Dimensions of roof windows: take the dimensions well | VELUX Magazine

Dimensions of the roof windows: take the dimensions well

Whether it’s fitting out attics or simply replacing old windows – you need to measure your roof windows to determine what you need. Factors such as the angle of the roof slope, the minimum light gap or the statics of the building must be taken into account to determine the dimensions of your roof windows.

  • Have the following points in mind before taking the measurements for the roof windows
  • Determine the dimensions of the roof windows for existing windows
  • Correctly measure the roof windows
  • Calculate the size of the roof window – here’s how it works!
  • More light and open rooms thanks to larger roof windows
  • Important notes regarding the dimensions of the roof windows
  • Frequently asked questions

Have the following points in mind before taking the measurements for the roof windows

Sufficiently large roof windows give the room a completely different look. The more light a room is bathed in, the larger and more welcoming it seems. If you replace your old roof windows, you can kill two birds with one stone by improving your energy balance. Indeed, modern windows are significantly better insulated.

There are some important points that you should consider when choosing your new roof windows. Indeed, the slope of the roof as well as the type of window have an influence on the dimensions of the roof windows. When replacing the windows, also ask yourself the question of whether the sizes of the models from various manufacturers are really compatible.

The common dimensions of roof windows are often between 78-160 cm high and 54-164 cm wide. In this sense, there are no common standards for several manufacturers, it is more each brand that has its own standard dimensions.

In addition, custom requests or special dimensions are to be planned in consultation with your window manufacturer or roofer. The professional will be able to support you in determining the odds.

Determine the dimensions of the roof windows for existing windows

If you want to replace your old roof windows, it is very easy to determine the dimensions of the corresponding roof windows. To do this, you just need to know your window type. The VELUX window identification plate necessary for this purpose is located on the leaf of the open window. It indicates both the size of the window and the type, so that the replacement of an old roof window does not pose a problem with this information. This information is also practical in that it allows you to quickly and easily buy the accessories that match your roof window, such as sun or heat protection products.

If it so happens that you cannot determine the type of window (the sticker has become illegible or has been detached), you can take the measurements of the window – empty leaf, inner dimension of the frame groove, external dimensions of the applique frame – and compare the measured dimensions online with those of the common VELUX window types.

Correctly measure the roof windows

If the identification plate is no longer present on your window or is illegible, you must measure the roof window yourself.

To measure the dimensions of the window, you will only need a tape measure or a folding meter.

If you want to replace your roof window, it is above all the four dimensions shown in the image that are important, you will need them both transversely and longitudinally:

Calculate the size of the roof window – here’s how it works!

  1. Measure the inner dimension of the groove of the frame: measure the distance of the grooves, i. e. the “edges” of the inner opening. Write down the width and height in millimeters. This dimension is sufficient most of the time to order the window.
  2. Measure the external dimensions of the applique frame: Also measure the distance from the external finishes of the applique frame. These dimensions are above all important if you want to replace the interior opening.
  3. Measure the voids: first measure the dimension of the illuminating glass surface (clear glazing). This is the distance from one edge of the leaf frame to the opposite edge. Also measure the leaf gap, i. e. the length and width of the leaf.
  4. Measure the opening width: Finally measure the opening width. This is the maximum distance between the lower edge of the sash and the upper edge of the window frame in the open position.

More light and open rooms thanks to larger roof windows

If you do not want to replace your old roof window identically and you prefer a replacement of the window with a new interior opening, or even an enlargement of the roof window, this is also possible thanks to the type number of your window. If a new interior opening improves the penetration of light, a completely new larger window of course guarantees even more light in the room.

Rafter width and rafter tangle

In the event of an extension of the window facade, however, you must ensure that the construction operations are in accordance with the situation on site. In general, the roof windows are located between two roof rafters. The basic rule states that between two rafters should pass the width of the window plus 4 to 6 cm. If this distance is no longer respected with the new roof window or if your house (especially old buildings) has many rafters with little distance, it is possible to adapt the rafters. Roofers and carpenters can both enlarge and reduce the width of the rafters – these static interventions must be carried out only by a professional and may make it necessary to check the static. Moreover, the roof must be dismantled for the cutting of the roof window, then the roof covered again. Take these cost elements into account when replacing or enlarging a window.

Important notes regarding the dimensions of the roof windows

The important thing here is to know which window model you want to install. The windows with opening by rotation have at best a lower edge of the window of 90 cm, while the windows with opening by projection require a lower edge of the window of 120 cm due to the control at the bottom.

Other concepts for the dimensions of the windows:

Other dimensions follow from the types of windows and are not necessary to order a window, do not be confused, these dimensions are important only in certain conditions.

  • External dimensions: To obtain all the relevant external dimensions, you must measure the external dimension of the leaf frame (distance from the outer edges of the leaf frame), the external dimension of the thermal insulation (distance from the outer edges of the insulation) and the external dimension of the applique frame (distance from the outer finishes of the applique frame).
  • Voids (see graphic): in addition to the internal dimension of the groove of the frame and the internal dimension of the wall frame, the leaf void and the dimension of the illuminating glazed surface (clear glazing) are part of the voids.
    • Dimension of the illuminating glass surface: length and width of the glass without the leaf.
    • Sash void: length and width of the sash (movable surface for closing a window).

    Dimensions of windows for flat roof

    If you want modern flat roof windows and determine their dimensions, please consider the following points. The flat roof window is practical and of course does not depend on the angle of the wall. But if it is easily accessible (for example on a roof terrace), glazing with a safety glass is mandatory. If you don’t feel able to take the measurements on the roof, ask a professional.

    Frequently asked questions

    How to properly measure a roof window?

    The important thing in the measurement of a roof window is above all the internal dimension of the groove of the frame – this dimension is sufficient most of the time to order the window. To correctly measure the old roof window, the width and height are indicated in millimeters. For the inner dimension of the groove of the frame, measure the distance of the grooves, i. e. the “edges” of the inner opening.

    What are the types of windows that exist?

    • Roof window with rotary opening
    • Roof window with projection opening
    • Window for flat roof
    • Panorama
    • Balcony
    • Cabrio
    • Roof access window
    • EFC roof window
    • Roof window for passive house
    • Roof window for cold room
    • Roof window with heat recovery

    At what height to install the roof windows?

    To install a roof window at the ideal height, the upper edge of the window and the lower edge of the window are decisive data. The upper edge of the window must be at least 2 m from the floor. The lower edge of the roof window, on the other hand, should be 90 cm from the floor.

    Where can I find the VELUX identification plate?

    To determine the type of roof window, all you have to do is read what appears on the identification plate of your window. This is located to the right or left behind the handle when the window is open. The identification plate provides information about the type of window, the size of the window, the serial number as well as the material and the glazing variant.

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