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Glass Skylight Selection Guide
How to specify sloped overhead glazing for atriums, rooflights, saw-tooth and barrel-vault skylights — laminated inner ply, double/triple IGU construction, Low-E + argon thermal performance, condensation control, slope and drainage rules, and jumbo-size feasibility.
Sloped & overhead glazingBS 6262-4 · EN 1991 · ASTM E1300Jumbo IGU up to 3300×12000mmHST 554°F × 4 hrs
1. What Is a Skylight (and Why It Is Stricter Than a Window)
A skylight is a sloped or horizontal glazed roof opening that admits daylight into a space below. Common examples include shopping-mall and hotel atriums, airport and transit concourse rooflights, office saw-tooth roofs, barrel-vault canopies, residential roof windows and glazed walkway covers.
A skylight is not a vertical window and not a flat roof. Because the glass sits horizontal to near-horizontal and directly above occupied space, it is classified as overhead glazing — the same family as canopies and glass bridges, but with two extra constraints: thermal performance (U-value / condensation) and watertightness across the full roof assembly.
Rule of thumb: If glass is installed at more than a few degrees from vertical and there are people beneath it, specify a laminated inner ply and treat the assembly as overhead glazing. A monolithic tempered-only skylight is not acceptable for public or occupied spaces — if the outer pane breaks, broken glass and water must not reach the room below.
2. Code & Load Logic
Skylight design starts with the same load family as canopy glass, plus a thermal chapter. The governing standards differ by market, but the load types are universal:
Wind (±)
Wind load
Positive and negative (suction) pressure, often magnified on a roof. Driven by location, height, exposure and roof geometry.
Snow & ponding
Snow & imposed load
Self-weight plus snow drift, standing water and maintenance access. Flat or low-pitch skylights are especially vulnerable to ponding.
Self-weight
Self-weight & deflection
Glass weight acts continuously. Design deflection is commonly limited to L/60 for overhead glazing — tighter than vertical walls.
Thermal
Thermal & condensation
The unique skylight load case: U-value, surface temperature, dew point and thermal-gradient stress from a hot upper surface and cold cavity. Low-E + gas fill + warm-edge spacer are the controls.
Applicable standards by market
| Standard | Market | Scope |
|---|
| BS 6262-4 | UK | Glazing for buildings — safety related to human impact; overhead glazing requirements |
| EN 1991 (Eurocode 1) | EU / UK | Actions on structures — wind, snow, imposed loads |
| EN 673 | EU / UK | U-value calculation for glazing |
| EN 1279 | EU / UK | Insulating glass units — long-term performance |
| ASTM E1300 | USA | Determining load resistance of glass in buildings |
| ASTM E2190 | USA | Standard specification for IGU |
| NFRC 100 / 200 / 300 | USA | U-factor / SHGC / VLT determination |
| AS/NZS 2208 | Australia / NZ | Safety glazing materials
Deflection limit: For overhead glazing, design deflection is commonly restricted to L/60 of the clear span. A longer span needs a thicker build-up or closer glazing-bar spacing. Confirm the project limit with the structural engineer — this single value drives the thickness table below.
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3. Skylight Assembly Anatomy
A skylight is almost always supplied as an insulating glass unit (IGU) with a laminated inner ply. Understanding the assembly is the key to every later decision — coating placement, gas fill, spacer and U-value all live in this sealed unit.
Double-glazed skylight IGU: ┌──────────────────────────────────────────┐ │ Outer lite — tempered (exposed to weather) ├──────────────────────────────────────────┤ │ Spacer + Argon/Krypton cavity (12–20mm) ├──────────────────────────────────────────┤ │ Inner lite — laminated (6+0.76PVB+6) ← post-breakage retention └──────────────────────────────────────────┘ Low-E coating: surface #2 (standard double IGU) Triple-glazed (cold climate): Outer lite │ Spacer + Cavity │ Mid lite │ Spacer + Cavity │ Inner (laminated) (Low-E #2) (Low-E #3 optional)
Why the inner ply is laminated: The inner lite is the last barrier between the occupied space and any failure of the outer pane. A PVB- or SGP-laminated inner ply holds broken glass in place, so a crack does not become falling debris or an open path for water. This is the defining safety feature of a sloped skylight.
