Velux Size Chart Guide: Choosing the Right Roof Window Sizes

Searching for a Velux size chart is usually the first step people take when a project needs a rooflight, but the codes on that chart rarely explain what actually matters for your roof. Width and height numbers mean little without knowing your structural opening.

That gap between a size code and a real, buildable opening causes more delays than almost anything else in a rooflight project. As experts, Skylights Roof Lanterns works through this with customers daily, translating size charts into practical, orderable dimensions for UK roofs.

This guide explains how a Velux size chart is built, how roof window sizes generally work across brands, and how to choose or custom-order the right size for your specific opening.

Why People Search for a Velux Size Chart First

Velux is the best-known name in roof windows, so a Velux size chart is often the natural starting point for anyone comparing options. According to VELUX's own company history, the brand was founded in 1941 and patented its first roof window the following year.

That long history is why so many size charts online reference Velux codes as a kind of shorthand for roof window sizes generally, even when the final product ordered comes from a different manufacturer entirely.

What a Velux Size Chart Actually Shows

A typical Velux size chart lists a short code, usually two letters followed by two numbers, next to a width and height in centimetres. The letters usually indicate the frame width, while the numbers relate to height.

Crucially, these figures describe the external frame dimension of the window, not the finished hole needed in the roof deck. Flashing kits and trims add extra allowance around that frame, which is easy to miss when comparing a chart to your own roof.

Roof Window Sizes: Standard Options Explained

Rather than relying on codes from a single brand, it helps to understand roof window sizes as ranges rather than fixed points. The table below shows how Skylights Roof Lanterns structures its own size options.

These standard sizes cover many common roof openings, while bespoke dimensions provide a practical solution when the existing structure falls outside those measurements.

Why Standard Rooflight Sizes Matter Before You Order

A Velux size chart can help you compare frame dimensions, but standard rooflight sizes are often a better reference point when you're planning an actual installation. The dimensions that matter most are the relationship between the top pane, bottom pane, and structural opening rather than the product code printed beside them.

That's why two rooflights with similar overall measurements can still require different opening sizes once the frame and glazing allowances are considered.

Understanding standard rooflight sizes before ordering makes it much easier to identify whether your project suits a stock size or whether a bespoke unit will be the better fit. It also helps prevent one of the most common ordering mistakes: matching the rooflight to the visible opening instead of the structural dimensions underneath.

How to Measure Your Roof Opening Correctly

Getting an accurate measurement before ordering prevents almost every sizing mistake. A few careful checks make the difference between a smooth fit and a costly reorder:

• Measure the structural opening, not the visible glass, since the frame sits on top of the timber upstand.
• Allow at least 5mm tolerance all around so the bottom pane slots in without forcing.
• Check upstand height and width, since this affects the final frame size once fitted.
• Note down both metric measurements, since UK suppliers work almost entirely in millimetres rather than the imperial figures often used abroad.
• Photograph the opening before ordering, in case your installer needs to double-check details later.

Taking ten extra minutes here avoids the most common reason custom orders get delayed.

Velux Size Chart vs Custom Roof Window Sizes

A Velux size chart works well if your opening happens to match one of the standard codes exactly. Many older or converted properties, though, have openings that fall between standard sizes.

In those cases, a custom-made unit is usually simpler than altering the roof structure to fit a fixed code. The custom flat glass rooflight allows you to specify the exact width and length required, rather than rounding up or down to the nearest standard size.

Choosing the Right Size for Your Project

Once you understand how roof window sizes are built up from frame, upstand and glazing allowances, choosing becomes far more straightforward. Smaller openings suit hallways and utility rooms, while larger formats work best in extensions and open-plan kitchens.

For projects needing airflow alongside daylight, checking size against ventilation needs matters too. Our opening rooflights range includes manual and electric options across the most commonly ordered sizes.

A Note on Commercial and Trade Orders

Developers and contractors often need consistent sizing across multiple units on one project, rather than a single bespoke piece. Bulk ordering standard sizes usually keeps both cost and lead time more predictable than repeated custom runs.

For larger projects, commercial rooflights can provide a consistent specification across multiple openings while accommodating the requirements of larger builds.

Final Thoughts

A Velux size chart is a useful starting reference, but the real work is matching a size to your actual structural opening. Standard sizes cover most straightforward projects, while custom dimensions solve the awkward gaps that standard charts cannot.

If you are unsure which size fits your opening, call Skylights Roof Lanterns on 020 4538 3079 or visit the contact page for a fast, accurate quote.

Frequently Asked Questions