For a flat rooflight in a room you use, choose glass: it is clearer, much warmer, quieter in rain, it does not yellow, and it lasts decades. Polycarbonate is cheaper, lighter and more impact-resistant when new, which keeps it the right choice for garages, outbuildings and low-budget replacements. The gap is widest on the things you live with daily. Glass gives a clear view and even light where polycarbonate diffuses it, glass holds around 1.1 to 1.4 W/m²K against a multi-skin polycarbonate unit’s 2.8 to 3.5, and glass stays clear where polycarbonate yellows and crazes under UV in 15 to 20 years. Where budget and a utility space are the drivers, polycarbonate still earns its place.
This page weighs the two materials. The related choice of shape, dome against flush glass, is on dome rooflight or flat glass, and the degradation of old polycarbonate is on yellowed and brittle dome rooflights. The wider job is on flat roof skylight installation.
The comparison
| Measure | Glass | Polycarbonate |
|---|---|---|
| Clarity | Clear, a view of the sky | Diffused, no view through multi-wall |
| U-value | Around 1.1 to 1.4 W/m²K | Around 2.8 to 3.5 for multi-skin |
| Longevity | Decades, does not yellow | Yellows and crazes in 15 to 20 years |
| Rain noise | Quiet, mass damps it | Louder, the light sheet drums |
| Weight | Heavier | Light, easy to handle |
| Impact strength | Toughened and laminated, safe | Very high when new, brittle when old |
| Cost | Higher | Lower |
| Best for | Habitable rooms | Garages, outbuildings, budget swaps |
Clarity
Multi-wall polycarbonate diffuses light through its internal flutes, so a room gets brightness with no view of the sky, and the flutes collect condensation and dust over the years that cannot be cleaned out. Glass passes light cleanly and gives both the daylight and the sky. In a kitchen or living room this is the difference you notice every day, and it is the strongest single argument for glass.
Warmth
A multi-skin polycarbonate unit at around 3 W/m²K loses heat more than twice as fast as a glass unit at 1.2. In a heated room through a Suffolk winter that is real money and a colder space. Solid single-skin polycarbonate is worse still on heat. Glass, with argon and a low-emissivity coating, holds warmth in and meets Part L comfortably.
Longevity and discolouration
This is where glass wins outright over time. Polycarbonate yellows and crazes under UV, going cloudy at 12 to 15 years and brittle by around 20. Glass does not degrade under sunlight at all, so a glass unit lasts decades with only the perimeter seal as a wearing part. Modern polycarbonate is better UV-stabilised than the 1980s material, so it lasts longer than it used to, but it still yellows eventually where glass never does.
Rain noise
A light polycarbonate sheet drums in heavy rain, which some people find pleasant and others find intrusive, especially over a bedroom or a quiet room. Glass has mass, which damps the noise, so it is markedly quieter. On a flat unit facing straight up, rain lands directly on it, so this difference is more noticeable than on a pitched one.
Impact and safety
New polycarbonate is extremely tough, which is its traditional selling point, but that toughness fades with UV so an old sheet becomes brittle. Glass rooflights use a toughened outer pane and a laminated inner throughout their life, so the outer resists impact and the inner holds together if broken, meaning nothing falls into the room. For overhead safety a modern glass unit is the more predictable material across its whole life.
Where polycarbonate is still the right call
Polycarbonate earns its place on utility spaces and tight budgets. On a garage, workshop, outbuilding or utility room, the clarity and warmth of glass buy little you will benefit from, and polycarbonate does the daylight job for less. It is also light, which simplifies handling on an awkward roof. Where the space is functional and the budget is the driver, we fit polycarbonate and it is the honest choice.
- Garages and workshops: daylight matters, a view does not, and cost does. Polycarbonate.
- Outbuildings and stores: the same. Glass would be over-specified.
- Low-budget replacements: where a glass unit is not proportionate to the room, a new UV-stabilised polycarbonate unit is a sensible swap.
- Poorly drained roofs: a polycarbonate dome sheds by shape, which is forgiving where the fall is wrong, though correcting the fall and fitting glass is the better outcome.
Suffolk and north Essex specifics
Most of the polycarbonate we replace across Ipswich, Kesgrave and Chantry is 1980s and 1990s material that has yellowed and gone brittle, and on habitable rooms we steer those to glass because the improvement in light and warmth is large. On garages and the outbuildings common on rural Suffolk plots, polycarbonate stays the sensible fit. On the coast at Felixstowe, glass units take the marine specification, and polycarbonate is fine on functional coastal outbuildings where the salt attacks the fixings rather than the sheet, so the fixings are what we upgrade.
Common questions
Is glass or polycarbonate better for a flat rooflight?
Glass for a habitable room, on clarity, warmth, quiet, longevity and safety. Polycarbonate for garages, outbuildings and tight budgets, where it is cheaper and lighter and the room does not need a clear view or low heat loss. Glass costs more up front and lasts decades without yellowing, so on any room you use it is the better long-run choice.
Does polycarbonate yellow like it used to?
Less than the old material, because modern polycarbonate is better UV-stabilised, so it lasts longer before clouding. It still yellows eventually, where glass does not degrade under sunlight at all. Most of the badly yellowed units we replace are 1980s and 1990s polycarbonate that has had decades of UV.
Is polycarbonate noisier in the rain?
Yes. A light polycarbonate sheet drums in heavy rain, which is more noticeable on a flat unit facing straight up. Glass has mass that damps the noise, so it is markedly quieter, which matters over a bedroom or a quiet room. Some people like the sound of rain on polycarbonate, so it is a matter of taste as well as room use.
Get the right material specified
Tell us the room, the opening size, whether it is a new opening or a replacement, and where the property sits. We will survey it, recommend glass or polycarbonate by how the space is used and the budget, and give you one written fixed price with the glazing specification stated. Every installation carries a 10-year workmanship guarantee.
Request a fixed quote in Ipswich, Suffolk or north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.