

Maintenance and Inspection

A correctly built sill needs little maintenance, but it needs some, and the maintenance is almost entirely about keeping the drainage path open. Weeps and drip edges should be checked and cleared at the start of each wet season and after any event that deposits debris, such as nearby construction, paving works or a windstorm. Sealant joints should be inspected annually for adhesion, cracking and compression set, and replaced when they have lost section rather than when they have visibly failed.

Render and cladding should be checked for cracks at the sill line and around the opening, because a crack in the render above a flashing can deliver water behind it. Internal linings, skirtings and floor edges should be checked for staining or swelling, particularly after prolonged rain, since these are often the first indicators that the external path has been compromised. Fixings should be checked for corrosion and for loss of isolation, and any fixing that has been sealed should be re-inspected at each maintenance cycle.

Standards, Regulation and the Compliance Context

In Australia, sill and window installation work sits within the National Construction Code, published by the Australian Building Codes Board, together with the ABCB Housing Provisions for residential work. The NCC establishes the performance requirements for weatherproofing and for structural adequacy; the referenced standards provide the means of demonstrating them. AS 2047 covers windows and external glazed doors in buildings and is the primary reference for the product and its installation. AS 4284 covers the testing of building facades. AS 4666 covers insulating glass units, AS 1288 covers glass selection and installation, AS 2208 covers safety glazing materials, AS 3700 covers masonry structures, and AS/NZS 2904 covers damp-proof courses and flashings.

Compliance is not established by the presence of a standard number on a drawing. It is established by the installation matching the tested system, by the flashing being continuous and correctly lapped, by the drainage path being open, and by the work being inspected at the hold points. Where a building consent or development approval applies, the relevant authority should be consulted before work begins, particularly where the replacement changes the appearance of the facade or the position of the frame.

In New Zealand the framework differs but the principles do not. Clause E2 of the Building Code sets the external moisture performance requirement, and E2/AS1 provides an Acceptable Solution for timber-framed construction. NZS 3604 covers timber-framed buildings, NZS 4223 covers glazing in buildings, and SNZ TS 4211:2022 covers windows and doors performance. The drained cavity, the flashing, the end dam and the weep remain the same physical response to the same physical problem, and a detail that works in one country will generally work in the other provided the exposure, the wall type and the specific system are considered.

Frequently Asked Questions

Can the sill be replaced without removing the window frame?

Sometimes, but rarely well. A sill trim or cover can be replaced with the frame in place, but the pan, the membrane lap and the end dams sit beneath and behind the frame. Working around a fixed frame usually means the flashing is cut, lapped in the wrong direction, or sealed instead of drained. Where the pan is being renewed, removing the frame is the reliable approach.

How do I know whether the existing flashing can be reused?

Expose it and test it. If it is continuous, falls to the exterior, has formed ends, is not perforated by fixings and is correctly lapped by the membrane, it can generally be retained and cleaned. If it is interrupted, reversed, corroded through or bonded to the frame with sealant, it should be replaced.

Why does water still appear inside after the sill was replaced?

The most common reasons are an unsealed end dam, a membrane terminated above the upstand, a weep blocked by render or sealant, and a frame drainage slot that discharges behind the pan. Each is a continuity defect rather than a material defect, and each is found by testing the assembly in stages.

Should the joint between the sill and the wall be sealed?

The visible joint between the frame and the wall finish should be sealed, with a joint designed for movement. The drainage openings and the lap between the flashing and the membrane should not. Sealing everything is not a strategy; it is a way of hiding the path that water needs.

Does a new sill need to be tested?

It should be. A pan test before the frame is re-fixed and a hose or chamber test before the internal lining is installed will find most defects while they are still inexpensive to correct. The test method and results should be recorded as part of the project documentation.

What is different on a rendered wall?

The flashing must be set out to clear the finished render, not the substrate, and the render must terminate above the flashing with a gap and a drip rather than being brought down over it. Render thickness is the most common reason a correctly made flashing ends up buried.

Conclusion

Aluminium window sill replacement is a small scope of work with an unusually large number of dependencies. The visible sill is the least significant part of it. What matters is the substrate that receives the flashing, the continuity of the pan, the integrity of the end dams, the openness of the weeps, the lap of the membrane and the order in which the trades work.

The decision between repair and replacement should follow the water path rather than the visible damage. The survey should establish what the new detail will be fixed to, and should be recorded well enough that the person detailing the work can see the same conditions. The pan should be continuous and tested before it is covered. The membrane should lap over the upstand. The frame should be supported on sound packers and fixed without puncturing the water line. The cladding and render should terminate clear of the flashing. The sealant should be applied last and only where it belongs. And the assembly should be tested and inspected before the lining closes it.

None of those steps requires unusual skill or unusual material. They require that the water line be treated as one continuous element from the wall to the exterior, and that every trade that touches it understands where it runs. A sill that is built that way will quietly do its job for the life of the window. A sill that is not will be opened up again within a season, and the second repair will be a larger job than the first.

References

Australian Building Codes Board. National Construction Code 2025, Volume One and Volume Two, including the ABCB Housing Provisions. https://ncc.abcb.gov.au
Standards Australia. AS 2047, Windows and external glazed doors in buildings. https://www.standards.org.au
Standards Australia. AS 4284, Testing of building facades. https://www.standards.org.au
Standards Australia. AS 4666, Insulating glass units. https://www.standards.org.au
Standards Australia. AS 1288, Glass in buildings – Selection and installation. https://www.standards.org.au
Standards Australia. AS 2208, Safety glazing materials in buildings. https://www.standards.org.au
Standards Australia. AS 3700, Masonry structures. https://www.standards.org.au
Standards Australia and Standards New Zealand. AS/NZS 2904, Damp-proof courses and flashings. https://www.standards.org.au
Standards New Zealand. NZS 3604, Timber-framed buildings. https://www.standards.govt.nz
Standards New Zealand. NZS 4223, Glazing in buildings. https://www.standards.govt.nz
Standards New Zealand. SNZ TS 4211:2022, Windows and doors performance. https://www.standards.govt.nz
Ministry of Business, Innovation and Employment. New Zealand Building Code, Clause E2 External moisture, and E2/AS1. https://www.building.govt.nz

Aluminium Window Sill Replacement: Substrates, Flashing and Drainage Continuity