Glass, aluminium frames and how they connect: where building durability is won or lost

When a window or glazed façade fails, the glass is almost never the problem. What fails is the connection: the seal that has split, the fixing that has corroded, the frame that was packed wrongly, or the flashing that sends water into the wall instead of out of it. That is also why the problem is expensive. The leak is hidden, it runs for years, and by the time it shows, the lining, the framing and sometimes the structure behind it are damaged.

This article is for building managers, strata committees, builders and trades. It covers how a glazed frame connects to the building, which materials and details matter, and what poor choices cost you in the short term (maintenance and call-outs) and the long term (durability and building integrity).

A note on what we have and have not verified. The NCC clauses and standards named below were checked against the published text or the standard's own scope. Where a point is site experience or engineering reasoning, we say so.

Why the connection matters more than the glass

A glazed assembly has four jobs at the edge of the building:

1. Carry wind loads to the structure without passing building loads into the glass or frame.
2. Keep water out at every joint.
3. Move with heat, wind and building settlement without cracking seals or fixings.
4. Last in the environment it is in: sun, salt air, cleaning chemicals, wet cement.

Most failures come from one of those four being ignored at the connection, not from the glass or the frame itself.

What the code and standards say

• NCC 2022 Housing Provisions, Section 8 covers windows and external glazed doors (Part 8.2), glass (Part 8.3) and glazing human impact (Part 8.4). It applies to Class 1 and 10 buildings; apartment and commercial buildings are covered by NCC Volume One, with their own performance requirements.
• Clause 8.2.2, installation of windows, requires that structural building loads are not transferred to the window assembly, that a gap of at least 10 mm is left between the top of the assembly and any loadbearing framing or masonry (more over wide openings, to allow for frame settlement), and that packing is located along each side and the bottom, fixed so the sides and bottom stay straight, and clear of any flashing material. The explanatory notes link correct installation to weathertightness: it prevents buckling, diagonal distortion and twisting, and it limits gaps for water to enter. They also say sills should be isolated from masonry, for example with a bituminous membrane.
• AS 2047 (windows and external glazed doors in buildings) sets the performance tests for window assemblies. A window that is design-tested to it has been tested for structure, water and air performance as a complete unit, not only as parts. Part 8.3 of the NCC covers glass selection only and does not cover window installation, so the manufacturer's installation requirements still apply.
• AS 1288 (glass in buildings, selection and installation) covers glass sizing and safety glass. It also applies to glass balustrades and to sloped overhead glazing. Glass thickness under Part 8.3 depends on the site's wind classification and the ULS wind pressure from AS 4055, and on the panel dimensions.

The practical point: the code does not tell the installer which sealant, fixing or isolation material to use. It sets the outcome (weathertight, structurally sound, safe) and the standards, and the manufacturer's installation instructions and the designer fill in the rest. That gap is where the problems start.

The five connection points that fail most often

These are drawn from the mechanics of the joint and our experience of what shows up in inspections. They are not a measured failure rate.

1. Dissimilar metals and wet cement

Aluminium is stable in air, but it corrodes quickly when it touches certain metals in the presence of water, or when it sits against wet cement, mortar or render. Corrosion needs three things together: different metals, water as the electrolyte, and electrical contact between them. Remove any one and the problem stops.

• Stainless steel fixings in aluminium are generally acceptable in mild environments, but the risk rises near the coast and in places where moisture is trapped. Anodising on its own is usually not enough to separate the metals, and a drilled or screwed fixing can damage the thin anodised layer.
• Isolation is done with non-absorbent plastic or tape, a compatible sealant, or non-conductive washers and spacers of suitable thickness.
• Wet cement and render are alkaline and attack aluminium. Frames embedded in or poured against masonry need a coating, wrap or separator, and the sill needs to be isolated from the masonry.
• Cleaning chemicals matter too. Acid cleaners for brick and render can stain and etch frames; see our article on cleaning chemicals and building materials.

Our article on material compatibility, sequence and junctions explains the "upstream" rule for metals in more detail.

2. Sealant: the wrong type, or sealant doing the flashing's job

Sealant is the last line of defence, not the first. A properly lapped flashing sheds water. Sealant only closes the gap that remains.

• Use the right chemistry. Acetoxy bathroom silicone is not suitable around aluminium frames, stone or masonry; neutral-cure products are. Our silicone sealant guide explains the choice.
• Where a sealant is structural (holding glass to a frame), it is a certified system, not a cartridge off the shelf. Ask for the compliance information for the system installed.
• Sealant joints need a correct depth-to-width ratio and a backing rod so the bead can stretch. A bead stuck to three sides cannot move and will tear.
• Sealant has a service life. Plan to inspect and replace it, rather than treating it as permanent.

3. Packing, fixing and frame distortion

A frame that is wedged, over-tightened or packed unevenly will bow. A bowed frame means gaps at the seals, sashes that rub or will not latch, and stress on the glass.

• Packing sits on each side and the bottom, and is kept clear of flashing, as Clause 8.2.2 requires.
• Fixings go where the manufacturer specifies, at the specified spacing, into the structure and not only into the lining.
• Where fixings pass through the frame, they need to be compatible with it, sealed where they penetrate, and isolated where the metals differ.
• Leaving the head gap lets the building settle without loading the window. Filling it tight with mortar or foam that cures hard defeats the purpose.

4. Flashing and the sill

The sill is where most window water ends up. The flashing sequence has to send it out, not in.

• The sill flashing should be installed to fall outward, turn up at the jambs, and be lapped by the wall wrap or membrane above and below in the right order.
• Head flashings go over the frame, and the wall wrap laps over the head flashing.
• Weep holes and drainage paths in the frame must remain open. Sealing them to stop whistling or draughts traps water inside the frame.

Our article on where buildings leak walks through this sequence for each junction.

5. Movement: thermal, wind and settlement

Aluminium moves with temperature more than glass does, and long runs of frame and large panes move noticeably. Fixings and seals need to allow that.

• Slotted or floating fixings and movement joints at the right spacing let the frame expand without distorting.
• Rigid grouting or hard mortar around a frame cracks as the building moves, and cracks become water paths.
• In tall or exposed buildings, wind movement adds to this. That is design and engineering work, not something to decide on site.

What poor connections cost: short and long term

In the short term (months to a few years):

• repeat call-outs for leaks, draughts, sticking sashes and failed seals;
• repainting and patching stained linings and reveals;
• mould treatment and remediation (see damp patches and mould: what they are telling you);
• strata levies spent on repeat repairs instead of planned maintenance.

In the long term (5 to 30 years):

• rot in timber framing, corrosion of steel framing and fixings, and swollen or failed linings behind the wall;
• corrosion of the frame itself at fixings and sills, which cannot be fixed by cleaning;
• full frame replacement, which often means removing cladding, reinstating waterproofing and repainting, far beyond the cost of the original window;
• defect claims and disputes, where the question becomes who installed it and whether it was built to the standard (see statutory warranties and defects after handover).

The cost pattern is the same as most envelope failures: a small amount of money and care at installation, versus a much larger repair after the damage is hidden inside the wall.

What to specify and check, by role

Builders and trades:

• Install to the manufacturer's instructions and the details on the drawings. Where they conflict, ask before you proceed.
• Confirm the window or glazing system is tested to AS 2047 and that you are using the tested installation method.
• Isolate dissimilar metals and masonry. Check fixing material against the frame and the environment.
• Photograph the flashing, packing and fixings before they are covered. We treat that as a hold point on every job.

Strata and building managers:

• Ask for the installation records and photos for glazed façade and window replacement works, not just the invoice.
• Include windows, glazed doors, sealants and flashing in the maintenance schedule, with an inspection cycle and a replacement allowance in the capital works fund (see capital works fund 101 and our maintenance schedule guide).
• Look for early signs: stained reveals, bubbling paint near a sill, sticking sashes, white corrosion powder on frames, split or hardened sealant.
• Treat a repeat leak as a connection problem, not a sealant problem, and have it inspected before it is patched again.

Owners and developers:

• Ask who is responsible for the window system, the installation and the flashing, and what warranty covers each.
• Make sure the design documents show the sill, head and jamb details, not only the window schedule.

A pre-installation checklist

1. Is the glazing system tested to AS 2047, and are the installation instructions on site?
2. Is the glass selected for the site's wind classification and use (AS 1288 and Part 8.3 for Class 1 and 10 buildings; the relevant Volume One provisions for others)?
3. What is the frame finish, and what exposure category is the site in (coastal, industrial, sheltered)?
4. Which fixings are specified, and are they compatible with the frame and the substrate?
5. Where are dissimilar metals or wet masonry in contact, and how are they isolated?
6. Is the flashing sequence (sill, jambs, head, wall wrap) drawn and understood by the installer?
7. Where are the movement joints and the head gap, and what sealant and backing rod go in them?
8. What are the hold points, and who signs off the flashing and fixings before lining or cladding goes over them?

How we approach it

At Precision Projects Group we work across new construction, renovation and strata building maintenance, so we see both ends of this problem: how glazing connections are built and how they fail ten years later. We treat the connection details as hold points, check the flashing, packing and isolation before they are covered, and record them in the handover record. For existing buildings we inspect frames, seals and flashings, work out whether a failure is local or systemic, and recommend repair or replacement by cost over the life of the building.

If you manage a building with ageing windows or glazed façades, or you are planning new work and want the connections specified properly, get in touch or see our services.

Sources

• NCC 2022 Housing Provisions, Section 8 Glazing: Part 8.1 Scope and application, Australian Building Codes Board
• NCC 2022 Housing Provisions, Part 8.2 Windows and external glazed doors, Australian Building Codes Board (clause 8.2.2)
• NCC 2022 Housing Provisions, Part 8.3 Glass, Australian Building Codes Board
• NCC 2022 requirements for glass in buildings, Housing Industry Association
• AS 2047, Windows and external glazed doors in buildings; AS 1288, Glass in buildings: selection and installation; AS 4055, Wind loads for housing, Standards Australia
• Compatibility of aluminium and steel, BRANZ

This article is general information, not technical or legal advice for your building. Glazing selection, fixing and flashing details depend on the design, the product's tested installation method, the site's exposure and the building class, and should be confirmed with your designer, certifier and the manufacturer. Full website disclaimer