Curtain walling

A typical aluminium pressure plate system is shown below. To exert effective pressure to the gasket, the pressure plate must be tightened to the correct torque (specified by the designer). Pressure plates are usually secured into the mullion screw fixing channel, but repeated insertion and renewal can lead to a weakening of the fixing and it is sometimes necessary to re-drill and re-secure a plate to achieve sufficient pressure.

In most cases a snap-on cover bead is provided. This is usually colour coated, although stainless steel varieties are available. The range of profiles is wide but they all rely upon a friction grip. The level of friction is governed by the closeness of fit (or interference) between the cover and the pressure plate. Both fittings are (usually) extrusions that will have been created using different die, the manufacture of which is subject to manufacturing tolerances. Problems can sometimes occur in batches supplied where the tolerance in dimension errs towards the low side, leading to a poor interference fit and therefore a loose pressure plate.

Section showing standard stick system with pressure plate

The figure below shows an aerofoil profile cover cap with a projection of around 100mm. The manufacturer states that this type of cap must only be used in conjunction with a pressure plate of a particular profile. However, in this case, the aluminium profile has been omitted and instead a rectangular glass fibre pressure plate (resembling a strip of hardwood) has been used instead (an example of the installer mixing different components to save costs). The cover cap is a tight fit against the pressure plate, but there is little strength in the glass fibre to prevent the cap from being torn out.

Example of an incorrectly fitted cover plate - here the aerofoil-shaped cover has been wedged against a glass fibre pressure plate of inferior quality

The caps suffer from what might be loosely termed a 'Friday and Saturday Night' syndrome, as gangs of late night revellers have found that the covers at low level can be dislodged easily with a sharp kick. The more they are put back, the looser they become.

Of more concern is the fixity of the covers at higher level where lateral pressure (for example, from a window cradle) could pull the cover away. The pressure plates will need to be replaced with those of the correct specification.