Kevin Underwood, technical director at the British Woodworking Federation.

Window casements detaching on high-rise buildings is not a mechanical failure that the industry can afford to ignore.

Yet, despite its importance, mechanical performance is often overshadowed by discussions around a window’s thermal efficiency or other attributes. Recent events however have brought mechanical performance at the forefront of the industry’s conversations.

Two reports published by the Collaborative Reporting for Safer Structures UK (CROSS-UK) have detailed mechanical window failures in high-rise residential buildings where window casements became detached from their hinges – in one case falling inward into the apartment, in another three incidents casements fell outward.

These are not minor defects. While an outward fall poses the greater hazard, windows detaching from the building can cause serious injury regardless of direction.

Both reports observed hinge failures and detachment incidents occurring with apparent increased frequency. The key takeaway from these reports is clear – hardware must be matched to the size and weight of the casement, with wind loads, corrosion resistance and long-term durability all factored into specification from the outset.

The relevant standards
Dependant on the types of window, the hardware requirements and test methods are set out in BS EN 13126-1:2022, which covers requirements common to all hardware types; BS EN 13126-8:2017 for tilt and turn, tilt-first and turn-only hardware, and BS EN 13126-9:2025 for horizontal and vertical pivot windows.

For the broader mechanical performance of windows, the key standard is BS 6375-2:2009, Performance of windows and doors – Classification for operation and strength characteristics and guidance on selection and specification.

This covers six characteristics, each with its own test and classification standard: operating forces, resistance to static torsion, racking, load-bearing capacity of safety devices, impact resistance, and resistance to repeated opening and closing.

What the tests involve
Each test standard assesses a specific aspect of how a window performs mechanically. Resistance to repeated opening and closing is evaluated under BS EN 1191, which cycles the full operation of a window; releasing the fasteners, opening and closing the casement or sash, and re-engaging the fasteners – between 5,000 and 20,000 times. The resulting durability class under BS EN 12400 reflects how the product holds up over its working life.

Operating forces are assessed under BS EN 12046-1, measuring the force required to engage or release hardware and to initiate movement of the casement or sash in both opening and closing directions.

Impact resistance testing under BS EN 13049 uses the same 50kg dual tyre pendulum impactor used to classify safety glass under BS EN 12600. It is swung to strike a critical point on the window from drop heights between 100mm and 950mm.

Racking and torsion tests, under BS EN 14608 and BS EN 14609 respectively, assess how a casement responds to the distorting forces it will face throughout its service life.

Compliance responsibilities
Mechanical performance is fundamental to a window’s safety throughout its lifespan, and it’s vital that the relevant standards are applied correctly based on the specific hardware selection and conditions the window will face to ensure long-term durability.

Maintaining a clear focus on mechanical performance and its practical application as window technology continues to develop will be essential to keeping building occupants and the wider public safe.

For more information visit: www.bwf.org.uk