A Joiner’s Guide: How to Manufacture French Casement Windows Without a Flying or Fixed Mullion

When it comes to manufacturing timber French casement windows, joiners often face the challenge of achieving a wide, unobstructed opening without compromising on appearance or security. At SDH, we aim to support joinery professionals by offering practical, reliable solutions to real workshop problems.

Many joiners are already familiar with using a flying mullion to maximise the clear opening in French casement windows. In fact, we previously explored the benefits of this method in our post Why manufacture French casement windows with a flying mullion?

But while a flying mullion remains a popular option, it isn’t always the right one.

Why Avoid a Flying Mullion?

There are several project-specific reasons a joiner may choose not to use a flying mullion when manufacturing French casement windows:

1. Desire for a flush external meeting line

Some designs require both sashes to align perfectly on the outside when closed. A mullion, flying or fixed, introduces a visible mullion profile that may not suit the desired look.

2. Traditional appearances

Heritage-style or conservation projects often call for clean, uninterrupted sightlines.

3. Hardware limitations

Traditional methods (such as standard fasteners, stays, and flush bolts) can deliver a flush finish, but they do have drawbacks:

• Visual clutter inside the room.
• Lower security compared with modern espagnolette systems.
• Less smooth operation.

For joiners producing modern performance windows, these limitations are often unacceptable.

A Modern Joinery Solution: Offset Excalibur Espag Locks

A practical alternative is to use offset Excalibur espagnolette locks, which allow you to retain a flush stile design without sacrificing security or function.

How this setup works:

• Fit an Excalibur Claw Espag Lock in the master sash.
• Fit the non-claw Excalibur Espag Lock in the slave sash.
• Both locks shoot into thimble keeps fitted directly in the frame.

This configuration delivers:

• A flush look where the two sashes meet
• A secure multi-point locking system
• A cleaner internal appearance compared to traditional flush bolts
• A familiar setup for joiners already comfortable with espag hardware

For section details and visual clarification, please refer to the drawings above.

Benefits for Joiners Choosing This Approach

This solution offers several advantages for professional joiners manufacturing timber French casement windows:

✔ Improved aesthetics

Achieve a clean, mullion-free meeting line externally.

✔ Enhanced security

Modern espag technology ensures the window meets contemporary performance expectations.

✔ Ease of manufacturing

Offset espag placement is a straightforward operation for most workshops.

✔ Better user experience

Smooth operation without additional internal hardware clutter.

Supporting Joiners With Practical, Real-World Solutions

While the drawings provided are not formal specifications, they are intended to give joiners clear, functional ideas for solving common manufacturing challenges. Every workshop faces different problems when building timber French casement windows, and our goal is to share solutions that help you deliver high-quality results.If you’re exploring other joinery challenges, such as hardware selection, sash construction, or window performance upgrades, we’re always happy to help as a trusted supplier to the joinery industry.