1. The 3 Key Definitions
- Model Size: The nominal size of the unit (e.g., “1000 × 1000”). For Surespan, this usually aligns with the Clear Opening.
- Clear Opening (CO): The usable pass-through dimension (internal frame to internal frame). This dictates what equipment or people can fit through.
- Structural Opening (SO): The builder’s work aperture (trimmer-to-trimmer). This is the hole you cut in the roof.
- Overall Size: The total external footprint including the frame and flange. Used for checking clashes with parapets, plants, or walkways.
Note: While the Structural Opening is often the same as the Model Size/Clear Opening, you must verify this on the specific General Arrangement (GA) drawing for your unit.
2. Comparison Table
Term |
Where to measure |
Used for |
|---|---|---|
Structural Opening (SO) |
Trimmer-to-trimmer / structural aperture line |
The Builder: Setting out the hole in the roof. |
Clear Opening (CO) |
Internal frame opening line |
The User: Ensuring access for people/equipment. |
Overall Size |
Outside of frame/lid (see GA) |
The Architect: Clash checks (parapets, handrails). |
3. Worked Examples
Example A: SRHP75 Roof Hatch (Standard)
If the schedule says SRHP75 1000 × 1000:
- Model Size: 1000 × 1000 (Treat this as the Clear Opening).
- Structural Opening: Typically 1000 × 1000 (Verify on GA).
- Overall Footprint: 1240 × 1240 (SO + 240mm).
Risk: If you only check the opening size and ignore the overall footprint, you may clash with nearby parapets.
Example B: Smoke Vent (SRH-PAOV)
Smoke vents add a layer of complexity regarding airflow performance.
- Structural Opening: The builder’s aperture.
- Geometric Area: Physical area (without wind deflectors).
- Aerodynamic Area: The tested airflow performance (with wind deflectors).
Critical: A “1.0m²” requirement usually refers to Aerodynamic Area, not the physical opening size. Always check the datasheet performance table.
4. Site & Specification Checklist
To get an accurate quote and a unit that fits, ensure these details are confirmed:
- Define the Dimension: Explicitly state if your numbers are Structural Opening (SO) or Clear Opening (CO).
- Check the Footprint: Ensure the Overall Size fits between parapets/plant (don’t just measure the hole).
- Don’t Measure Finishes: Measure the structural aperture (timber/steel/concrete), not the internal plasterboard or upstand finish.
- Check Orientation: Confirm hinge side and ladder position to avoid trapping hazards.
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Complete the Roof Hatch Specification Form
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