What Size VELUX Window Do I Need?

Choosing the right VELUX window size can make a big difference to the look and feel of your home.

Whether you're renovating an attic, adding natural light to a bedroom or planning a new extension, getting the size right is an important part of the project.

With so many VELUX window sizes available, it's not always obvious which one will work best.

Should you choose a larger window to maximise daylight?

Would two smaller windows be better?

And what do codes like MK04 and MK06 actually mean?

In this guide, we'll take you through the most common VELUX window sizes, what to consider before buying and the additional products you may need for a successful installation.

Understanding VELUX window sizes

VELUX roof windows come in a range of standard sizes designed to suit different roof structures and room layouts.

Each window has a size code, such as CK04, MK06 or SK06, which identifies its dimensions.

For example, an MK06 VELUX window measures 78cm wide by 118cm high, while an MK08 measures 78cm wide by 140cm high.

These codes make it easier to identify the correct window, whether you're planning a new installation or replacing an existing roof window.

Common VELUX window sizes

Here are some standard VELUX window sizes you'll come across when shopping for roof windows in Ireland.

VELUX size code	window dimensions (width × height)
CK02	55 × 78 cm
CK04	55 × 98 cm
CK06	55 × 118 cm
FK06	66 × 118 cm
MK04	78 × 98 cm
MK06	78 × 118 cm
MK08	78 × 140 cm
PK06	94 × 118 cm
SK06	114 × 118 cm

These are nominal window dimensions, not the exact size of the roof opening required for installation.

Always check the manufacturer's specifications before making any alterations to your roof.

You can browse our range of VELUX windows to explore different sizes and styles.

How to choose the right VELUX window size

There's no single window size that suits every home.

The best choice depends on the room, how much natural light you'd like and the available roof space.

Here are some important things to consider before ordering.

1. Consider the size of your room

One of the first things to think about is how much natural light your room needs.

A small attic bedroom may only need one well-positioned roof window, while a larger open-plan living space could benefit from several windows.

As a general guide, VELUX recommends that the total roof window area should be approximately 15–20% of the room's floor area for pitched roofs to achieve good levels of daylight.

For example, if your room measures 20m², that would suggest aiming for approximately 3–4m² of total roof window area.

This is a starting point rather than a strict rule.

The amount of daylight entering a room will also depend on the position of the windows, roof orientation and surrounding buildings or trees.

It's also important to remember that the overall window dimensions are not the same as the actual glazed area, so check the manufacturer's specifications when comparing options.

2. Think about your roof pitch

Your roof's pitch, or slope, plays an important role in choosing a suitable VELUX window.

Not every roof window is designed for every roof pitch.

Some models are intended for standard pitched roofs, while others are specifically designed for lower-pitched roofs.

If you're working on a bungalow, extension or another property with a shallow roof slope, it's particularly important to check which windows are suitable.

Choosing a compatible window and installation system helps ensure proper drainage and weather protection, especially with Ireland's frequent rainfall.

3. Consider where the window will be positioned

Window placement is just as important as window size.

A roof window positioned higher up may help bring daylight further into a room, while a lower position can provide a better view outside.

For example, if you're planning an attic bedroom, you might want your VELUX window positioned so you can enjoy the view while sitting or standing.

In a kitchen extension, you may be more interested in bringing natural light into the centre of the room.

The height of the ceiling, roof structure and surrounding furniture can all influence the best position.

It's worth discussing the available options with your builder or installer before deciding on a size.

One large VELUX window or two smaller windows?

This is a common question when planning a renovation or extension.

A single large VELUX window can create an attractive feature and offer a generous view of the sky.

It may be a good choice for a room where you want one main source of natural light.

However, installing two smaller windows in different positions can sometimes help distribute daylight more evenly throughout the space.

For example, in a long attic room, two roof windows positioned apart may brighten more of the room than one window placed at one end.

The best choice depends on the available roof space, structural requirements and the overall design you're hoping to achieve.

Bear in mind that multiple windows may also require additional flashing components and installation work.

Which VELUX window size is best for different rooms?

Every room has different requirements, so it's worth thinking about how the space will be used.

Attic bedrooms

For attic bedrooms, natural daylight and ventilation can make the space feel more comfortable and welcoming.

Depending on the room's dimensions and roof structure, sizes such as MK04, MK06 or MK08 may be worth considering.

The right choice will depend on how much light you want, where the window can be positioned and whether you'd like a view outside.

Bathrooms

A VELUX window can be a useful addition to an attic bathroom, helping bring in natural light and allowing fresh air to circulate.

In a compact bathroom, a smaller window may be sufficient, although ventilation, privacy and moisture-resistant finishes should also be considered.

Kitchen extensions

Kitchen extensions often benefit from generous natural daylight, particularly where the new space extends further away from existing windows.

A larger VELUX window or a combination of roof windows can help create a brighter kitchen or dining area.

For these projects, consider where the daylight will fall throughout the day and whether multiple windows would provide a more balanced result.

Home offices

If you're creating a home office in an attic or converted space, natural daylight can make the room a more pleasant place to work.

A well-positioned roof window can help brighten your desk area, but it's also worth considering potential glare on computer screens.

