Glazing Packers Window Spacers and Door Shims
If you have ever watched a professional tradesperson install a window or door, you’ve likely seen them slide small, brightly coloured plastic tabs into the gaps before securing the frame. These are glazing packers (frequently referred to as spacers or shims), and while they might look like simple pieces of plastic, they are the unsung heroes of structural alignment and glass safety.
Whether you are a DIY enthusiast tackling your first home improvement project or an experienced installer, understanding how these little tools work is essential. Let’s dive into everything you need to know about them.
What Are Glazing Packers?
Glazing packers are flat, heavy duty plastic spacers designed to level, align, and support heavy loads during construction and installation. They are manufactured to precise thicknesses and feature a textured or ribbed surface to prevent them from slipping once stacked or placed under pressure.
In window and door installations, they bridge the gap between the rough structural opening (the brickwork or timber frame) and the outer frame of the window or door.
Why Do I Need Them?
No structural opening in a building is perfectly square, level, or plumb. If you were to screw a window frame directly into a skewed masonry opening, the frame would twist, distort, and warp.
You need packers to:
- Achieve Perfect Alignment: They ensure the frame sits perfectly level (horizontal) and plumb (vertical). Without them, doors will swing open on their own, and windows won't lock smoothly.
- Transfer Weight Correctly: Double and triple-glazed sealed units are incredibly heavy. Packers transfer the immense weight of the glass directly down to the structure, preventing the upvc or aluminium frame from bowing under the load.
- Prevent Glass Contact: Glass must never touch the frame or the fixing screws directly. Doing so creates stress points that lead to spontaneous glass breakage. Packers maintain the necessary clearance.
- Allow for Movement: Buildings expand and contract with temperature changes. Packers provide a secure bridge while leaving room for perimeter expansion foam and sealant.
What Are They Made Of?
Modern glazing packers are made from high density, high quality plastics, typically polypropylene (PP) or high density polyethylene (HDPE).
They are engineered this way for highly specific reasons:
- Rot and Water Proof: Unlike traditional wood shims, plastic will never rot, warp, split, or absorb moisture when exposed to the elements behind a window frame.
- Incredible Compressive Strength: They can withstand hundreds of kilograms of pressure without compressing or flattening over time.
- Chemical Resistance: They won't degrade when they come into contact with silicone sealants, expanding foam, or brickwork cleaner.
Why Are There 6 Standard Sizes (1mm to 6mm)?
Standard flat packers are manufactured in six precise thicknesses: 1mm, 2mm, 3mm, 4mm, 5mm and 6mm.
The reason for these specific sizes is modularity. Gaps in construction vary wildly. By having a baseline set of sizes ranging from 1mm to 6mm, an installer can perfectly stack them together to achieve any millimetre measurement required.
> *Need an 11mm lift? Stack a 6mm and a 5mm packer together.
> *Need 7mm? Combine a 4mm and a 3mm.
The precise increments take all the guesswork out of levelling.
Why Are They Different Colours?
The bright colours aren't for aesthetics—they are a strictly standardised industry colour-coding system. When a tradesperson is balanced on a ladder holding a heavy piece of glass, they don't have time to use a calliper or read tiny print to find the right thickness. They rely on the colour:
| Colour | Thickness |
| Green | 1mm |
| Black | 2mm |
| White | 3mm |
| Grey | 4mm |
| Blue | 5mm |
| Red | 6mm |
Because of this system, an installer can look into their tool bag, grab a white and a grey packer, and instantly know they have exactly 7mm of packing material ready to go.
Are There Any Other Uses for Them?
While they are designed for glazing, their durability, rot-proof nature, and exact measurements make them incredibly versatile "problem solvers" across dozens of DIY and building applications. Some common alternative uses include:
- Levelling Kitchen Cabinets & Countertops: Ideal for shimming base units on uneven concrete floors before fixing them to the wall.
- Laying Garden Decking: Used underneath timber or composite joists to lift them off damp ground and level the subframe.
- Drylining and Stud Walls: Placed behind timber battens when fixing them to uneven masonry walls to ensure a perfectly flat surface for plasterboard.
- Paneling and Cladding: Ensuring alignment when installing internal wall panelling or external cladding boards.
- General Workshop Shims: Handy for levelling heavy machinery, workbenches, or large pieces of furniture on uneven flooring.
Summary
Whether you call them packers, shims, or spacers, these colour-coded plastic tabs are essential for any precision levelling job. They guarantee that your installations remain structurally sound, perfectly aligned, and built to last for decades. The next time you see a green or red tab tucked under a window frame, you'll know exactly what it's doing!
To view our range spacers, click the link below:
