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What size of rooflight do I need?

Courtyard view of Tew Farmhouse and Barns for the What Size Rooflight Do I Need? Blog - Contractor Hub header. Helpful Guides: What size of rooflight do I need?

Whether it’s a large roof window or a small lantern roof extension, the size and shape of your rooflight will have a significant impact on your home or building project.

Fortunately, choosing the right rooflight size is made simple with bespoke solutions and a wide range of sizes and models available.

Read on and discover more about how to choose the right rooflight size, common rooflight sizes in the UK, and bespoke solutions from The Rooflight Co.

From visually expanding cramped spaces to brightening dark rooms, rooflights allow natural light to pour in throughout the day – much more than traditional windows. There are plenty of rooflight and skylight options in regard to style, function, budget, and personal preference. From varied, quality rooflight ranges to bespoke skylight creations, it’s easy to find the perfect rooflight for your home.
Below, our experts will guide you through picking the right rooflight for your home. We consider the pitch of your roof, the location of the room, desired aesthetics, and bespoke features to customise your rooflight exactly as you would like.

How do I choose the right skylight size?

Rooflights bring in sunlight, naturally heat homes, and give a bright and airy aesthetic to any room they are installed in. Differently sized rooflights will be able to provide different advantages and offer unique features. This makes choosing the right-sized skylight an important aspect of creating the perfect look, feel, and function of your building or home.

In order to pick the right rooflight size, you will first need to pick the right model. Here at The Rooflight Co, we offer a wide range of flat and pitched rooflights created to fit perfectly into almost any home – be it a conservation building or a sleek and modern home.  All of these models come with standard rooflight sizes and shapes. Depending on the type of roof you’re working with, the kind of building you have, and any unique requirements (such as glazing or opening method) – one of these standard sizes may be the right one for you.

See our complete guide to rooflights for a look into all our available models.

What are the standard rooflight sizes in the UK?

Our UK rooflight models and shapes follow a range of standard sizes. The rooflight width and length options will vary depending on the shape of the rooflight, opening options, and whether they are pitched or flat.

Our popular rooflight and skylight standard sizes cover all common rooflight sizes. Below, we share the most common size rooflights at The Rooflight Co.

The Conservation Rooflight

Flash Slate Installation
Width: 532mm – 1141mm
Length: 780mm – 1895mm

Standard Clay ‘On the rafters’
Width: 412mm – 1021mm
Length: 520mm – 1635mm

The Neo® Rooflight

Flash Slate Installation
Width: 847mm – 1922mm
Length: 1142mm – 1977mm

Standard Clay ‘On the rafters’
Width: 727mm – 1802mm
Length: 882mm – 1802mm

The Conservation Plateau Rooflight

Width: 625mm – 2025mm
Length: 625mm – 2025mm

The standard measurements shown above are the structural widths and lengths as opposed to the viewable dimensions.

The Neo Advance® flat rooflight with external kerb

Width: 1000mm – 3000mm
Length: 1000mm – 1500mm

The Neo Advance measurements shown are the Finished External Kerb sizes, as opposed to the structural sizes stated on the other products.

Interior shot of Neo Rooflight at Wraxall yard
A pyramid rooflight with a conservation rooflight behind.
Cuddymoss internal space
Mottisfont Abbey - Case Study
Bespoke Conservation Plateau Rooflight
A pleated blind on a Neo Rooflight in a bathroom

How do you measure a rooflight?

Rooflights are normally measured by using structural widths and lengths, or by using the viewable dimensions or the external finished kerb dimensions.

The structural width dimension is typically defined as being the distance between the inside faces of the structural rafters for the width, whilst the structural length is the distance between the inside faces of trimmers. The viewable dimensions are the area that you can see through the glazed area of the rooflight once it is installed. While this measurement may sometimes be offered, rooflights and skylights are more frequently measured with structural dimensions.

For flat rooflights, the dimensions often include the finished external kerb as well as the opening in your roof for your rooflight. The external kerb is the framework or mounting for your skylight to sit on, and it provides the interface between the roof and window to allow for proper water run-off. Our neo Advance Flat Roof range is measured in dimensions that include the width of the external kerb as well as the roof finish.

What should you consider when choosing rooflight sizes?

Once you understand the dimensions and options available to you, there are still a number of factors you should consider before settling on a specific rooflight size. For example, you may want a specific rooflight size and shape, but building regulations or planning permission may not allow it. This can be especially true if you live in a world heritage site or in a conservation area. Although most skylights and rooflights are permitted development, it is always best to double-check with your local planning authorities.