4. Glass Construction Matrix
| Construction | Build-up example | Laminated inner | Thermal (U-value) | Best for skylight type |
|---|
| Monolithic tempered (single lite) | 12 / 15 / 19mm single lite | ❌ No — shatters | Baseline (poor) | Not acceptable for public overhead glazing |
| Double IGU + laminated inner | 6T + 16Ar + 6.76Lami (6+0.76PVB+6) | ✅ Yes | ~1.0 W/m²K | Standard atrium / rooflight (recommended) |
| Double IGU + Low-E + laminated inner | 6T (Low-E #2) + 16Ar + 6.76Lami | ✅ Yes | 0.8–1.1 W/m²K | Energy-code / commercial atrium |
| Triple-glazed IGU + laminated inner | 6T + 12Ar + 6T + 12Ar + 6.76Lami | ✅ Yes | 0.5–0.8 W/m²K | Cold climate / passive house |
| Acoustic / security skylight | 6T + 16Ar + 8.76Lami (1.52PVB or SGP) | ✅ Yes (thicker) | ~0.9–1.1 W/m²K | Airport, rail, high-security rooflight
Why double IGU + laminated inner is the default: It solves the three skylight problems at once — the IGU cavity controls U-value and condensation, the tempered outer lite resists wind/snow/impact, and the laminated inner ply provides post-breakage retention. Add a Low-E coating on surface #2 and argon fill, and the unit is code-ready for most commercial atriums.
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5. Thickness & Span Selection Table
The table below is a starting point for preliminary design, assuming a simply supported rectangular panel, standard wind/snow loads and L/60 deflection. Final thickness must be confirmed by a qualified engineer using ASTM E1300 / EN 1991 for the actual site loads.
Outer lite (tempered) by clear span
| Typical clear span | Recommended outer lite | Support note |
|---|
| Up to ~1.2 m | 6 mm tempered | Small rooflight, residential |
| ~1.2 – 1.8 m | 8 mm tempered | Standard unit skylight |
| ~1.8 – 2.5 m | 10 mm tempered | Commercial atrium (most common range) |
| ~2.5 – 3.5 m | 12 mm tempered | Large rooflight / barrel vault |
| Above 3.5 m | 12 / 15 / 19 mm or structural | Full engineering required |
Inner lite (laminated) — post-breakage retention
| Application | Laminated inner build-up | Overall | Interlayer |
|---|
| Standard skylight | 6+0.76PVB+6 | 12.76 mm | PVB |
| Commercial atrium | 6+1.52PVB+6 | 13.52 mm | PVB (reinforced) |
| Long-span / high-load | 8+1.52PVB+8 or 10+2.28SGP+10 | 17.52 / 22.28 mm | PVB or SGP |
| Walkable / trafficable roof | Multi-ply SGP | 30.76 mm+ | SGP
Span is king: Thickness is driven primarily by the clear span between glazing bars, not by the overall panel size. A 6 m panel supported every 1.5 m behaves like a 1.5 m span. Provide the glazing-bar grid (support spacing) with your RFQ — it changes the answer more than any other variable.
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6. Thermal Performance (U-Value, Condensation & Coating)
Thermal performance is where skylights differ most from canopies. The objective is to keep the interior glass surface warm enough to avoid condensation while rejecting unwanted solar gain.
| Configuration | Cavity / gas | Coating | U-value | SHGC (typical) | Best for |
|---|
| 6+12Ar+6 (single Ag) | 12mm / Argon | Single silver | 1.2–1.4 W/m²K | 0.42 | Cold / mild climate |
| 6+16Ar+6 (double Ag) | 16mm / Argon | Double silver | 0.9–1.1 W/m²K | 0.32 | Standard commercial atrium |
| 6+20Ar+6 (double Ag) | 20mm / Argon | Double silver | 0.8–1.0 W/m²K | 0.30 | Enhanced |
| 6+16Kr+6 (double Ag) | 16mm / Krypton | Double silver | 0.7–0.9 W/m²K | 0.30 | Premium (krypton) |
| Triple 6+12Ar+6+12Ar+6 | Two cavities / Argon | Double silver | 0.6–0.8 W/m²K | 0.28 | Cold / passive |
| Triple + krypton | Two cavities / Krypton | Triple silver | 0.5–0.6 W/m²K | 0.22 | Passive house |
Condensation control — the three controls
- Low-E coating (surface #2 / #3): Reflects long-wave infrared back into the room, raising the interior surface temperature.
- Argon / krypton gas fill: Reduces convective heat transfer in the cavity (argon ~10–15% better than air).
- Warm-edge spacer: Reduces the thermal bridge at the IGU perimeter — the most common site of edge condensation.
- Design note: Skylights are more condensation-prone than vertical glazing because the horizontal surface collects less radiant warmth and the cavity sits directly in the cold roof zone. Specifying all three controls (Low-E + gas + warm-edge) is the standard defence, and the slope / ventilation strategy should be confirmed with the design team.
7. Slope, Drainage & Ventilation
- Slope: A minimum slope (commonly 5°–15°, often ~1:12 to 1:4 depending on the system) is required to shed water and prevent ponding. Flat "skylights" that can pool water are a design red flag — ponding load can double the design case.
