An explanation of the industry-recognised guideline from 2009
The Guideline for Assessing the Visual Quality of Glass for Building Applications is a guideline developed by the Federal Guild Association of the Glazing Trade (Bundesinnungsverband des Glashandwerks) and the Federal Flat Glass Association (Bundesverband Flachglas e.V.). It contains important criteria for assessing the clear glass area that remains visible once the glass has been installed. The guideline relates exclusively to the visual assessment of glass products. Product-specific characteristics such as sound insulation or attack-resistant glazing are not part of this inspection guideline.
Scope of the guideline
The guideline applies to glass products of various types:
- coated glass
- body-tinted glass
- laminated glass
- toughened glass such as toughened safety glass or heat-strengthened glass
It does not apply to the following glass products in special versions, nor to the assessment of the visual quality of unframed edges:
- glass in composite assemblies
- glass with elements built into the cavity between the panes
- glass products using patterned glass
- wired glass
- special safety glazing such as attack-resistant glazing
- fire-resistant glazing
- all non-transparent glass products
Carrying out the inspection
The view through the glazing is inspected. The glazing covered by the guideline is inspected from a distance of at least 1 metre, from both inside and outside.
The inspection is carried out
- from a viewing angle appropriate to the use of the room
- in diffuse daylight without direct sunlight or artificial lighting
- perpendicular to the surface
For inspecting interior glazing, lighting that is normal for the rooms concerned is used.
Visual properties of glass products as part of the inspection
The visual properties of glass products include the inherent colour and colour differences in coatings. These are subject to variations inherent in the production process and are not the subject of the visual inspection.
If, within the visible area of an insulating glass edge seal, there is a visible deviation in the parallelism of the spacer relative to the straight glass edge, this can be the subject of a complaint if the maximum deviations in relation to the limiting edge length are exceeded.
- Triple thermal insulation glass: up to 2.5 metres limiting edge length, maximum 4 mm
- Triple thermal insulation glass: over 2.5 metres limiting edge length, maximum 6 mm
- Double insulating glass: up to 3.5 metres limiting edge length, maximum 4 mm
- Double insulating glass: over 3.5 metres limiting edge length, maximum 6 mm
Mechanical or chemical damage to the outer surface is often only discovered during the inspection after glazing. In such cases, the cause must be established. The extent of the damage is assessed and whether it is a valid ground for complaint is determined in accordance with the detailed table "Permissible tolerances for the visual quality of glass products" (contained in the full version, which you can download below).
Defects smaller than 0.5 mm are not taken into account during the visual inspection. The maximum size of 3 mm per defect area, in the form of spots, inclusions, blemishes and shell-shaped chips, must not be exceeded.
If a glass surface exhibits local waviness, this must not exceed 0.3 mm over a measuring length of 300 mm. Warping must not exceed 3 mm over a glass edge length of 1000 mm. Square and approximately square shapes and single panes with a nominal thickness of less than 6 mm are an exception. Here, greater warping is possible.
Full version of the Guideline for Assessing the Visual Quality of Glass for Building Applications - DOWNLOAD.
