How to Prevent Window Dropping or Bowing Using Packers
Why Windows Drop or Bow Over Time
A dropped or bowed window is one of the most common problems in uPVC installations. It usually develops weeks or months after fitting and causes:
- Difficulty closing the window
- Lock misalignment
- Visible gaps between sash and frame
- Sash rubbing on the bottom
- Draughts or water leakage
The root cause is almost always incorrect packing, especially incorrect toe-and-heeling or poor weight distribution of the glass.
Correct use of glazing packers prevents nearly all of these issues.
How Packers Support the Sash
A sealed glass unit is heavy and must be supported at specific points to stop the sash from twisting or sagging.
Packers ensure:
- The weight of the glass is transferred correctly to the hinges
- The sash remains square
- The frame is not distorted
- The opening and closing action stays smooth
- The lock lines up correctly
Proper packing is not optional — it is essential for long-term performance.
The Most Important Step: Toe-and-Heeling
Toe-and-heeling is the process of placing packers diagonally opposite each other to counteract the weight of the sash.
This creates a rigid support triangle that prevents the sash from:
- Dropping
- Twisting
- Bowing
- Rubbing on the frame
For a Side-Hung Window
Toe-and-heel in this pattern:
- Bottom hinge corner: pack under the glass
- Top lock corner: pack on the side of the glass
This stops the sash from sagging towards the lock side.
For a Top-Hung Window
Toe-and-heel like this:
- Bottom hinge corner
- Bottom lock corner
This prevents the sash from bowing outward when opened.
Nearly all window dropping problems happen because toe-and-heeling was not done or done incorrectly.
Use the Correct Thickness Packers
Choosing the right thickness ensures the frame stays square and the glass sits correctly.
Incorrect thickness can cause the sash to bow outward or pull inward.
Typical toe-and-heel thicknesses:
- 3 mm
- 4 mm
- 5 mm
Fine adjustments can be made with:
- 1 mm
- 2 mm
- Wedge packers
Never force a packer into place — over-packing causes pressure points.
Support the Glass on Both Bottom Corners
The bottom of the glass should always be supported by two setting blocks:
- One under the hinge side
- One under the lock side
This distributes the weight evenly and prevents the unit from slipping.
Incorrect bottom support leads to:
- Dropping
- Bowing outward
- Seal failure
- Misaligned locking points
Bottom support is just as important as toe-and-heel.
Add Side Packers Where Needed (But Don’t Over-Pack)
Side packers are used to:
- Balance the glass
- Prevent sideways movement
- Maintain consistent gaps
- Keep drainage channels clear
Use just enough to stabilise — over-packing can distort the sash.
Avoid Packing on Drainage Points
Never put packers where they will block drainage holes or interrupt water flow.
A blocked drainage path can cause swelling and subtle frame distortion, which leads to bowing.
Keep drainage areas completely clear.
How to Prevent Bowing Specifically
Bowing happens when the glass is either:
- Packed too tightly on one side
- Not supported enough on the opposite side
- Supported against a fixing screw or uneven surface
To prevent bowing:
- Use bridge packers over screws
- Ensure packers sit flat and fully support the edge
- Avoid stacking too many packers on one corner
- Make sure the sash remains square when checked with a level
- Do not use improvised materials like wood or cardboard
Even small pressure imbalances can cause long-term bowing, especially on large casements.
Test the Window After Packing
After bead installation:
- Open and close the sash several times
- Check for smooth locking
- Ensure even gaps all around
- Tap the glass gently to confirm no rattling
- Confirm the sash doesn’t lift or drop at the lock
If anything feels off, adjust packers immediately — not after sealing the job.
Common Installer Mistakes That Lead to Dropping or Bowing
- Not toe-and-heeling at all
- Toe-and-heeling on the wrong corners
- Using the wrong thickness packers
- Over-packing one side
- Forgetting to support the bottom corners
- Packing directly over a screw head instead of using a bridge packer
- Using non-plastic improvised shims
- Blocking drainage channels
Avoiding these mistakes eliminates almost all future call-backs.
Final Thoughts
Correct packing is the key to preventing dropped or bowed windows. By using the right packer thickness, supporting the glass at the correct points, and toe-and-heeling properly, you ensure the sash stays square, aligned, and trouble-free for years.
For installer-grade packers, explore: