Types of skylights for flat roofs: Plastic or Glass?

Plastic or Glass?

When it comes to choosing skylights for flat roofs, many people are unsure as to what the best type of rooflight actually is. While most of us can agree that the best kinds of rooflights are either polycarbonate plastic ones or glass, it can sometimes be difficult to decide which one is the best for your project. We’re taking a look at some of the differences between the two, so you can see which is better when it comes to certain features.

The aesthetics

To begin with, we should look at what the two types of rooflight can provide in terms of aesthetics. While both would look attractive when installed, especially from below. The main difference in terms of aesthetics is that glass rooflights are generally flat as opposed to a thermoformed shape such as a dome or pyramid. While polycarbonate offers a cheaper alternative, the light transmission isn’t sacrificed and the overall aesthetics are similar to glass from inside.

Light transmission properties

When it comes to light transmission in skylights for flat roofs, both the polycarbonate and glass rooflights seem to be evenly matched. The quality and quantity of light which comes through the rooflights is always an important consideration, as you will obviously want as much pure and uninterrupted light to come through as possible. Both a plastic rooflight and a glass one provide the same levels of light when used, as they are both clear materials. Alternatively, with the polycarbonate skylights you can get a tint put into the plastic, as so to prevent glare from the sun on very bright days.

Material Light Transmission Impact Resistance Thermal Performance
Polycarbonate 80 – 90% Non-Fragile 2.2 W/m²K
Glass > 95% Fragile 1.5 W/m²K

Impact Resistance

Another thing you will want to look at is the amount of impact resistance each type of material can take. Polycarbonate Plastic is one of the strongest, durable and long lasting materials, and can take a good deal of impact before it fractures and shatters. Glass is required to meet certain regulations which means they tend to be toughened and laminated to prevent fall through and glass shattering. Polycarbonate is often classed as Non-Fragile. In the event of damage to fixing points or UV coatings, it is inevitable that this will be a catalyst to product degradation and thereby the period of time that the assembly will remain non-fragile will be reduced.

Thermal Performance

With the increased demand for energy efficiency, this can prove a crucial element to consider when deciding between either glass or plastic. The efficiency of rooflights can be measured in a number of ways, the most common is to consider the U-Value. This is a measurement of the thermal performance as whole and can be used to calculate heat loss in a building. The lower the U-Value, the better the material performs thermally. Part L Regulations specify a recommended U-Value for rooflights of no less than 2.2 W/m²K. Generally plastic needs to have at least three layers to reach a U-Value of around 2.2. This is in comparison to glass which will reach values of 1.5 and less when created as double glazed sealed units. Obviously, the more you do to modify each material, the better U-Values you’re going to get.


Overall, these are just a few of the features that measure each of the features of both types of rooflights. While one may perform slightly better than another in certain circumstances, our personal conclusion is that both materials are exceptional choices for rooflights. They’ve both been designed with peak efficiency in mind, and they are both great choices to use in your rooflights.


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