⚖️ Weight Distribution
It sounds simple, but there is a critical detail: the glazing unit weighs a lot. A double or triple energy glass unit for a balcony door can weigh from 50 to over 100 kg! It is also an absolutely rigid material.
Why Does the Window Sag? The Secret of Correct Glass Packing
Have you ever tried to close a window or balcony door, only to find that instead of closing smoothly, it creaks at the bottom, drags against the frame or won't close at the corners? In the trade, this is called "the sash has dropped".
The vast majority of people (and quite a few tradesmen!) blame the hinges or request adjustment. The truth, however, in about 90% of cases, lies somewhere entirely unexpected: in the glass packing (shimming).
In this article we explain in detail this lesser-known but critical aspect of window installation.
1. The Anatomy: How the glass is held inside the sash
An opening aluminium (or PVC) sash consists essentially of an aluminium frame (surround), inside which the glazing unit sits. The glass is held in position by small plastic packers (shims) and secured with a bead (glazing cap).
📐 Critical: Where It Rests
If we take that heavy, rigid glass and simply "rest" it loosely inside the lightweight aluminium frame, gravity does its work. The enormous weight pulls the handle side downward. The rectangular frame deforms into a rhombus. This is precisely what "sagging" is.
🔑 The Role of Glass Packing
And this is where glass packing (shimming) comes in: the science of exactly where the packers are placed inside the sash so that the weight of the glass is put to work for stability.
2. The Correct Method: The "Diagonal" Principle
The basic rule is beautifully simple in theory: the packers are placed diagonally.
📏 Hinge Side (Bottom)
At the bottom, on the hinge side, a packer is placed. This "lifts" one side of the glass and transfers its weight directly onto the hinge (the pivot axis).
📏 Lock Side (Top)
At the top, on the opposite side (lock side), a second packer is placed. This "locks" the glass into the corner and creates a force that pushes the sash corner upward, counteracting gravity.
🛡️ The Result
These two packers create an invisible "force zone" along the diagonal. The rigid glass works like a huge diagonal brace (like the beam placed diagonally in a wooden garden gate). When the handle side tries to fall, it "hits" the top packer, presses the glass diagonally, and the pressure ends up at the bottom packer and ultimately at the hinge. The sash becomes "rock solid" and will never sag.
3. What Happens Without Correct Packing: The "Rhombus"
If the packers are placed wrongly (or not placed at all!), the weight of the glass offers no support. Gravity then starts to "loosen" the internal corners, and the frame slowly shifts from a rectangle into a rhombus shape.
📉 Visible Symptoms
The bottom corner (handle side) drops, the top corner rises. The door won't close evenly - it drags, rubs, doesn't latch, or leaves gaps.
🔓 Loss of Insulation
The gaskets don't "press" evenly against the frame. Result: draughts, noise, possible water leaks.
❌ Mechanism Damage
The hinge takes uneven load. Over time it wears prematurely and the situation worsens.
❓ Why Does It Happen?
Usually due to: hasty construction (packers not in diagonal corners), wrong material (soft packers that compressed in the summer heat), or transport (a packer slipped during delivery to the building site).
4. The Fix: Correction & Prevention
If your window already "sags", the solution is not always replacing the hinges. Often, an experienced technician can remove the glass, re-position the packers correctly and re-level the sashes.
🔧 Re-packing (4-Step Process)
1. Remove beads with a plastic tool (not a screwdriver, it scratches the aluminium). 2. Lift the glazing unit with a glazing shovel. 3. Place packers correctly along the diagonal. 4. Re-fit the beads. The job takes roughly 10 minutes for an experienced technician.
🏗️ Factory Prevention
In a serious window factory, glass packing is done before the window leaves the production line. If the installer removes the packers during fitting (e.g. for "convenience"), they must always re-install them.
✅ Self-Check
A quick self-check: with the door closed, check whether it leaves a smaller gap at the top (handle side) than at the bottom. If so, it probably needs re-packing.
💡 Tip: If your balcony door "sags", don't rush to replace the hinges. Ask the technician first to check whether the packers are correctly placed along the diagonal. The "cure" costs very little and fixes the majority of cases!
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