Our guide on rooflight planning permission goes into more detail about government guidelines and when you may need planning permission.


Another vital consideration is the positioning of your rooflight and how much privacy and natural light you would like throughout the day. A large rooflight may be ideal, but it may subtract from the peace and comfort of a room if your neighbours can easily peer in. You may want to consider motorised blinds or perhaps a different location or size for your rooflight altogether.

Natural Light

The way light enters your building can also be vastly different depending on where you install your rooflights. For example, south-facing rooms often receive a strong amount of sun throughout the day, and a smaller-sized rooflight may be more practical than an overly large one. If you want to install a rooflight in a north-facing room, which is the coolest and darkest room in a house, you may want a large roof window to bring in as much warm light as possible.

The structural integrity of the rooflight

When you install a rooflight, it’s important to ensure that the existing roof structure is suitable to take the weight of any rooflights installed in the roof. This is more of a consideration for larger rooflight sizes as they are naturally heavier than their smaller counterparts.

What to consider when choosing a rooflight size

If you are installing a pitched rooflight in a roof for the first time, you will also need to consider the extra space required to install pitched rooflights flush with the roof surface. This typically means that an extra 120mm needs to be added to the width for ‘On the Rafter’calculations, and at least 260mm added to the calculations for the length. This is compared to the measurements for ‘On the Rafter’ installation.

It’s also important to consider a rafter depth of at least 150mm that’s required for any ‘Between the Rafter’ installation in order for it to be viable. Sufficient rafter depth is required to allow the rooflight to be sunken down into the roof surface, whilst maintaining the necessary structural support for the rooflight weight.

Depending on the complexity of your project you may need a structural engineer to assist with determining suitability.

What does ‘On the Rafter’ and ‘Between the Rafter’ mean?

‘On the Rafter’ and ‘Between the Rafter’ refers to how the rooflight interacts with the structural rafters on the roof. As its name suggests ‘On the Rafter’ means that the rooflight is installed directly on top of your roof rafters, which typically leads to a proud installation where the rooflight casement sits above the level of the roof material.

Conversely, ‘Between the Rafter’ means that the rooflight is installed in the space between the roof rafters. This allows for the rooflight baseplate to be sunken into the roof surface to create the extra depth required to allow a flush finish. A flush finish is where the rooflight casement sits level with the surrounding roof surface. When installing a rooflight in this way, the rooflight will need to be secured to a sunken set of secondary bearer structures that are formed on the inside faces of the roof rafters at a depth to give the clean flush finish.

How big can a rooflight be?

The most common size for a rooflight is normally somewhere between 412mm to 3000mm in width and between 520mm to 2025mm in length. However, there are many choices and a great variety in styles shapes and sizes. If you are looking for a size or shape that is not usually offered, there are many bespoke rooflight solutions that can create the perfect skylight for your project no matter the size.

Bespoke rooflight solutions

With bespoke rooflights, your skylight can be as large as you would like and as building regulations allow. Bespoke rooflights can accommodate almost any size, shape, or style, creating the perfect solutions for architects or homeowners who need flexibility in design.

If you go bespoke, you will be able to completely customise your rooflight to satisfy any unique building requirements or project designs. Our UK-made bespoke rooflights can offer you unconventionally large sizes and complete freedom of choice when it comes to shaping, colours, finished, and materials.

What size rooflight is best for me?

The incredible variety and wide range of rooflights and skylights available make finding the perfectly sized rooflight for your project straightforward. The most common rooflight sizes chosen by homeowners vary, as it depends on the dimensions of your existing structure. The right-size rooflight for your project may come in a standard model and be ready-made and all-set for installation. This will lower costs and possibly cut down much of the lead time. However, if you require a rooflight that is either uniquely large or in an unorthodox shape, you will easily be able to find the perfect bespoke solution for your home.

Bespoke and standard rooflights and skylights for your project

Here at the Rooflight Company, we have nearly 30 years of experience in delivering high-quality and innovative rooflights to our customers. Whatever project you have in mind, our passionate designers and expert British manufacturers are able to craft it into a reality.

See our case studies for inspiration and a look into the bespoke solutions we have provided to satisfy our customers’ every preference. Explore our range of rooflights and skylights today to see if one of our standard models may be the perfect fit.

Please get in touch if you have any questions. Our friendly team would be happy to talk and help you find the perfect rooflight size for your project.

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