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CAN CGSB 12.3-M91 (2017) Technical Overview: Flat Clear Sheet Glass Specification
A Comprehensive Guide to the Canadian Standard for Flat Clear Sheet Glass
1. Scope and Application
CAN CGSB 12.3-M91 (2017) is the definitive Canadian standard governing flat, clear sheet glass intended for general building glazing, mirror manufacture, and picture framing. Developed by the Canadian General Standards Board (CGSB), this standard establishes the minimum material, dimensional, quality, and workmanship criteria for soda-lime silicate glass produced by the vertical drawing or horizontal flat-drawn process. It covers both single strength and double strength designations, as well as heavier sheet glass commonly used in residential windows, storm doors, and display cases.
The standard applies to glass with a thickness range of approximately 1.5 mm to 6.0 mm, and categorizes the product by its intended end use — primarily by the visual quality classification (Glazing Quality or Mirror Quality). The 2017 reaffirmation confirmed the technical content of the original 1991 edition, ensuring ongoing alignment with modern fabrication practices while retaining the metric and imperial dimensions that are integral to the Canadian construction industry. It remains widely referenced in provincial building codes, retrofit specifications, and procurement documents for fenestration products.
2. Technical Requirements
2.1 Types and Dimensions
CAN CGSB 12.3-M91 defines glass by nominal thickness, permitted thickness tolerances, and length/width tolerances. The standard requires that the glass be “flat clear sheet glass” — free from surface haze and with a reasonably parallel surface finish across the pane. Thickness designations are based on traditional sheet glass trade sizes; the following table summarises the key dimensional requirements:
| Thickness Designation | Nominal Thickness (mm) | Thickness Tolerance (mm) | Maximum Overall Size (mm × mm) | Typical Use |
|---|---|---|---|---|
| Single Strength | 1.5 – 2.5 | ±0.15 | 1372 × 2134 | Small pictures, storm storm windows |
| Double Strength | 2.5 – 3.0 | ±0.20 | 1524 × 2438 | General glazing, sliding doors |
| Heavy Sheet (3/16 in) | 4.8 – 5.0 | ±0.25 | 1829 × 2438 | Large windows, display cases |
| Extra Heavy Sheet (1/4 in) | 6.0 | ±0.30 | 1829 × 3048 | Commercial glazing, storefronts |
2.2 Quality Classification and Defects
The standard provides two distinct quality grades:
- Glazing Quality (GQ) — Permits the limited presence of cosmetic imperfections such as fine scratches, tiny seeds (bubbles ≤ 1.5 mm), and small stones that do not impair the structural integrity. Imperfections are allowed only outside a central “vision area” or if their size is below defined thresholds.
- Mirror Quality (MQ) — Requires a superior surface finish free of visible scratches, seeds, stones, or cord. Striae and nip (localised thumb-like distortions) are strictly limited. This grade is intended for high-reflectivity mirror applications.
Tip: When selecting sheet glass for mirror manufacturing, always specify Mirror Quality (MQ) to avoid visual distortion. Glazing Quality may exhibit fine surface imperfections that become noticeable after silvering.
2.3 Edgework and Cutting
CAN CGSB 12.3-M91 requires that all cut edges be reasonably straight and free from chipping that would compromise handling safety or final seal performance. The standard permits a maximum edge chip depth of 3 mm for thicknesses up to 3.0 mm and 5 mm for thicker panes. All edges must be smooth enough to avoid immediate injury during normal installation. For glazing applications, edges are to be cut to a tolerance of ±1.6 mm on length and width from the specified dimensions.
3. Implementation Highlights
When using CAN CGSB 12.3-M91 as a procurement or quality specification, the following implementation points are critical:
- Verification of glass thickness — Because sheet glass thickness can vary across the sheet, the standard requires an average of four measurements (one near each corner, excluding the edge 12 mm). Ensure your incoming inspection follows this procedure to avoid rejections due to out-of-tolerance glass.
- Handling and storage — The standard assumes the glass is delivered in factory-fresh condition with a protective interleaving powder (typically soda–lime glass fines). Do not wash this powder off until just before installation; it prevents scratching during stacking.
- Annealing quality — While the standard is primarily dimensional and visual, it also references residual stress limits. Glass that is poorly annealed may exhibit excessive bow, warp, or spontaneous breakage. A polariscope inspection is recommended for high-stress applications such as sealed insulating glass units.
