Specifying Glass Skylights: A Technical Guide for Architects on Performance, Safety, and Compliance

This technical guide provides a comprehensive framework for specifying glass skylights that meet the rigorous demands of contemporary projects. We will cover the critical aspects of glass composition, non-fragility testing, thermal performance, and application-specific requirements to ensure your design is safe, efficient, and flawlessly executed.

Part 1: The Foundation – Glass Composition and Safety Hierarchy

The first and most critical decision is the type of glass. Its composition directly dictates its safety profile and suitability for overhead applications. The following hierarchy moves from least to most safe for overhead glazing.

• Annealed Glass: The basic, untreated product of glass manufacturing. Due to its low bending strength and its tendency to fracture into large, dangerous shards, it is fundamentally unsuitable and non-compliant for any overhead glazing application.
• Toughened (or Tempered) Glass: This glass is heat-treated to increase its strength significantly. It qualifies as a “safety glass” because it shatters into small, granular fragments. However, these fragments can still fall in clumps, posing a risk. Furthermore, it is susceptible to rare but catastrophic spontaneous breakage from nickel sulphide inclusions.
• Laminated Glass: This is the superior choice for overhead safety. It consists of two or more panes of glass bonded together by a durable polymer interlayer (typically PVB). If the glass breaks, the interlayer holds the fragments in place, preventing them from falling and causing injury. For most overhead applications, particularly in residential and commercial settings, a laminated inner pane is the minimum acceptable standard.

Part 2: Mandating Safety – Specifying Non-Fragility

For any skylight installed on a roof where maintenance personnel or other individuals may require access, specifying “non-fragility” is a legal requirement under CDM Regulations. It is a critical error to assume that simply using laminated glass makes a skylight non-fragile.

A skylight assembly can only be classified as non-fragile if it has passed rigorous independent testing. The key standards in the UK are:

• ACR[M]001 ‘Red Book’ Test: The industry-standard test from the Advisory Committee for Roofsafety, which involves soft body impact tests simulating a person falling onto the unit.
• CWCT TN66 & TN67: Tests from the Centre for Window and Cladding Technology involving hard and soft body impacts.

To achieve non-fragility, the entire system – including the glass specification, interlayer thickness, frame, and fixings – must work in concert. A typical specification for a non-fragile unit includes a laminated inner pane of at least 9.5mm thickness, fully supported by a robust frame. Sunsquare products are rigorously tested and certified as non-fragile systems.

Part 3: Performance Specification – Thermal Efficiency (U-values)

A skylight must perform as an effective part of the insulated building envelope. The key metric for this is the U-value (W/m²K), which measures heat loss. A lower U-value indicates better thermal performance. To achieve excellent U-values that comply with Part L of the UK Building Regulations, the following components must be specified:

1. Low-Emissivity (Low-E) Coatings: A microscopically thin, transparent metallic coating applied to the glass. It reflects long-wave infrared heat back into the building, dramatically reducing heat loss without impacting light transmission.
2. Inert Gas Filling: The cavity in a double or triple-glazed unit is filled with a dense, inert gas like Argon. As Argon is a poorer conductor of heat than air, it significantly reduces thermal transfer through the unit.
3. Warm Edge Spacer Bars: Traditional aluminium spacer bars create a thermal bridge at the edge of the glass, leading to heat loss and condensation. “Warm edge” spacers made from insulating composite materials must be specified to eliminate this weak point.

At Sunsquare, all our insulated glass units feature Low-E coatings, argon gas filling, and warm edge spacers as standard, ensuring exceptional thermal performance across our entire range.

Part 4: Performance Specification – Solar Control (g-value)

While U-values address heat loss, the g-value (or Solar Heat Gain Coefficient – SHGC) addresses unwanted heat gain from the sun. This is critical for preventing overheating in summer.

• Solar Control Coatings: Similar to Low-E coatings, these advanced metallic coatings are spectrally selective. They are engineered to reflect a high proportion of solar infrared radiation (heat) while still allowing maximum visible light to enter. This is the most effective way to control solar gain without compromising daylight levels.
• Body Tints: Tinting the glass can reduce solar gain, but this comes at the direct expense of light transmission and can alter colour rendition internally. It is often a less sophisticated solution than a modern solar control coating.

Part 5: Application-Specific Specification – Walk-On Skylights

Specifying walk-on skylights for terraces, internal floors, or green roofs requires an even higher level of scrutiny, focusing on load-bearing capacity.

The relevant British Standard (BS EN 1991-1-1:2002) mandates minimum load requirements. For domestic applications, this is typically a uniformly distributed load (UDL) of 1.5 kN/m² and a concentrated load of 2.0 kN. These requirements increase significantly for commercial applications.

A robust walk-on specification includes:

• A multi-laminated glass construction, often incorporating toughened and heat-strengthened panes.
• An anti-slip finish applied to the top surface.
• A structural frame engineered to support the specified loads without deflection.

Sunsquare’s walk-on units are independently tested and certified to meet and exceed these stringent load-bearing requirements, with full supporting documentation for specifiers.

Conclusion: A Holistic Approach to Specification

Specifying a glass skylight correctly is a holistic process. It requires a clear understanding of the interplay between glass composition for safety, system testing for non-fragility, and component technology for thermal and solar performance. By moving beyond aesthetics to a performance-led specification, architects can deliver projects that are not only beautiful but also safe, compliant, and highly efficient.

Sunsquare is a trusted partner to architects and specifiers, offering expert technical guidance, fully tested systems, and bespoke solutions for the most demanding projects. All our products are BSI Kitemarked and manufactured in the UK to exceed industry standards. Download our technical whitepaper or contact our specification team to discuss your project’s unique requirements.

Contact our technical team at +44 (0)1922 714087 or email sales@sunsquare.co.uk.