National Audit Office

National Audit Office

The refurbishment of the National Audit Office in London required the manufacture, installation and replacement of almost 700 W40 and W20 windows, doors and screens.

This demanding project was orchestrated by top architectural companies HOK International and TP Bennet. We fabricated new frames for all of the openings in the property, which is situated within a very prominent London area.

Jean Letherby, one of the architects for HOK who was involved in the project, recalls “Imperial Airways originally commissioned the NAO offices in 1939, and they very much incorporated the Art-Deco style of the times. In the ‘60s, the two wings were added.” She went on to comment that the use of the W20 and W40 steel windows were considered appropriate for the refurbishment of the older structure. "This was a large and particularly challenging project and West Leigh performed well on it with their fantastic window services”.

W20 windows contain both vertical and horizontal pivots, and also side hung casements and bottom hung vents - all aspects of which exactly matched the Grade II listed part of the building.

As a direct result of the two “wings” of the property not being listed, the architects had more creative freedom and decided on a dramatic, large pane fixed frame. The structural concrete columns were masked by reflective spandrel panels.


W20 is the traditional heavy-duty framing system, 32mm wide with 5mm thick flanges, originally developed for commercial and industrial applications. It accommodates single and insulating glass units up to 16mm thick in the same profiles simply with a change of bead. All glazing, whether in fixed lights or casements, is in the same plane. It can be divided into small panes with tee glazing bars, having a table face width of no more than 29mm. Hinged casements, hung at top, bottom or side, and swing windows pivoted horizontally or vertically, can all be constructed from the same kit of profiles. Elemental coupling is achieved with simple slender transoms and mullions.


An evolutionary development from W20, W40 offers continuity of style with the same casement sightlines as W20 but the 40mm wide profiles permit double glazing with 26mm thick units. The insulating glass units are drained and ventilated in accordance with modern best practice and gasket glazed to ensure consistent neatness and reliability of seal. In addition to improved thermal performance, weathertightness is significantly enhanced and fittings can be less obtrusive with variable geometry hinges, friction pivots and multi-point locking cremone bolts concealed within the profiles. W40 achieves the Secured by Design standard.

Contact our team for more information or request a quote today.