Glass vs Polycarbonate Rooflights: The Trade Spec Decision Made Simple

Introduction

Glass versus polycarbonate comes up on almost every rooflight project. The client wants glass; the budget says polycarbonate; the thermal target sits somewhere in the middle. If you are a roofer, builder, or specifier trying to make a defensible call, this guide gives you the working comparison — without the marketing.

The Core Difference

Both glass and polycarbonate are structural, transparent (or translucent) materials used to form the dome or flat panel of a rooflight. The differences that matter on a project are:

Clarity: Glass maintains optical clarity indefinitely. Polycarbonate, without UV protection, yellows and hazes over years. Modern UV-coated polycarbonate is considerably better than older materials — but glass remains superior long-term.
Weight: Polycarbonate is significantly lighter than glass. A double-skin polycarbonate dome weighs a fraction of an equivalent glass panel. This matters for installation at height, for structural loading, and for handling on a roof in wind.
Impact resistance: Polycarbonate absorbs impact — it dents rather than shatters. Toughened glass is impact resistant, but a direct impact will crack it. Polycarbonate is the safer choice on sites with high hail exposure or risk of falling objects.
Thermal performance: Both materials can achieve comparable U-values in multiwall or double/triple-glazed configurations. The key difference is that glass coatings (low-e, argon filling) have a more mature and predictable performance profile. Polycarbonate multiwall systems rely on air gap conductance, which varies by wall thickness and configuration.
Cost: Polycarbonate is cheaper. Unit for unit, at equivalent size and configuration, polycarbonate rooflights come in below glass. For volume projects or where budget is tight, polycarbonate is the practical choice.
Longevity: Glass, properly installed, lasts the life of the building. Polycarbonate has a shorter effective lifespan, typically 20–25 years before UV degradation becomes noticeable — UV-coated products extend this significantly.

When to Specify Glass

Glass rooflights are the correct specification when:

Aesthetics matter — residential extensions, premium commercial interiors, visible ceiling applications
The client wants a long-term, low-maintenance solution
Planning conditions require a specific glazed appearance
Acoustic performance is a consideration — glass provides better sound attenuation than polycarbonate
The project is within a conservation area or involves a listed building

When to Specify Polycarbonate

Polycarbonate rooflights are the right call when:

The rooflight is in an industrial, agricultural, or functional commercial setting where aesthetics are secondary
Weight loading is a constraint — lightweight structures, older buildings with limited structural capacity
Budget is tight and the performance requirements can be met with polycarbonate
High impact risk — hail, industrial operations, maintenance traffic on the roof
The rooflight is a certified AOV smoke vent — most certified dome smoke vents use polycarbonate

Note on AOV smoke vents: certified EN 12101-2 smoke vents typically use polycarbonate dome units. The non-fragile classification of the dome is part of the certification. Do not substitute a glass dome into a certified AOV specification without confirming compliance with the manufacturer.

U-Value Comparison

For UK building regulations compliance under Part L, the limiting U-value for rooflights in a new dwelling is 1.2 W/m²K (2021 Part L1A — confirm against current approved documents). Both glass and polycarbonate products can achieve this, but the configuration matters:

Single skin polycarbonate: Typically 5.5–6.5 W/m²K — not Part L compliant for new dwellings in isolation.
Double skin polycarbonate: Typically 2.8–3.5 W/m²K — approaching compliance but typically below 1.2 W/m²K threshold. Not suitable for new dwellings at this configuration.
Triple skin polycarbonate: Can achieve approximately 1.5–1.8 W/m²K — better, but still may not meet the 1.2 W/m²K limit without additional specification.
Double-glazed glass (low-e + argon): Typically 1.0–1.3 W/m²K — at or below the Part L limit, depending on frame and edge configuration.
Triple-glazed glass: Typically 0.6–0.9 W/m²K — comfortably within Part L requirements.

Always obtain the specific U-value from the product data sheet for the unit being specified. Do not rely on generic class figures. RamSpec AOV product pages include technical specifications.

Non-Fragile Classification

For any rooflight in a working roof — i.e., where operatives may be present at roof level — non-fragile classification matters under CDM regulations. ACR[M]001 sets the test standard.

The Brett Martin Mardome Smoke Vent carries Class B non-fragile classification to ACR[M]001 when new and fully installed in accordance with the manufacturer’s instructions.

Note the conditions: ‘when new and fully installed’. Non-fragile classification is not a permanent property of a rooflight — ageing, UV degradation, impact damage, and incorrect installation can all compromise it. Confirm the non-fragile status of any rooflight before allowing operatives to work in proximity to it.

Ordering Rooflights — Glass and Polycarbonate — from RamSpec AOV

RamSpec AOV Solutions supplies both glass and polycarbonate rooflights through Brett Martin, Ventlux, and Vent Trade (Vent Engineering). The full product range is available to order online at ramspecaov.co.uk — with product data sheets accessible from each product page.

Standard ordering terms:

Pro-forma payment — cleared payment before despatch
10–15 working day lead time from confirmed order
Direct delivery to site from the supplier

No account opening, no minimum order. Order what you need, when you need it.