How to Measure Flat Roof Window?

Getting roof windows finally installed can be an exciting project. After all, such windows attract attention with their non-trivial design, adding originality to your house’s overall appearance, and are capable of completely transforming the way your home looks on the outside and feels inside, allowing more natural light into the interior.

The main difficulty here is the size of the design, which should be chosen with the utmost care – taking the right measurements means creating an exterior that enhances the external nobility of the house. However, buying or replacing roof lanterns isn’t something a homeowner would do on a regular basis, so it’s only natural if you’re puzzled about the measuring process.

Moreover, it is a critical step that should be taken seriously, as getting incorrect measurements can turn out to be a costly mistake. This is especially true considering that getting flat glass rooflights isn’t the cheapest home improvement in the first place.

Before ordering a new roof window or a replacement rooflight, it is crucial to understand how to go about measuring for flat roof windows and make sure you have the correct sizes. To help you out, we’ve prepared this handy guide that will get your measurements right. Read on!

Roof Window

Is It a New Roof Window or a Replacement Rooflight?

This is the first and most important question to answer before you start measuring. The type of window you’re getting installed determines how you will take the measurements.

👉 Replacing Existing Rooflights

If you’re replacing an existing rooflight, the process is relatively straightforward and all you need is a tape measure. All you need to do is take the measurements of the existing window and order a replacement of the same size.

You most likely already have an upstand (or kerb) on your roof. In such a case, there are two key measurements you need to take here:

  • External measurement. You can take it by measuring the distance from one external edge of the upstand to the other.
  • Internal measurement. Conversely, you can take this one by measuring the distance from one internal edge of the upstand to the other.

Finally, take a look at the existing frame and check how it is installed. If it is recessed into the ceiling, make sure to measure the depth of the recess as well. This will ensure that your new window will fit snugly into place.

Now that you have these measurements, you can shop for a replacement skylight. The majority of manufacturers will have a flat roof window sizes chart or list the external upstand size suitable for the roof lanterns from their offer. You should check your measurements and compare them to those provided by the manufacturer to see if there’s anything suitable for you.

Of course, you can also directly contact a store and give them both your internal and external upstand size for further advice on the right window for your measurements.

It is also worth mentioning that if you’re going to replace your current rooflight with the same type, it might be enough to check your existing window. At the top of the window sash, there should be an information plate (it can be either on the left or the right side of the frame) with the model and size of that exact roof window.

👉 Installing New Flat Roof Windows

If you are installing a brand-new rooflight and starting from scratch, the process will be a bit more complicated.

The most important thing to remember here is to refer to a particular manufacturer’s installation details before constructing an upstand in order to ensure the best appearance and fit.

Some companies have the size of their roof windows measured to the external upstand size, while others measure them by the internal kerb size. Oftentimes, you will also find brands that have roof opening as their key measurement.

So, when taking measurements, it would be best to take notes of all of them.

You can create a timber upstand yourself, but it is a good idea to contact the manufacturer to confirm the dimensions first. Timber upstands must always be at least 150mm high, but widths will vary depending on the product being installed.

The three key measurements you’ll have to take here include:

  • finished kerb length in mm,
  • finished kerb height in mm,
  • width of kerb in mm.

If you have trouble finding a suitable product according to the measurements you’ve got, you can always contact the manufacturer directly, and they’ll be able to navigate you through their offer or provide advice on the dimensions they need to know or the measuring process in general.


Below, we answer the most commonly asked questions regarding the flat roof window measuring process.

Can you put windows in a flat roof?

Yes, you can. Roof windows can be installed on virtually any type of roof, whether it’s a flat roof or a pitched roof.

However, keep in mind that the installation angle of the flat roof window should be elevated by around 15° in relation to the roof pitch.

Are all skylights the same size?

No, they are not. The size of the window will depend on the specific model and manufacturer. That is why it is essential to take your measurements before ordering a new window or a replacement.

What is the standard size of a flat roof window?

There is no such thing as a “standard” size of a flat roof window. The dimensions will vary depending on the model, manufacturer, and, in some cases, the specific installation requirements.

Roof windows are represented in different styles and sizes, with each manufacturer usually having at least 6 size grids in their ranks. For example, the sizes of Velux roof windows are quite large, represented rather in the large and medium grid, so if you’re looking for a small skylight, you would want to check other manufacturers.

Using roofing sizes charts provided by manufacturers can help you find exactly what you need, according to your own measurements.

What is the difference between a skylight and a roof window?

A skylight is a window installed on the roof to let in natural light. A roof window is a type of skylight that can be opened to ventilate the room.

You can find both manually and electrically operated roof windows.

Do I need planning permission for a flat roof window?

It depends. You might need planning permission if the window is going to be installed in a conservation area, listed building, or if it will protrude from the roof more than 0.15m.

It is always best to check with your local planning authority before starting any work.

How do I know if I need an internal or external upstand size?

This will depend on a specific model of the window you’re getting. You can check with the manufacturer to see if they have any recommendations.