Window Mullions and Transoms

12th Jan 2017
Slimline windows and composite door in white

What’s the difference between a mullion and a transom?

The key difference between a transom and a mullion is the direction of their alignment on a window, but both mullions and transoms provide additional aesthetics to divide a window’s pane of glass into smaller sections.

What is a window mullion?

Window mullion uPVC slimline windowA window mullion is a vertical beam that divides a window into smaller glass units. The purpose of a window on a mullion is to provide support to the glazing of a window, but they may also be required to provide structural support to a window arch. When used to support glazing, window mullions are often paired with transoms for extra support.

What is a window transom?

Window transom on flush timber casementsA transom is a horizontal beam that is paired with a mullion that divides a window’s upper part into additional lights. When transoms are positioned, generally they will be placed above head height to avoid obstructing the view from a window.

Why do my windows need mullions and transoms?Tilt and turn windows

Mullions and transoms can be found on a variety of window materials, including uPVCtimber and aluminium windows. Often building regulations, structural implications or simply budget constraints mean that it would be impractical to fill a window space with one large pane of glass. Mullions and transoms make it possible to divide windows into adjacent units or panes. With the use of mullions and transoms, windows can be better tailored to suit a homeowner’s needs, for example, meeting the requirements of their interior space or their preference for external access.

The use of angles, inserts, support metal and drilled anchors fix the mullion structure to the building securely. This transfers weather force away from the window glazing and onto the structure of the building. It is crucial that new or replacement windows are supported by strong mullions and transoms, and that the mullion system is installed by accredited industry experts.

Friction stay hinges are another crucial element of a window specification that assists the opening of the window. Take a read of this article for more information on friction stay hinges.

Do all window styles need mullions and transoms?

flush windowsOn most window styles, mullions and transforms are incorporated into the design for decorative reasons. They are most commonly used when the homeowner is looking for a heritage aesthetic as it separates the glazing into smaller window openings which were a popular trend on older properties. Still to this day, many double glazed window styles are split into smaller sections for added decor or to adhere to building regulations.

Many of our replacement window styles can be fitted with mullions and transoms, including our unique slimline windows, as well as popular styles like casement, flush casement, sash, mock sash and tilt and turn windows.

We’re leading replacement window designers and installers, so can create your dream replacement window solution to meet your exact requirements with transoms and mullions to achieve both modern and traditional designs.

Sheerwater Glass – 50 years of glazing experience

If you’re looking for replacement windows or more information on window mullions and transoms, get in contact with a member of the team who will be more than happy to help. We have nearly 50 years of experience in the double glazing industry, so our experts will be able to answer any questions you may have.

Related to this post:

What are the standard UK window sizes?

Our guide to the different types of windows (and which are the best!)

What are dummy sashes?

Should I repair or replace my broken window?