A good aluminium window installation is invisible. The window opens easily, the plaster line is clean, and nothing leaks when the southeaster drives rain against the wall for three days. A bad one announces itself within a winter.
This guide walks through the eight steps a proper installation follows, what each one should look like, and the specific failures that show up in Cape Town houses a year or two later.
Key takeaways
- A single-window swap takes 2 to 4 hours; a full house is usually 2 to 4 days.
- Installation on a sound opening adds roughly R900 to R2,200 per window.
- The frame must be mechanically fixed, not held in place by foam or sealant alone.
- Almost every leak traces back to the seal or a missing drip detail, not the window.
- Get the making-good included in writing, or you will be left with a rough opening.
The eight steps
The eight steps of a proper aluminium window installation
The order matters. Skipping or reordering any of these is where problems start.
Measure and order
The brick opening is measured in three places for width and height, and the smallest reading governs. The frame is ordered about 10mm smaller all round. Anyone who measures the old frame instead of the opening is guessing.
Protect the room
Furniture moved or covered, floors sheeted, a dust screen on the doorway. Cutting out a steel frame produces grinding dust that travels through a whole house.
Remove the old frame
Glass out first, then the frame cut free in sections rather than levered. Levering pulls brickwork and doubles the making-good.
Prepare the opening
Loose plaster chipped back to sound material, the sill checked for fall to the outside, and the lintel inspected. Any of these wrong and the new frame inherits the problem.
Set and level
The frame is packed on shims and checked for level, plumb and square across both diagonals. A frame out of square by 3mm will bind on the vent within a year.
Fix to the structure
Screws or lugs into brick at roughly 450mm centres, never into mortar joints alone. Foam is an insulator and a gap filler, not a fixing.
Seal and form the drip
Backing rod then a neutral-cure silicone joint outside, sized so it can move. The sill needs a drip so water leaves the wall instead of tracking back under the frame.
Make good and hand over
Plaster repaired inside and out, paint touched in, hardware adjusted, protective film removed, and the vent opened and closed in front of you.
The two details that decide whether it leaks
Almost every window leak we are called to fix comes down to one of two things, and neither is the window.
Silicone applied straight into a deep gap with no backing rod bonds on three sides. It cannot stretch, so it tears the first time the frame expands in summer heat. The joint needs backing rod behind it and a width roughly twice its depth.
Without a drip groove or a projecting sill, water runs down the wall, reaches the frame, and travels back underneath it by surface tension. The wall looks wet inside and everyone blames the window.
There is a third, rarer one worth knowing about: blocked drainage slots. Aluminium windows are designed to let water that gets past the outer seal drain back out through small slots in the bottom of the frame. Paint them over or pack them with silicone and the frame fills up like a gutter.
What aluminium window installation costs and how long it takes
| Job type | Time per window | Install cost | Notes |
|---|---|---|---|
| Like-for-like, ground floor | 2 – 3 hours | R900 – R1,500 | Sound opening, easy access |
| Like-for-like, upper floor | 3 – 4 hours | R1,400 – R2,200 | Access and protection add time |
| Steel frame removal | 4 – 5 hours | R1,600 – R2,600 | Cutting out, heavier making-good |
| Opening widened | 1 – 2 days | R4,500 – R12,000 | New lintel, propping, brickwork |
| New opening in solid wall | 2 – 3 days | R8,000 – R20,000 | Structural, may need approval |
| Installation only, excluding the window unit. Whole-house jobs are cheaper per opening because the team, scaffolding and plasterer come once. | |||
Unit prices sit on top of these. Our guide to aluminium windows sizes and prices covers frame costs by size, and aluminium window prices in Cape Town puts the two together.
Do you need council approval to replace windows?
Replacing a window like for like in the same opening does not need plans. You are maintaining, not altering.
The moment the opening changes size, it is a different conversation. Widening an opening in a load-bearing wall, adding a new window, or changing anything on a heritage-protected facade needs approval from the City of Cape Town before work starts. Energy-efficiency requirements under SANS 10400-XA also apply to new and altered glazing, and the technical standards behind that sit with the South African Bureau of Standards.
