Bridge Packers vs Flat Packers: Which One Do You Need?

Both bridge packers and flat packers are essential glazing tools, but they serve different purposes. Using the wrong type on site wastes time and can compromise the integrity of the installation. Here's how to tell them apart and know when to use each.

What Is a Flat Packer?

A flat packer is exactly what it sounds like — a flat, solid plastic shim of a precise thickness. They are manufactured in standard widths (28mm, 30mm, or 32mm) and a range of thicknesses, typically 1mm, 2mm, 3mm, 4mm, 5mm, and 6mm. Most suppliers (including us) colour-code them by thickness so you can identify the right one at a glance.

Flat packers are used to raise, level, and support components — whether you're levelling a window frame off an uneven cill, adjusting the height of a door threshold, or shimming a structural element to plumb.

What Is a Bridge Packer?

A bridge packer (also called a horseshoe packer) has an open slot cut through it. This slot allows the packer to be slid sideways around a bead, frame member, or glazing bar — rather than having to be placed underneath the glass before installation. They come in widths of 15mm, 20mm, and 28mm.

Bridge packers are primarily used for toe and heel glazing — positioning the glass within the frame to control how the load is distributed. Industry glazing standards specify precisely where toe and heel packers should be placed for different window types.

The Key Difference: Access

The defining functional difference is access:

• Flat packers must be in position before the glass or component is placed on top of them
• Bridge packers can be inserted after the glass is in position, by sliding them in around the bead or frame lip

This makes bridge packers essential for corrective adjustments once the glass is already in the frame, or in situations where toe/heel positions are specified but only accessible from the side.

Using Them Together

In practice, most glaziers use both types on the same job. A typical window installation might use:

1. Flat packers on the cill to achieve the correct height before the frame goes in
2. Bridge packers inserted at the toe and heel positions after the glass is bedded in

Sizing: How to Choose the Right Packer

Width

Match the packer width to the frame depth. For standard 70mm uPVC frames, 28mm packers are common. For deeper commercial profiles, 30mm or 32mm may be specified. Bridge packer width (15mm, 20mm, 28mm) is typically chosen to match the clearance available on either side of the glazing bead.

Thickness

Use the thinnest combination that achieves the required clearance. Stacking two 2mm packers is preferable to a single 4mm packer that may deform under load. Always check the frame manufacturer's specification for minimum and maximum clearances.

Quick Comparison

Both types are available in multi-colour mixed packs — ideal for keeping a selection on the van so you always have the right size to hand.

Browse our flat glazing packers and bridge packers. Need advice? Get in touch and we'll help you find the right product for your application.