Specialist cladding surveys are considered to be outside the scope of a normal building condition survey, and are usually required where a large proportion of the facade is clad with curtain walling or other complex system that is beyond the technical ability of most competent building surveyors to inspect.
However, when surveyors are required to undertake assessments without access to a cladding consultant, they need to know the way such walls work. Understanding the framing system is the first step.
Standardised styles
Curtain walls from the early 1970s tended to follow a standardised style of construction, typically based on a stick framework of mullions and transoms assembled on site. By the early 1980s, these had been supplemented by panellised systems and then from the 1990s by storey-height, factory-assembled unitised systems that were fully glazed.
- framing system
- glass or panel support system
- glazing
- panelling
- metalwork.
It will often be difficult from a casual visual inspection to determine how a cladding system is constructed, and system documentation may need to be reviewed or intrusive investigation carried out to understand this.
This is because the cladding package on a project may often have involved multiple subcontractors and different suppliers for each of the various individual elements. Glass may even have been manufactured, processed, fabricated and installed by four different companies.
Stick systems
The most common type of curtain wall from the 1970s onwards has been the stick system. This comprises a proprietary system of aluminium box-section mullions and transoms assembled on site, into which glazing and panels can be installed.
The systems are typically drained and pressure-equalised so any excess rainwater that enters should exit harmlessly through mullion or transom drainage without any internal leakage.
The former is suitable for low- to mid-rise buildings where any rainwater that enters the system migrates laterally along transoms to the mullions. It then falls no more than two storey heights to leave via a drainage spout, which is concealed from view behind either the external covercaps or seals.
Transom drainage meanwhile is suitable for all buildings, and is much more weather-resistant because the concealed drainage to the perimeter of each grid of glazing or panel is an airtight, pressure-equalised void; any rainwater entering the system is channelled out by visible slots in the transom pressure plates and covercaps.
The assembly of stick systems, particularly those that are transom-drained, relies on the intensive use and correct application of gaskets and sealants for watertightness. Unfortunately, most of these components and the standard of installation cannot be determined by visual inspection alone, as an intrusive investigation is required to assess critical components that are otherwise concealed from view.
'The most common type of curtain wall from the 1970s onwards has been the stick system'
Unitised and panellised systems
Increasingly used since the 1990s, a unitised curtain wall consists of typically 1.5m-wide frames of aluminium box-section construction, spanning from floor to floor, and supporting various combinations of either windows, spandrel glass, and metal or stone panels.
The panel-to-panel joints are weather-sealed using a series of multiple gaskets to form a pressure-equalised and drained chamber to the perimeter of each unitised panel. The gaskets typically form three lines of defence against rainwater and wind-driven air infiltration.
The first line of defence consists of an outer synthetic rubber flap gasket, which is visible externally, and overlaps with a similar gasket on the adjoining panel to prevent the majority of driving rainwater entering the system.
The second and third line of defence consists of a solid synthetic rubber bulb gasket, which is concealed from view, as it interlocks into the immediately adjacent panel at the time of installation. It is these second and third level gaskets that form the principal rainwater and air infiltration barrier as any rainwater that bypasses the outer first line of defence flap gaskets should harmlessly drain down to the base of the system and exit to the outside. Provided these gaskets interlock correctly at the time of installation then the system will not leak.
The unitised framework is assembled with its glazing in factory-controlled conditions and then transported to site for installation in a systematic manner, requiring minimal intervention. There is therefore much less that can go wrong at installation, and hence the overall standard of work is much higher than with stick systems.
Unitised systems are these days routinely specified for most prestigious new city-centre developments. However, stick systems remain popular for all types of new development elsewhere.
Briefly popular in the 1980s for certain high-profile developments but rarely used now, panellised systems are similar to unitised systems. However, they are much wider – typically 6m to 9m between structural columns – and often incorporate more stone and metalwork as well as the glazing.
- Capped systems: Used only in stick systems, the most common retention method is for the glazing to be supported on the framework by rubber gaskets in aluminium pressure plates behind decorative covercaps. The plates are screw-fixed into the gridwork to compress the gaskets on the glass and keep it in position. As the first line of defence, this also prevents excess rainwater from entering. The covercaps, pressure plates and gaskets can be readily removed for future maintenance to rectify any rainwater leaks or replace glass.
- Structurally glazed systems: Likewise, only stick systems can be structurally glazed, with the glass retained by a system of toggles that are screw-fixed into the gridwork and engaged in slots along the edge of the pane. The glazing joints are then sealed externally with silicone to provide a neat and simple appearance, often used at ground level in particular.
- Structural silicone glazed systems: This method is mainly used in unitised systems, with the glazing or panel fixed directly to the system's frame by structural-grade silicone. It can also be attached to a carrier subframe in either a stick or unitised system. The glass is bonded to the supporting frame in factory conditions, given that surface preparation and silicone application and curing must be to a high standard. Lack of adhesion between silicone and glass or incompatibility between the structural silicone and the hermetic silicone seal of the insulated glass unit can cause problems. Replacement of failed glazing in service is straightforward when it has been applied to a carrier subframe, as this can be unbolted as a complete entity and the glass resealed off site in factory conditions. In the absence of a subframe, the glazing will require very careful resealing in situ by a specialist.
- Structurally bolted systems: These are commonly referred to by Pilkington's proprietary name of Planar and often used as a feature to a main entrance, given their simple glass wall appearance with clean sight lines. The system may be either single pane or double-glazed toughened glass, supported on structural steel spider brackets with stainless steel countersunk screws in each corner and at mid height. Flush-tooled structural silicone to each glass joint provides the sole weather seal. Other than glass breakage or deterioration of the silicone seals, there is very little that can wrong with this system. Maintenance of the Planar stainless steel screw fixings is however required every three years, as it is likely that some torque relaxation will have occurred.
Panelling
Spandrel panels of either opaque glass or coated aluminium can be supported in the framing system to disguise the floor slab edge and ceiling zone. Glazed panels that are either single panes or insulated units must consist of toughened glass to resist the thermal stress from heat build-up, given its much superior tensile strength to laminated and/or monolithic annealed glass.
Toughened glass should also be heat soak tested, which is a process undertaken at the time of manufacture to reduce the risk of spontaneous failure from a nickel sulphide inclusion by up to 95%. Furthermore, the opaque coating, typically applied on the rear face of the inner glass, should be ceramic rather than back painted given the superior durability of ceramic coatings.
Common practice - prior to the Grenfell Tower fire - has been to use rigid board insulation behind a glazed spandrel panel, typically of polyurethane or polyisocyanurate composition, which vertically bypasses any horizontal fire break. Spandrel panels of coated aluminium, rather than being a glazed unit, will have insulation bonded to the rear typically of either polyurethane, polyisocyanurate, phenolic foam or polystyrene composition.
Metalwork
Decorative covercaps to the mullions and transoms are typically snap-fitted onto the pressure plates. A gap of around 1mm should be left at each end, otherwise restrained thermal expansion can cause the caps to become detached. This may also occur as a result of impact from rope access operatives such as for maintenance or window cleaning.
This article provides a snapshot of the most common typical framing systems. It is not however an exhaustive description of all of the various systems available, so be cautious when attempting to describe such systems and if in doubt consult a specialist. The next article will look at glass and glazing systems.
Andrew Tee is the owner and founder of cladding consultants and curtain walling specialists Tee Technical
Contact Andrew: Email
Related competencies include: Building pathology, Construction technology and environmental services, Inspection