If the house is over 60 years old, heritage rules may apply to anything visible from the street, even a like-for-like swap. Worth a phone call before the frames are ordered rather than after.
We measure, supply and install across greater Cape Town, and make good behind ourselves.
How to tell a good installer from a cheap one
You cannot judge an aluminium window installation by looking at the finished window, because a bad one looks fine for the first eight months. You judge it by what the installer says before they start.
- They measure the opening themselves rather than working off your numbers.
- They tell you how the frame will be fixed, and the answer involves screws.
- They mention backing rod when they talk about sealing.
- They ask which way the weather comes from.
- They include removal, disposal and making good in the written quote.
- They say how long the job takes and what will be open to the weather overnight.
The cheapest quote is usually cheapest because it stops at a clean opening. Someone still has to plaster and paint that, and if it is not the window installer it will be you.
Once the windows are in, the only ongoing job is keeping salt and grit off the hardware, covered in the coastal maintenance guide. You can see completed installations across the Northern Suburbs and Helderberg in our project gallery.
Preparing your house before the installers arrive
A little preparation shortens the job and protects your things. None of it is difficult, and installers will quietly thank you for it.
Clear a working zone of about a metre inside and outside each window. Take down curtains, blinds and rails, because they will come down anyway and it is better done by someone who knows how they go back up. Move anything breakable out of the room entirely rather than covering it, particularly where a steel frame is being cut out.
Outside, clear plants, pots and furniture away from the wall and make sure there is a path to bring frames in. A 1800 x 1800 unit with glass in it is heavy and awkward, and dragging it across a flowerbed helps nobody.
Think about pets and about dust. Grinding out a steel frame is the dustiest thing that will ever happen in that room, and fine dust travels. A closed door with a damp towel at the base is worth more than it sounds.
Finally, agree up front what happens at the end of each day. On a multi-window job there should never be an opening left unglazed overnight, and if the weather turns, the sequence should change rather than the house being left open.
What to check before you sign the job off
Handover is the last point at which small faults are free to fix. Walk the job properly rather than nodding from the doorway.
- Open and close every vent and lock every catch. Anything stiff now will be worse in a year.
- Look along the frame from the side. It should be dead straight, with an even gap around the vent all the way round.
- Check the outside sealant joint is continuous, smooth and pressed into the gap, not a thin bead skimmed over the top.
- Find the drainage slots on the bottom of the frame and confirm they are open.
- Check the plaster is made good inside and out and the paint is touched in.
- Confirm the protective film has been peeled off. Left in sun for months it bakes on and becomes very hard to remove.
- Ask for the guarantee in writing, covering both the unit and the installation.
If anything is not right, say so at handover. A good aluminium window installation team will fix it the same day. Once the invoice is settled and the van has gone, the same fault becomes a favour rather than an obligation.
Frequently asked questions
How long does an aluminium window installation take?
A straightforward like-for-like replacement takes two to four hours per window. A full house of ten to fifteen windows is usually two to four days, and the team works room by room so the house is never fully open.
How much does it cost to install an aluminium window?
Installation on a sound existing opening runs about R900 to R2,200 per window in 2026, on top of the unit price. Steel frame removal pushes that to R1,600 to R2,600, and widening an opening is a separate structural job.
Can I install an aluminium window myself?
On a newer house with a square standard opening, a competent DIYer can manage it. The risk is the seal and the fixing, which are the two things that cause leaks, and neither is obvious until the first driving rain.
Do I need planning permission to replace a window?
Not for a like-for-like replacement in the same opening. You do need approval if the opening changes size, if a new opening is created, or if the property is heritage protected.
Should the window be fixed with foam or screws?
Screws or fixing lugs into brick, at roughly 450mm centres. Foam fills the gap and helps with insulation and draughts, but it is not a structural fixing and a frame held in by foam alone will move.
Why is my new aluminium window leaking?
Usually the sealant joint has failed because it was applied without backing rod, or there is no drip detail on the sill so water tracks back under the frame. Blocked drainage slots in the bottom of the frame are the third common cause.
Can aluminium windows be fitted in winter?
Yes, though silicone needs a dry surface to bond properly, so heavy rain stops sealing work. Good installers plan the openings so nothing is left unglazed overnight.