The size and position of your VELUX window should work together with the layout of the room.

How to measure for a VELUX Window

Before ordering, it's important to establish whether you're replacing an existing VELUX window or installing a new one.

If you're replacing an existing window, start by checking the identification plate on the window. This normally contains the model and size code, which can help identify the correct replacement. You should also check the full product specification to confirm compatibility.

If you're installing a new window, your installer will need to assess the available roof space, roof pitch, rafters and any structural changes that might be required.

Avoid ordering a window based solely on an approximate measurement taken from inside the room.

The required opening and installation clearances must match the manufacturer's instructions for the chosen window and flashing system.

Don't forget your VELUX flashing kit and installation products

Once you've chosen the right VELUX window size, there's one more important thing to consider: the products needed to install it correctly.

A VELUX window doesn't simply fit into your roof on its own.

Depending on the window and roof type, you'll also need a compatible flashing kit and may benefit from additional installation products to improve insulation and weather protection.

VELUX flashing kits

A VELUX flashing kit is an essential part of most pitched roof window installations.

It helps create a weathertight connection between the window and the surrounding roofing material, directing rainwater safely away from the opening.

This is particularly important in Ireland, where roofs are regularly exposed to heavy rain and strong winds.

Different flashing kits are designed for different roofing materials, including slates and various types of roof tiles.

When choosing your flashing kit, make sure it matches:

Your VELUX window size: For example, an MK06 window requires a compatible MK06 flashing kit.
Your roofing material: Slate and different types of roof tiles may require different flashing systems.
Your roof pitch: The flashing must be suitable for the roof slope and window model.
Your installation type: Installing one window may require a different flashing arrangement from installing two windows side by side.

You can explore our range of VELUX flashing kits to find an option suitable for your roof window and roofing material.

VELUX installation products

Alongside the flashing kit, it's worth considering additional installation products that help improve the overall performance of your roof window.

One example is the VELUX BDX insulation collar, which is designed to provide insulation around the window frame and help reduce heat loss at the junction between the window and roof.

Proper insulation around a roof window is especially important when you're trying to keep your home warm and energy efficient throughout the colder months.

Depending on your chosen installation system, other components, such as underfelt collars, can also help provide additional weather protection.

Explore our range of VELUX installation products to see the available options.

Before ordering: Always check exactly what's included with your chosen VELUX flashing kit, as some kits may already include insulation and underfelt collars.

This helps avoid ordering additional components unnecessarily.

Taking the time to choose the correct window, flashing kit and installation products together can help make the installation process smoother and ensure everything is compatible from the start.

Common mistakes to avoid when choosing VELUX window sizes

A little planning can help you avoid problems later in the project.

Choosing a window that's too small: This can leave a room feeling darker than expected, particularly in larger spaces.
Ignoring the roof structure: Rafters and other structural elements may affect where a window can be installed.
Forgetting about roof pitch: Always check that the window model and flashing kit are suitable for the slope of your roof.
Overlooking window position: Even a large window may not provide the result you want if it's installed in the wrong location.
Not checking the full product code: VELUX window codes contain information beyond the size, so check the complete specification before ordering.
Forgetting the flashing kit: The correct flashing and installation components are essential for a weathertight fit.
Ordering unnecessary installation extras: Check what's already included with your chosen products to avoid buying duplicate components.

If you're unsure, getting advice before ordering is much easier than trying to resolve a sizing problem during installation.

Frequently asked questions about VELUX window sizes

What is the most common VELUX window size?

VELUX offers several popular sizes, including MK04 (78 × 98cm), MK06 (78 × 118cm) and MK08 (78 × 140cm).

The best choice depends on your room dimensions, roof structure and daylight requirements.

Can I replace my VELUX window with a larger one?

Yes, it may be possible to replace an existing VELUX window with a larger model.

However, this can require alterations to the roof structure and a new flashing system.

A qualified installer should assess the roof before you order.

Do I need a flashing kit for my VELUX window?

Yes, a compatible flashing system is generally required for a pitched roof window installation.

The correct kit depends on your window model, size, roof pitch and roofing material.

Are VELUX windows and flashing kits sold separately?

VELUX windows and flashing kits are often sold separately, although some packages may include additional installation components.

Always check the product description to see exactly what's included.

How do I find the size of my existing VELUX window?

Check the identification plate on your existing window, usually located on the window sash.

It should contain the model and size code.

This information can help you identify a suitable replacement.

Find the Right VELUX Window for Your Home

Whether you're planning an attic conversion, renovating an older property or adding natural light to a new extension, choosing the right VELUX window size is an important step.

By considering the size of your room, your roof pitch and how you want to use the space, you'll be in a better position to select a window that suits your project.

At MyBuildingSupplies.ie, we stock a range of VELUX windows in different sizes and styles to suit a variety of building and renovation projects.

We also supply VELUX flashing kits and VELUX installation products to help you plan your roof window installation.

Not sure which VELUX window is right for your project?

Our knowledgeable team is here to help.

Whether you need advice on window sizes, choosing the correct flashing kit or finding compatible installation products, we're happy to guide you through the available options.

Explore our range online or get in touch with our team for expert advice to help you choose the right VELUX products for your home.