Building pathology: Schools windows

Windows that can be opened manually may be specified for new school designs on cost and environmental grounds, as the sole method of natural ventilation or to provide extra ventilation.

Such windows require hinges and handles as a minimum, but a more sophisticated ironmongery package is the norm, including locks, stays, espagnolettes, shoot bolts and restrictors. The range of options is enormous with different types, sizes, grades, materials and finishes giving specifiers flexibility to balance performance requirements and cost.

Failures often result when this balance tends towards lower cost rather than performance. Yet windows in schools can be subject to high levels of use, careless use or misuse. They also tend to be larger than domestic windows and will be double-glazed, if not triple-glazed, so their weight leads to more stress on the ironmongery.

Heavy handling and misuse

Excessive force or misuse can cause failures such as loosening and fracture of fixings, detachment, fracture or permanent distortion of the ironmongery, or misalignment of hinges and locking mechanisms.

Heavy-handed use is often a response to defects occuring early in the window’s life. For instance, heavy opening vents that have dropped slightly on their hinges or variable geometry hinges that have not been cleaned of debris will lead to the window being slammed to close it. Tilt-and-turn windows are particularly prone to this.

Damage to locking mechanisms can also occur if the handles are operated before the window is fully closed.

Prompt maintenance should be carried out to rectify problems before the window becomes unusable or requires replacement.

To minimise the risk of unintended loading resulting in failures, windows should be specified to BS 6375-2:2009.

Corrosion

Corroded ironmongery, particularly fixings, can impede the smooth action of windows.

Bimetallic corrosion can happen when dissimilar metals are in direct contact. A common scenario is when tiny cost savings are made by using low-carbon steel fixing with stainless steel friction hinges. Rapid corrosion may occur to the fixing, which acts as an anode and has a small area compared with the hinge, which acts as the cathode.

The risk of corrosion can be reduced by:

• Selecting designs that drain water away from ironmongery
• Ensuring bimetallic corrosion is not a problem by using metals with similar electrode potentials, or ensuring surfaces are adequately protected to avoid the risk of bimetallic corrosion. See PD 6484 for alleviation of bimetallic corrosion
• Specifying stainless steel hinges and fixings to BS EN 10088-2 grade 1.4301 (UK type 304) or ironmongery with high or very high corrosion resistance confirmed by testing to grade 3 or 4 of BS EN 1670:2007

Mechanical wear

Repeated movement of metal on metal leads to a gradual loss of surface material and can cause binding and impaired operation of the ironmongery. Debris such as metal or plastic particles, or swarf, grit or grime in the hinge, gearbox or lock mechanisms will accelerate mechanical wear. The rate of wear depends on the compressive forces between adjacent metal surfaces, intensity of use and wind dynamics. Once components have begun to wear, the rate of deterioration increases.

Friction hinges tend to wear at pivot points, the end-point and end-cap junction. The bottom hinge of side-hung casements carries up to two thirds of the mass of an open window, making it especially prone to wear.

Hinges with plastics components to prevent direct metal-to-metal contact will reduce the risk of mechanical wear. Materials such as modified nylon are resistant to wear and can compress slightly under load, thus ensuring a continually tight fit between components.

Maintenance

Actions to prolong the life of the window ironmongery include cleaning, lubrication and regular adjustment, typically annually.

A light engineering oil with low viscosity and no additives should be used for lubrication. Water-repellent oil containing silicone may cause surface damage to the ironmongery, while viscous lubricants such as grease or Vaseline hold dust or grime, which will accelerate wear.