Modern Relevance: Although originally issued in 1991, the 2017 reaffirmation confirms that the thickness tolerances and quality grades remain appropriate for most residential and light commercial uses. The standard is still actively cited by Canadian window manufacturers and is compatible with modern edge-deletion and wet-glazing systems.
4. Compliance Notes
Compliance with CAN CGSB 12.3-M91 is typically demonstrated through factory certification and periodic audit testing. The standard does not require third-party certification for every shipment, but many procurement contracts specify that the manufacturer shall provide a Certificate of Compliance referencing the standard year (2017).
Caution: CAN CGSB 12.3-M91 is not a safety glazing standard. It does not assess impact resistance, tempered strength, or laminated glass properties. For applications requiring human-impact safety (e.g., doors, tub enclosures), refer to CAN/CGSB 12.1-M90 or building code requirements. This standard is solely for annealed flat clear sheet glass.
Key compliance parameters to verify during inspection:
- Thickness: Use a calibrated micrometer in at least four locations per sheet. Reject any sheet where measurements fall outside the tolerance listed in the standard.
- Optical quality: Conduct a visual inspection under diffused lighting (at approximately 600–700 lux) from a distance of 300 mm. Note any seeds, stones, scratches, or cord with the aid of a magnifying comparator if specified.
- Edge chipping: Measure the depth of any chip from the original cut line. Chips deeper than the allowable limit (3 mm or 5 mm, depending on thickness) constitute a nonconformity.
Critical: Pay particular attention to the “½-inch edge exclusion zone” for defect assessment. Most quality standards allow minor cosmetic imperfections within 12 mm of the cut edge. Failing to account for this zone can lead to false rejections during incoming inspection.
Frequently Asked Questions
Q: What is the difference between CAN CGSB 12.3-M91 and ASTM C1036?
A: CAN CGSB 12.3-M91 is the primary Canadian specification for flat clear sheet glass, while ASTM C1036 is the analogous US standard. Both define similar thickness grades and quality limits, but the Canadian standard uses different defect measurement protocols (e.g., edge-chip allowable depths) and is referenced directly by the National Building Code of Canada. For projects within Canada, specifying CAN CGSB 12.3-M91 ensures regulatory alignment.
A: CAN CGSB 12.3-M91 is the primary Canadian specification for flat clear sheet glass, while ASTM C1036 is the analogous US standard. Both define similar thickness grades and quality limits, but the Canadian standard uses different defect measurement protocols (e.g., edge-chip allowable depths) and is referenced directly by the National Building Code of Canada. For projects within Canada, specifying CAN CGSB 12.3-M91 ensures regulatory alignment.
Q: Does this standard cover coloured or patterned glass?
A: No. CAN CGSB 12.3-M91 is limited to flat clear (colourless) sheet glass. Tinted, pattern, or textured glass is covered by separate CGSB or ASTM standards. The “clear” requirement also excludes any surface coatings, ceramic frits, or applied films.
A: No. CAN CGSB 12.3-M91 is limited to flat clear (colourless) sheet glass. Tinted, pattern, or textured glass is covered by separate CGSB or ASTM standards. The “clear” requirement also excludes any surface coatings, ceramic frits, or applied films.
Q: Can I use sheet glass meeting CAN CGSB 12.3 in an insulating glass unit (IGU)?
A: Yes, sheet glass meeting the standard is commonly used as the inboard or outboard lite in sealed IGUs, provided the glass thickness matches the unit design. However, for better thermal stress resistance, many IGU manufacturers now prefer float glass to drawn sheet glass. The standard does not address seal compatibility — that is covered by CAN/CGSB 12.8 (for IG units) or ASTM E2190.
A: Yes, sheet glass meeting the standard is commonly used as the inboard or outboard lite in sealed IGUs, provided the glass thickness matches the unit design. However, for better thermal stress resistance, many IGU manufacturers now prefer float glass to drawn sheet glass. The standard does not address seal compatibility — that is covered by CAN/CGSB 12.8 (for IG units) or ASTM E2190.
Q: What year of the standard is currently in force?
A: The most current version is CAN CGSB 12.3-M91 (2017). The “M91” refers to the original publication year (1991), and “(2017)” denotes the reaffirmation year. All technical content from the 1991 edition remains unchanged; only editorial updates were made in 2017 to align with current CGSB formatting guidelines.
A: The most current version is CAN CGSB 12.3-M91 (2017). The “M91” refers to the original publication year (1991), and “(2017)” denotes the reaffirmation year. All technical content from the 1991 edition remains unchanged; only editorial updates were made in 2017 to align with current CGSB formatting guidelines.