Packing a uPVC window is one of the most important steps during installation. When done correctly, it prevents the window from dropping, bowing, or sticking, and ensures smooth long-term operation. When done incorrectly, the window may fail within months—leading to draughts, difficulty closing, or even cracked glass.

This guide explains exactly how professional UK installers pack a uPVC window, following best practices used across the glazing industry.

Why Packing a uPVC Window Is Essential

Correct packing ensures:

• The glass unit is fully supported
• Weight is distributed evenly
• Sashes do not drop over time
• Hinges and locks stay aligned
• Drainage pathways remain clear
• No pressure points develop on the glass
• The frame stays square inside the opening

Packing is not optional—it’s a critical part of every window installation.

Tools and Materials You’ll Need

• Glazing packers (flat packers in various thicknesses; wedge packers if needed)
• Setting blocks (for bottom support)
• Glazing shovel or paddle
• Tape measure
• Suction cups (optional but recommended)
• Rubber mallet (light use only)
• Safety gloves

Understanding the Key Packing Points

In uPVC windows, the glass must be supported at specific points, not randomly placed.

There are two main types of packers used:

1. Setting blocks (bottom packers)
   These support the weight of the glass on the hinge side and lock side.
2. Side packers (toe & heel packers)
   These keep the sash square and prevent the window from dropping.

Correct placement is the difference between a perfect install and a callback job.

Step-by-Step Guide to Packing a uPVC Window

Step 1: Insert the Window Frame Into the Opening

Position the uPVC frame in the aperture and ensure:

• It is level
• It is plumb
• It is square

Do all major adjustments before inserting the glass.

Step 2: Place the Bottom Setting Blocks

Setting blocks go at the bottom of the frame to support the weight of the glass.

Place them:

• One on the hinge side bottom corner
• One on the lock side bottom corner

These packers carry nearly all the weight of the glazed unit.

Do not place glass directly on the uPVC frame.
Packers prevent damage and maintain drainage.

Step 3: Insert the Glass Unit Carefully

Lift the double-glazed unit into the frame, resting it on the setting blocks.

Make sure:

• The glass is centred
• It is sitting flat on both blocks
• It has equal clearance around the edges

If needed, use a glazing shovel for fine adjustments.

Step 4: Toe and Heel the Sash (Critical Step)

This is the step that prevents dropped sashes.

Toe and heel means:

Placing packers diagonally opposite each other to counteract the sash’s weight and movement.

For a side-hung casement window:

• Pack bottom hinge corner (under glass)
• Pack top lock corner (side of glass)

This creates a structural “triangle of support,” ensuring the sash cannot drop.

For a top-hung window:

• Pack bottom hinge corner
• Pack bottom lock corner

This prevents bowing when the sash opens.

Packer placement:

• Use flat packers for firm support
• Use wedge packers only if micro-adjustments are needed
• Ensure packers fully support the glass edge, not just the frame

Step 5: Insert Additional Side Packers If Needed

On some frames, you may need to add side packers to:

• Keep drainage channels clear
• Prevent the glass from moving sideways
• Remove rattles or minor gaps
• Maintain equal reveal gaps

Do not over-pack—excessive pressure can stress the glass.

Step 6: Snap the Beads Back In (In the Correct Order)

Beading must be replaced correctly:

1. Top bead
2. Bottom bead
3. Hinge-side bead
4. Lock-side bead

This ensures the glazing unit locks firmly into place.

Use a glazing paddle to guide beads without damaging the frame.

Step 7: Test the Window Operation

Open and close the sash several times.

Check for:

• Smooth operation
• Consistent gaps
• No catching on the frame
• Correct compression on the seals
• Lock engagement without force

If the sash drops or catches—add, remove, or adjust packers as needed.

Step 8: Check Final Alignment and Drainage

Verify:

• Sash is square
• Drainage holes are not blocked
• Glass sits evenly on the packers
• Frame hasn’t twisted during glazing
• The lock operates smoothly

A well-packed window should maintain alignment for years.

Common Mistakes to Avoid

❌ Using cardboard or wooden shims (they absorb moisture and fail)
❌ Packing only the bottom and ignoring toe-and-heel
❌ Over-packing, causing pressure on the sealed unit
❌ Blocking drainage channels
❌ Using the wrong packer thickness
❌ Not supporting both bottom corners

Avoiding these mistakes prevents most future call-backs.

Frequently Asked Questions

Do all uPVC windows need toe-and-heel packing?

Yes — especially side-hung or top-hung casement windows.
Failing to do so often results in dropped sashes.

Can packing fix a dropped window?

In most cases, yes. Re-packing toe-and-heel can lift the sash back into alignment.

Which packers should I use?

Use rigid, colour-coded flat glazing packers for support.
Use wedge packers only for fine adjustments.

What thickness should I choose?

Thickness depends on the frame, but common sizes are 1–6mm (industry standard colour coding).

Final Thoughts

Packing a uPVC window correctly is essential to prevent dropped sashes, misaligned locks, or stress on the glass. Using the correct packers and placing them in the right positions ensures a long-lasting, smooth-operating window.

For installer-grade glazing packers, explore:

1. Flat Packers
2. Wedge Packers
3. Bridge Packers
4. Mixed Packer Sets