Sizing & Measurements
Sizing & Measurements
Rooflights come in different shapes and sizes, and there are multiple ways for manufacturers to define rooflights sizes. Some manufacturers measure their rooflights based on the external frame, others measure the external upstand, and some measure the glazing size.

Vario by VELUX bespoke rooflight sizes are defined as the clear internal measurement after final installation, which means that the measurement of our rooflights is equal to the glazing area and daylight area obtained after final installation and plastering.
size vario by velux rooflight
Clear Internal Measurement

Specify More Daylight For Your Clients

What is the difference between clear internal and external kerb measurements? 56% more daylight! We use the clear internal measurement (the size you select), which is equal to the amount of natural light you'll get after completing the installation. To obtain a daylight area of 1000 x 1000 mm, you would have to specify a 1200 x 1200 mm if the rooflight manufacturer uses external kerb measurement. At Vario by VELUX, what you specify is what you get as we use clear internal measurements. Hence, if you need a daylight surface of a 1000 x 1000 mm this is exactly what you would be specifying.

Structural Opening & Opening Rooflights

Structural Opening & Opening Rooflights

When preparing the structural opening for a new bespoke rooflight, your starting point will be the rooflight measurements. You determine your structural opening by adding +30mm to the length and width of the clear internal rooflight measurements, this will leave sufficient space for the internal finishing: for example, installing 15mm plastering to create the lining.


When specifying an opening rooflight, it is important to position it with the motor at the lowest part of the roof (the width is equal to the motor positioning) and with the hinges at the highest part of the roof. This will ensure that the product becomes “top-hung and  the construction will not experience unnecessary gravitational stress pulling it the wrong way.

structural opening rooflight vario by velux
rooflight vario by velux weight
Rooflight Weight & Handling

Rooflight Weight & Handling

Compared to old plastic rooflights, more modern glass rooflights are significantly more durable. But with that enhanced durability comes additional weight, which is important to consider at the early design and specification phase.

The weight of a rooflight is very important. If you are specifying a rooflight for an existing building, structural considerations and calculations must be made to ensure that the roof structure can take the added weight of a new rooflight. When designing and specifying for new builds, weight and load must be defined for the entire roof structure and considered at the early stages of the build.

Modern rooflight weights depend on their glazing type (e.g., double layer glazing vs. triple layer glazing). You will always be able to obtain the exact weight of your rooflight order, but if an overall initial weight estimation is needed, the graph on the left is a very handy tool.

Design with daylight