- Drainage gutter: The frame system must include a perimeter condensation and leak drainage path. Water trapped at the panel edge is a leading cause of seal failure, staining and interior damage.
- Ventilation (operable skylights): If the skylight opens for smoke exhaust or comfort ventilation, the opening mechanism and weather sealing become part of the glass-specification discussion — provide the hardware interface with your RFQ.
- Thermal break: The framing system should include a thermal break; an uninsulated aluminium frame will condense even with a high-performance IGU, undermining the glass specification.
8. Jumbo & Oversize Skylight Glass
Skylights are one of the most natural applications for jumbo IGU. A single oversize panel can span an entire atrium bay without a central glazing bar — cleaner lines, fewer joints, fewer leak paths and a more dramatic daylight opening.
| Capability | Jumbo Glass Group |
|---|
| Jumbo threshold | Width > 2440 mm and height > 3660 mm |
| Max jumbo IGU panel | 3300 × 12000 mm (11 × 40 ft) |
| Typical skylight jumbo size | 2000 × 4000 mm up to 3300 × 6000 mm per panel |
| IGU overall thickness | 24–60 mm (double to triple glazed) |
| Max panel area | ~40 m² per sealed unit |
| Cutting accuracy | Bystronic CNC ±0.01″ (±0.25 mm) |
| Tempering uniformity | Grenzebach furnace ±9°F (±5°C)
Why the jumbo limit is 12000mm for IGU (not 20000mm): Our maximum single laminated lite is 3300×20000mm, but a skylight is supplied as a sealed insulating unit. Once the panel is laminated and assembled with spacer, gas fill and secondary seal, handling and long-term seal integrity become the limiting factors — hence the jumbo IGU maximum of 3300×12000mm. This is the critical distinction between the skylight page and the canopy page, and it is why your RFQ must specify the full IGU build-up rather than just the glass dimensions.
When to go jumbo: If the atrium width exceeds 2440 mm and you want no glazing bar in the opening, a jumbo IGU is the answer. The trade-off is handling — a 3300×5000 mm triple IGU can weigh over 1,500 kg, so the crate, rigging and support frame must be engineered together. We provide free structural and thermal simulation to validate the build-up before production.
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9. Processing Rules & Heat Soak Testing (HST)
| Step | Rule for skylight glass |
|---|
| Cutting & edgework | Bystronic CNC ±0.01″; polished / beveled / arrissed edges. Edge quality is critical — edge damage is the dominant break origin in overhead glass. |
| Tempering (outer lite) | Fully tempered (EN 12150 / ASTM C1048), surface stress ≥95 MPa. |
| Lamination (inner lite) | PVB or SGP interlayer, autoclave bonded. SGP for high residual strength. |
| Heat soak test (HST) | 554°F (290°C) × 4 hours — strongly recommended for all overhead/skylight glass to eliminate nickel sulphide (NiS) risk. |
| Low-E coating | Applied to relevant lite surface before IGU assembly (see coating placement in §6). Edge deletion 8–10 mm. |
| IGU assembly | Spacer + desiccant + primary seal (PIB) + argon/krypton fill + secondary seal. Once sealed, an IGU cannot be disassembled or modified. |
| Ceramic frit / coating | Applied and fired before tempering; edge deletion if combined with coating. |
| Drilling / notching | Completed on monolithic plies before tempering and lamination — never after.
Why HST is non-negotiable for skylights: A skylight is fully exposed overhead — a spontaneous break is both a safety event and a water-ingress risk. HST forces NiS inclusions to fail inside the factory, reducing field breakage risk to below 0.01%. Jumbo Glass Group records a post-HST breakage rate below 0.08%, with SGS batch reports and EN 12150 traceability for every heat-treated lite. Combined with our 20-year breakage insurance, this is the trust foundation for every skylight we supply.
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10. Skylight Glass Decision Flow
① Site & use→② Occupied below?→③ Sloped / overhead?→④ Loads (wind / snow / ponding)→⑤ Clear span & glazing bars→⑥ IGU + laminated inner→⑦ U-value target (Low-E / gas / spacer)→⑧ Slope & drainage→⑨ HST + temper→⑩ RFQ + sim
Every skylight project routes through the same gates. The three that catch most specifiers out are ⑤ clear span (not panel length), ⑦ U-value / condensation (the skylight-specific chapter), and ⑧ slope & drainage (mandatory — a flat skylight that ponds is not a valid design).
