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We always ensure we follow the Glass and Glazing Federation (GGF) Code of Good Practice which sets out standards for members covering the sales process, materials, aftercare and more.

As you’d expect, glass manufacturing is a complex and specialised process. We believe that there needs to be more awareness about industry standards which detail certain allowances within the glass itself.  

When we consider that glass is effectively for looking through not at, it should be inspected as such as the guidelines outline. End users should be advised about these standards where possible, and the nature of the glass manufacturing process whereby very small blemishes can occur.  

We’ve outlined some of the key points from the Code of Good Practice so read on to find out more...  



Glass that is used to produce sealed units is the same as glass used in single glazing. 

The Code of Good Practice states: “Viewing sealed units for scratches on the outer faces of the panes must be carried out before any rendering, plastering or other works adjacent to the glazing takes place, and as early as reasonably practicable following installation.” 



To carry out a professional standard inspection, one should stand in a room no less than 3 metres away from a sealed unit and look directly through it. When inspecting toughened, laminated or coasted glass, one should stand no less than 3 metres away. 

Allowances are made within the guidelines in that if it isn’t possible to stand at the correct distance, then you are able to stand as far away as you can from a sealed unit.  

Inspections should be carried out in natural daylight but not directly towards the sun. There should be no visible moisture on the surface of the glass. Glass must be viewed at an angle of 90º and inspections should exclude a 50mm wide band around the edge of the glass.  (Glass should not be inspected with artificial light or by torch light)




1. View the glass from inside the room standing a minimum of 3m away facing the glass. This is because under normal everyday situations, looking through windows is usually at a distance and not from close up – say within 6 inches of the glass.

2. Look through the glass and not at it. This is expected as normal as the glass is there to enable a view beyond the window, not stopping at it.

3. Do not inspect in direct sunlight. This is because the direct sun will be refracted by the coating and any tiny imperfections in the glass surface, highlighting items that cannot be seen in normal conditions.

4. Ensure the glass is clean and dry inside and out. It is impossible to check glass that has rain, dust or condensation up on it, mixing up these elements with potential problems.

5. Ignore the 50mm perimeter of the glass. This is because when looking through windows, the eye is drawn to central areas and one doesn’t look through the edge of glazing when viewing. (Obviously large obtrusive marks in the perimeter are not acceptable.)



GGF guidelines specify that any flat transparent glass – such as laminated, toughened or coated, is acceptable if the following are neither obtrusive nor bunched: 

  1. Bubbles or blisters 

  1. Fine scratches no more than 25mm long 

  1. Minute particles  

An important point to note is the guidelines state that the obtrusiveness of blemish is to be judged when looking through the glass under natural light – not at it. They go so far as to state that users should understand that glass used in any sealed unit is a processed product and as a consequence blemishes are to be expected. 

The guidelines confirm that defects such as smears, fingerprints or other dirt on the faces of the glass, or extraneous material in the cavity are unacceptable, if they are visually disturbing. 



A natural phenomenon of toughened glass is that it may show visual distortions which are accentuated by reflections in sealed units. This isn’t a fault as such but can occur.  

Laminated glass can have a few more blemishes as it is made up of several layers. Low emissivity coatings can product transient visual effects. Coating may look like a transparent film or produce a haze in some lights. When lightly coloured objects such as net curtains are placed next to the glazing, it can look slightly darker.  



Sealed units will generally reduce the incidence of condensation, but users should be aware that it may still occur under certain conditions. Such as high internal humidity / moisture levels within a room. 



Occasionally small rainbow effects are produced by the glass refraction of light. This is due to high quality flat sheets of glass being placed parallel to each other.  



Multiple reflections can occur in certain light conditions. This is caused by multiple surface reflections within the sealed units which can vary from pane to pane. 

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