T6 Aluminium Grade: Why It Matters for Windows & Doors

T6 Aluminium Grade: Why It Matters for Windows, Doors & Facades

Walk onto almost any construction site in India and ask what grade of aluminium is going into the doors, windows, or facade, and you'll often get a vague answer — or none at all. That gap is exactly why this topic keeps coming up in the industry: T5 grade aluminium is still being used in situations where T6 grade aluminium for windows and doors is the more appropriate, safer choice — and very few people outside the fabrication world know to ask the question.

This blog looks at why that gap exists, what the standards actually say, and how you can make sure your project gets the right grade the first time.

The Problem: T5 Is Still Widely Used, Even at Height

For lower windows, small openings, or lightly loaded trims, T5 aluminium is a perfectly standard, code-recognized choice. The concern raised across the industry is more specific: facades, doors, and windows around 8–9 ft, 10 ft in height or larger are sometimes still fitted with T5-grade sections, when the load on that span calls for the higher strength of T6.

This isn't a fringe issue. In many mid-size Indian cities, fabricators default to whatever grade is cheaper and locally available, rather than what the specific window or facade actually needs. Because the difference between T5 and T6 isn't visible to the eye, it's easy for it to go unnoticed — until wind pressure, glass weight, or years of use expose the gap.

What Do T5 and T6 Actually Mean?

In simple terms, both are heat-treatment states of the same aluminium alloy:

• T5 is cooled and aged after extrusion, without an extra heat-treatment step.
• T6 goes through an additional solution heat-treatment and rapid quench before aging, which gives it noticeably higher strength and hardness.

That extra step is why T6 costs a bit more and takes more process control to manufacture — and why it's generally the safer default for anything carrying real structural load.

Why Height and Span Change Everything

A small kitchen window and a 10-foot facade panel are not the same engineering problem, even if both are made from the "same" aluminium. As height and span increase:

• Wind pressure on the panel increases
• The glass gets heavier and larger
• The frame has to resist more bending and deflection

This is precisely why industry guidance points to T6 once frames or facades reach roughly 8–10 ft or more in height — at that scale, the margin for error with a lower-strength temper narrows considerably.

Why India's Building Codes Don't Always Catch This

What NBC and BIS Actually Say

Here's the part that surprises most people: neither the National Building Code (NBC) nor BIS standards explicitly mandate T6 over T5 for a given window or facade height. IS 733 covers wrought aluminium bars, rods, and sections used in doors, windows, and curtain walls, and wind loads are calculated separately under IS 875 (Part 3) — but the specific temper for a specific opening is left to the structural calculation, not spelled out as a blanket rule.

That's not a flaw in the codes — structural design is meant to be calculated per project. But it does mean there's no automatic red flag stopping an under-specified T5 section from being installed at height, unless someone actively runs the numbers.

Why the Responsibility Falls on Engineers and Fabricators

Because the code doesn't hard-code the answer, the responsibility shifts to the people specifying and fabricating the work: structural engineers, facade consultants, and fabricators. If nobody checks the calculation, the wrong grade can pass through a project unnoticed — not because it's legal to skip the check, but because nothing forces the check to happen.

This is really the heart of the issue: it isn't that T5 is banned, it's that using it without a supporting calculation, at a height where it doesn't belong, is a decision nobody should be making by default.

What Happens When the Wrong Grade Is Used

Under-specifying temper doesn't usually cause an immediate, visible failure. The more common outcomes are:

• Excess deflection in high wind, which stresses glazing seals and sealants over time
• Reduced safety margin during storms or unusually high wind events
• Long-term fatigue or fitting issues that only show up years after installation

None of these are dramatic on day one — which is exactly why the issue is easy to overlook during construction and only becomes obvious later.

How to Know If Your Project Needs T6

Ask these questions before finalizing your aluminium specification:

• What is the height and span of the door, window, or facade panel?
• What wind load applies to this location, per IS 875 (Part 3) or the applicable wind code?
• How heavy is the glazing (single, double, or laminated glass)?
• Is this a primary structural member, or a secondary/decorative one?
• Has a structural engineer actually calculated deflection and strength for this specific section — or was the grade just assumed?

If the answer to that last question is "assumed," that's the point to pause and get it checked.

A Simple Checklist Before You Finalize Your Aluminium Grade

• Ask your fabricator which temper (T5 or T6) is being used, by name — not just "6063 aluminium."
• Request the mill test certificate confirming the actual grade supplied.
• For anything above roughly 8–9 ft in height, or any primary structural member, ask for the structural calculation that supports the chosen temper.
• Don't assume compliance with NBC or BIS means the temper has been checked — it usually means the load calculation needs to be checked separately.

Frequently Asked Questions

Is T5 aluminium illegal to use in India?

No. T5 is a recognized, standard temper. The issue isn't that it's illegal — it's that it's sometimes used at heights or spans where the load calculation would call for T6 instead.

Do NBC or BIS require T6 for all windows and facades?

No. Neither NBC nor BIS mandates a specific temper by height. Wind loads are calculated under IS 875 (Part 3), and the resulting temper choice is meant to come from a structural calculation, not a fixed code table.

At what height should I be asking about T6?

Industry practice generally flags facades, doors, and windows around 8–9 ft, 10 ft or taller as the point where the difference in strength between T5 and T6 becomes structurally significant enough to verify.

How can I check what grade was actually used in my project?

Ask your fabricator for the mill test certificate, which should state the alloy and temper (e.g., 6063-T5 or 6063-T6) supplied for the job.

Is T6 always more expensive?

Generally, yes, due to the extra heat-treatment step — but the cost difference is usually small relative to the cost of correcting a facade or window failure later.

Who is responsible for specifying the right grade?

The structural engineer or facade consultant on the project is typically responsible for confirming the correct temper through calculation, though fabricators and architects also play a role in raising the question.

Conclusion: Ask the Right Question Before You Sign Off

The T5-vs-T6 gap in Indian construction isn't really a code violation — it's a communication gap. Nothing stops the wrong grade from being used at height, because the standards leave that decision to project-specific calculation, and that calculation doesn't always happen. The fix is simple: ask what grade is being used, ask for the certificate, and for anything of real height or structural importance, ask for the calculation behind it. That one question is often the only thing standing between a facade that performs for decades and one that quietly falls short.