11. Standards & Compliance Checklist
- ✅ BS 6262-4 — overhead glazing and human-impact safety (UK projects)
- ✅ EN 1991 + EN 12150 + EN 1279 + EN 673 — actions on structures + toughened glass + IGU + U-value (EU/UK)
- ✅ ASTM E1300 + ASTM C1048 + ASTM E2190 + NFRC — load resistance + heat-treated + IGU + thermal ratings (USA)
- ✅ AS/NZS 2208 — safety glazing (Australia / NZ)
- ✅ HST to EN 14179 — heat-soaked glass for overhead applications
Compliance is project-specific. Confirm the target market and building code with the design team before finalising the specification.
12. Frequently Asked Questions
What is the best glass construction for a skylight?
For any sloped or overhead skylight, the recommended construction is an insulating glass unit (IGU) with a laminated inner ply — for example 6mm tempered + 16mm argon + 6.76mm laminated (6+0.76PVB+6). The laminated inner lite retains broken glass if the outer pane fails, while the IGU cavity with Low-E and argon controls U-value and condensation. Monolithic tempered-only skylights are not acceptable for public overhead glazing.
Why does a skylight need a laminated inner ply?
A skylight is sloped overhead glazing above occupied space. If the outer pane breaks, the laminated inner ply holds the broken fragments and any in-falling debris, preserving a safe barrier until replacement. This is the same post-breakage-retention logic that makes laminated glass mandatory for canopies and glass bridges — and it is addressed directly by BS 6262-4 and EN 1991 overhead-glazing rules.
What U-value can I achieve with a jumbo skylight IGU?
Double-glazed jumbo Low-E IGU typically reaches 0.9–1.4 W/m²K (around 1.0 W/m²K is the standard for a 6+16Ar+6 double-silver build-up). Triple-glazed units reach 0.5–0.8 W/m²K, and triple-glazed plus krypton can reach 0.5–0.6 W/m²K for passive-house projects. The actual U-value must be calculated per EN 673 / NFRC for the specific build-up.
How large can a jumbo skylight panel be?
Our maximum fabricated size for a jumbo skylight IGU is 3300×12000mm (11×40ft). This is smaller than our maximum single laminated lite (3300×20000mm) because once the panel is sealed into an IGU, handling and long-term seal integrity become the limiting factors. Always provide the support grid and glazing-bar spacing with your RFQ — they govern the practical size more than the headline maximum.
Why is heat soak testing (HST) important for skylights?
A skylight is fully exposed overhead, so a spontaneous break is both a safety hazard and a water-ingress risk. Heat soak testing at 554°F (290°C) for 4 hours forces nickel sulphide inclusions to fail inside the factory, reducing field breakage risk to below 0.01%. Jumbo Glass Group records a post-HST breakage rate below 0.08%, with SGS batch reports and EN 12150 traceability for every heat-treated lite.
How do I control condensation on a skylight?
Condensation is managed through the combination of Low-E coating (typically on surface #2 or #3), argon or krypton gas fill, and a warm-edge spacer that reduces the thermal bridge at the edge of the IGU. Together these raise the interior surface temperature above the dew point. The slope and ventilation strategy of the rooflight also affect moisture behaviour and should be confirmed with the design team.
Get Your Skylight Glass Specification
Send your layout, glazing-bar grid and project location for a free structural & thermal (U-value / condensation) simulation plus an FOB quotation — typically within 48 hours.
Required information:
- Project type & location
- Skylight span & glazing-bar grid (support spacing)
- Slope / pitch & drainage intent
- Design wind / snow / ponding loads
- Outer lite thickness / inner lamination
- IGU build-up (double / triple, cavity width)
- Low-E coating grade & surface placement
- Gas fill: argon / krypton
- Spacer type: warm-edge / aluminium
- Target U-value, SHGC, VLT
- HST required: yes / no
- Walkable? (anti-slip frit required)
- Edgework & any holes / notches
- Standard: BS / EN / ASTM / NFRC / AS-NZS
- Upload: PDF / DWG / layout / schedule
- Quantity, destination port & incoterm
📧 tomron1688@gmail.com | 🌐 Upload drawings at jumbotemperedglass.com
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← Selection Guides Hub
Back to the full selection-guide index.
Glass Canopy Selection Guide →
The closest sibling — sloped overhead glazing with a single laminated lite (up to 3300×20000mm).
Glass Bridge / Deck Guide →
When your skylight becomes walkable — concentrated load rules.
Jumbo Laminated Glass →
The inner-ply construction behind every safe skylight.
Jumbo Insulated Glass (IGU) →
The sealed thermal unit — U-value, argon, warm-edge spacer.
Jumbo Low-E Glass →
Coating grade and surface placement for energy-code skylights.
© Jumbo Glass Group. Specifications are for preliminary design guidance; final glass selection must be confirmed by a qualified structural engineer for each project. SGS batch reports, EN 12150 / EN 1279 traceability and 20-year breakage insurance available on request.
