Stepped Units — Flat Roof & Special Frames
A stepped (offset) sealed unit has inner and outer glass panes of different sizes, creating a step at one or more edges. The most common application is flat-roof rooflights, where the outer pane oversails the upstand so water is thrown clear of the kerb — steps typically run from 20 to 300mm depending on the detail.
Step Types
| Step Type | Description | Typical Step | Application |
|---|---|---|---|
| Single Bottom | Outer pane extends below inner | 20-300mm | Rooflight kerb oversail |
| Single Top | Inner pane extends above outer | 20-300mm | Deep head rebates |
| Double (Top+Bottom) | Steps on both parallel edges | 20-300mm | Opposing rebate details |
| L-Step | Steps on two adjacent edges | 20-300mm | Corner conditions |
| All-Round | Inner smaller on all four sides | 20-300mm | Deep-rebate frames |
| Reverse | Inner pane larger than outer | 20-300mm | Deeper internal rebate |
Flat Roof / Rooflight Applications
Stepped units for flat roof rooflights accommodate kerb profiles with different inner and outer rebate depths. Outer pane MUST be toughened, inner pane MUST be laminated (overhead glazing mandatory). Minimum slope 5° for self-draining.
Ordering — CRITICAL
Specify: (1) BOTH inner and outer pane dimensions separately, (2) which pane is larger, (3) step position (top/bottom/left/right), (4) step dimension in mm. A dimensioned drawing is STRONGLY RECOMMENDED. Errors cannot be corrected after manufacture.Gas Fill — Air as Standard, Argon Optional
CSU sealed units are air-filled as standard. Argon gas fill (minimum 90% concentration) is available as an optional upgrade. Argon has 34% lower thermal conductivity than air (0.018 vs 0.026 W/mK), improving thermal insulation by approximately 15%. Argon is an inert, non-toxic, odourless gas that makes up 0.93% of Earth’s atmosphere — it is inert and environmentally neutral. Krypton gas is available by special order for slim-cavity heritage applications where maximum performance is needed in a narrow gap.
| Property | Dry Air | Argon (Optional) | Krypton (Special) |
|---|---|---|---|
| Thermal Conductivity | 0.026 W/mK | 0.018 W/mK | 0.010 W/mK |
| U-Value Benefit | Reference | ~15% better | ~25% better |
| Optimal Cavity | 12-16mm | 16-20mm | 10-12mm |
| Depletion Rate | N/A | <1% per year | <1% per year |
| Relative Cost | 1× | ~1.2× (included free) | ~50× (premium) |
Spacer Bar Options — Warm Edge vs Aluminium
The spacer bar separates glass panes and creates the sealed cavity. Traditional aluminium spacers conduct heat rapidly (160 W/mK), creating a cold thermal bridge at the unit edge that causes condensation and heat loss. Warm edge spacers use low-conductivity materials, reducing this bridge by up to 94%. CSU recommends Swisspacer Ultimate or Super Spacer TriSeal for all projects.
| Spacer | Material | Conductivity | Psi Value | Edge Temp vs Alu | Colours |
|---|---|---|---|---|---|
| Aluminium | Aluminium alloy | 160.0 W/mK | 0.080 W/mK | Baseline (cold) | Silver, White, Black, Brown, Grey |
| Swisspacer Ultimate | Composite polymer | 0.19 W/mK | 0.030 W/mK | +3-4°C warmer | Black, White, Brown, Grey, Oak +15 |
| Super Spacer TriSeal | Silicone foam | 0.17 W/mK | 0.028 W/mK | +3-4°C warmer | Black, Grey |
| Thermix TX.N Plus | Stainless + plastic | 0.60 W/mK | 0.040 W/mK | +2-3°C warmer | Silver / Black |
| TGI Spacer | Stainless + polymer | 0.90 W/mK | 0.050 W/mK | +1-2°C warmer | Silver |
Warm edge improves Uw by 0.1-0.2 W/m²K, eliminates edge condensation, and can improve WER by one full band. Available in widths: 6, 8, 10, 12, 14, 16, 18, 20, 22, 24mm.
The Stepped Ledge Rule
One detail separates stepped units from everything else in the rack: the ledge never carries weight. Rest the larger pane’s edge on the A-frame — upright, covered, never flat — and keep the exposed step edge padded and clear of neighbouring units. Lift on the larger pane only: suckers above 25 kg, mechanical above 60 kg.
Beyond that, the usual discipline: glazed within 28 days, above 5°C, acclimatised first. Stepped units are made to your rebate or kerb drawing — allow 4-15 days depending on the step detail.
Related Products & Glass Types
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