Why manufacture French casement windows with a flying mullion

Joinery by: JTP Joinery

Timber French windows are a popular choice for many homes throughout the UK, as they provide a timeless look, especially for older period-styled properties. Producing French casement windows with two sashes and a fixed centre mullion is one option when manufacturing a French window. However, not only does the centre mullion obstruct your view when both windows are open, it can also obstruct access, especially if the opening is required as a fire escape route. A solution for this is to manufacture a French window with what is referred to as a "flying mullion", where instead of the mullion being part of the window frame, it is attached to a slave sash. When the window is closed, it looks like a normal French window and when both sashes are opened, there is an uninterrupted view and more importantly (when access is concerned) a wide enough opening to escape in an emergency. Fire regulations state that the minimum opening aperture should be 330mm width x 750mm height.

To achieve this style of window, during the manufacturing process the mullion would be machined separately and then screwed to the slave sash. Both the slave and master sashes would have their own locking mechanisms and be fixed to the frame using friction hinges. If the window is to be used as a fire escape, then egress friction hinges would be recommended to maximize the width of the opening. For the locking mechanisms, we would recommend using the Excalibur Shootbolt system. On the slave window, you would use the Excalibur no claw option. Using the no claw option will allow you to fix the mullion to the slave window with a spacer block in between, as shown in the section drawing below. This slave window should be locked into position by shootbolts into thimble keeps located in the frame. For locking the master window there are a few options. You could use the claw version of the Excalibur shootbolt system, with stormproof keeps fixed to the surface of the flying mullion. Other options include the Maco MK1, Maco Inline, or the Vanguard Standard.

​

In the section drawing above, there are suggestions for other products we recommend when producing this type of window. If you're externally glazing your windows, we would recommend using the Qwood QW-SD3 Clip in Glazing Bead, and if internally glazing we recommend the Qwood QW-SD4 Clip in Glazing Bead. If you don't currently internally glaze your windows please take a look at our previous blog post on the benefits of internal glazing here.

For more information on tooling for manufacturing the flying mullion section please contact Whitehill Tooling.