Max Span Calculator — Cheat Sheet
Quick-reference cheat sheet for the Max Span Calculator tool.
Published 27 May 2026
Max Span Calculator
Quick-reference companion for the free Portal tool
What This Tool Does
The Max Span Calculator is an essential tool for South African architects, designed to quickly and accurately determine the maximum permissible unsupported span for various glazing systems. It takes into account critical factors such as wind loads, glass type, and pane dimensions, ensuring compliance with SANS 10137 (The installation of glazing in buildings) and NBR (National Building Regulations) requirements.
This tool is indispensable during the preliminary design phase, tender preparation, and detailed design, allowing architects to specify appropriate glazing solutions that are structurally sound and cost-effective. It helps prevent over-specification of glass thickness, which can lead to increased material costs and reduced design flexibility, while simultaneously ensuring safety and performance under typical South African weather conditions, including high wind zones prevalent across the coast and certain inland areas.
Architects, structural engineers collaborating on façade design, and anyone involved in specifying fenestration for South African building projects will find this calculator invaluable for ensuring their designs meet local standards and perform reliably.
Step-by-Step Usage Guide
Start by inputting your project's location. The calculator will automatically suggest the relevant SANS 10160-3 wind zone (e.g., Wind Zone 3 for coastal areas like Cape Town or Durban, Wind Zone 1 for inland areas like Johannesburg). You can override this if specific site conditions dictate a different zone based on a detailed wind study.
Example: For a project in "Johannesburg, Gauteng", select "Wind Zone 1". For "Bloubergstrand, Western Cape", select "Wind Zone 3".
Enter the total height of the building in meters. Then, select the appropriate terrain category as defined by SANS 10160-3 (e.g., "Category 2: Open terrain with scattered obstructions" for suburban areas, "Category 4: Built-up urban areas" for dense city centres).
Example: Building height "15m", Terrain Category "Category 3: Terrain with numerous closely spaced obstructions".
Choose your desired glass type (e.g., "Annealed", "Toughened", "Laminated") and its nominal thickness in millimeters. Specify whether it's single glazing, double glazing (IGU), or triple glazing. If IGU, input the air/argon gap thickness.
Example: Glass Type "Toughened", Thickness "6mm", Configuration "Single Glazing". Or "Double Glazing", Outer "6mm Toughened", Inner "6mm Annealed", Gap "12mm Air".
Provide an initial estimated width and height of the glass pane. The calculator will use these to determine initial stress calculations. These values can be adjusted later if the calculated max span is too restrictive.
Example: Width "1200mm", Height "2400mm".
Select the support conditions for the glass pane. Common options include "Four-sided support", "Two-sided support (top/bottom)", or "Free edge (balustrade)". This significantly impacts the stress distribution and allowable span.
Example: For a typical window frame, select "Four-sided support". For a frameless glass balustrade, select "Free edge".
The calculator will display the "Maximum Allowable Span" for your chosen configuration. Review this against your design requirements. If the calculated span is too small, consider increasing glass thickness, changing to toughened/laminated glass, or adjusting support conditions. If it's larger than needed, you might be able to reduce glass thickness for cost savings.
Example: If the result is "Max Span: 2.8m" for a 6mm toughened pane, and your design calls for a 3.2m span, you might need to upgrade to 8mm toughened glass or a laminated option.
Common Scenarios & Expected Results
| Scenario | Input (Key Parameters) | Expected Output (Illustrative) |
|---|---|---|
| Standard Residential Window (Joburg) | Location: Johannesburg (Wind Zone 1) Building Height: 6m Terrain: Category 2 Glass: 6mm Annealed Single Glazing Support: Four-sided |
Max Span: Approx. 1800mm (height or width) (Note: Annealed glass is restrictive under wind loads) |
| Coastal Balustrade (Cape Town) | Location: Cape Town (Wind Zone 3) Building Height: 10m Terrain: Category 1 (Open coastal) Glass: 12.76mm Laminated (6mm Toughened + 0.76 PVB + 6mm Toughened) Support: Free edge (bottom supported) |
Max Span: Approx. 1000mm (height) (High wind load & free edge requires robust glass) |
| Commercial Curtain Wall (Durban) | Location: Durban (Wind Zone 3) Building Height: 40m Terrain: Category 4 Glass: 24mm IGU (6mm Toughened Outer + 12mm Air + 6mm Toughened Inner) Support: Four-sided |
Max Span: Approx. 3000mm (height or width) (Toughened IGU performs well in higher zones) |
| Large Shopfront Glazing (Pretoria) | Location: Pretoria (Wind Zone 1) Building Height: 4m Terrain: Category 3 Glass: 10mm Toughened Single Glazing Support: Four-sided |
Max Span: Approx. 3500mm (height or width) (Lower wind load, thicker toughened glass allows for large panes) |
Pro Tips
Wind Zones & SANS 10160-3
Always double-check the wind zone for your specific site. While the tool provides defaults, local site conditions and municipal overlays can sometimes necessitate a higher zone. Refer directly to SANS 10160-3 for definitive guidance, especially for critical structures or unusual geometries.
Laminated vs. Toughened
For safety glazing, consider laminated glass (e.g., 6.38mm, 8.38mm) as it remains in place when broken, reducing injury risk. Toughened glass, while stronger against impact, shatters into small pieces. The calculator accounts for the different structural properties of each.
IGU Performance
Insulated Glass Units (IGUs) offer superior thermal and acoustic performance. Remember that the structural capacity of an IGU is often dictated by the weakest pane, and the calculator considers the combined effect, but pay attention to individual pane thicknesses.
Frameless Applications
When designing frameless glass (e.g., balustrades, shower screens), the support conditions are critical. Ensure you accurately select "Free Edge" or "Two-sided" as appropriate, and consider the additional SANS requirements for balustrades regarding impact resistance and deflection limits, which may further restrict span.