08.08 Overhead glazing
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Construction
Overhead glazing may be constructed using one of the following forms:
Patent glazing
Patent glazing is described in BS 5516 Code of practice for design and installation of sloping and vertical patent glazing. BS 5516 defines:
‘Patent glazing is the term applied to a self-draining and ventilated system of dry glazing which does not rely necessarily for its watertightness upon external glazing seals’
‘Patent’ glazing traditionally comprises lapped or butt-jointed panes of glass fixed into metal glazing bars on the vertical edges, with horizontal supporting bars (cames) used only if needed. A supporting structure is required for the vertical glazing bars. The glazing is set on strip sealant or gasket materials and may be retained using beading, clips (sprung or screw-fix) or wings, with cosmetic snap-on aluminium or plastic caps. Sections of glazing may be overlapped or tiered, with draught excluders in the overlap providing a practical alternative to fully sealed systems. In the UK patent glazing systems have been extensively used in applications such as arcades, canopies and railway station roofs. Although patent glazing is traditionally unsealed it is not usually expected to perform to the same standards as recent designs of slope glazing system (for example the need for airtightness of a canopy roof is usually irrelevant). Importantly the vertical framing members are manufactured as a single piece and do not have joins along their length, reducing the probability of leakage due to unsealed joints, and this contributes to the successful use of patent glazing.
The Council for Aluminium in Building also publishes guidance on the specification and use of patent glazing.
Stick system slope glazing
This comprises an assembly of vertical and horizontal framing members, with infill retained using screw-fixed pressure plates or structural gaskets. Stick system curtain walls normally require modification for use on a slope. The modifications may involve reshaping of cover-plates to discourage water from ponding on the slope glazing system, improved drainage channels, provision to collect and drain condensate and improved jointing details. Curtain walling can rarely be used as slope glazing without modification to meet the specific performance requirements of sloped facades.
Water will collect on a roof if there are any horizontal protrusions (this is true of all slope glazing types, but is more likely to occur with modified curtain walling systems) and can also collect on a sloping wall. Ponding of water leads to dirt accumulation and eventual etching of the glazing surface. Consideration shall be given to the provision of suitable drainage paths - particularly if horizontal elements are deliberately used to intercept and drain water from the slope glazing system.
Structural sealant glazing
Structural sealant glazing systems are readily used in sloping applications, with modifications similar to those required for curtain walling. Improved drainage is essential, to eliminate the possibility of standing water on the structural sealant. Structural sealant glazing provides a flush surface, allowing water to drain freely. However, the volume of water flowing down a long slope may be very high, and suitable guttering and drainage must be provided.
Mechanical glazing is achieved by mechanically clamping or bolting-through the glazing units, with a site-applied weatherseal.
Mechanical glazing systems allow flush surfaces to be created. The gaps between panes are usually sealed with a wet-applied sealant. This type of system is often referred to as ‘Planar’; ‘Planar’ is actually a registered trademark of Pilkington Glass Limited and refers to one specific system.
Skylights
Skylights are any glazed punched openings in a conventional roof system which are primarily intended to allow light into the building. This includes non-openable features such as domelights, but does not include sheets of profiled translucent or transparent glazing materials interspersed in a profiled sheet metal or tiled roof. Skylights shall at least meet the same performance requirements for water penetration and air leakage as the roof in which they are mounted. However, the additional performance criteria and tests described in this standard shall not be blindly applied to predominantly non-glazed roofs; these roofs may have additional structural and thermal requirements.
Glazing safety
Glazing safety is considered one of the key issues in the design of slope glazing systems. The selection of the most appropriate glazing for slope glazing systems shall be based on a proper risk analysis, which deals with issues such as ease of access to the area beneath the glazing and the frequency of such access.
Glazing may break for a number of reasons, principally impact (either deliberate or accidental), thermal fracture and internal weakness (such as nickel sulfide inclusions in toughened glass).
Plain annealed glass has been used for more than a century in patent glazing and horticultural glasshouses, with very few reported incidents - annealed glass breaks more easily than other forms of glazing but often much of the glass remains in the frame. When it does fall out of the frame the risk of hitting a passer-by is often very small. However, the use of annealed glass in patent glazing has been discouraged since the mid-1970s.
The most difficult aspect of glazing safety to assess is not the actual risk of being hurt by falling glazing, which is small, but the perception of the risk of being hurt. Passers-by in a shopping arcade will feel uneasy if there is broken glass on the floor which has clearly fallen from the roof, but this effect is difficult to quantify.
Glazing materials
Single-glazed annealed glass is best restricted to applications where the risk of breakage is considered low, and where the consequences of glazing failure are deemed not to place the public or occupants at significant risk of injury. Typically this could allow use in areas which are:
- not normally accessible to the general public or building occupants, and
- no more than 5 metres above lowest floor level, and For example this would allow use in horticultural glasshouses where, although work may normally be carried out, it is unlikely that one person will be working in one place for a significant period of time.
- for commercial use only, or detached from any frequently-used structures, and
- not part of a normal working/living area.
Note that the usage of the building must also be considered. For example, the use of some kinds of glass over areas used for food preparation, or over swimming pools, must be considered very carefully, as particles of broken glass may cause a significant hazard.
Wired glass has raised concern about the effect of wire thickness on thermal fracture of wired glass - thicker wires may intercept more solar radiation, becoming hotter as a result, and have been suggested as a potential cause of premature failure. This effect is difficult to predict, given the complexity of the wire-glass interface and possible interactions with shading, and more experience is required before likely problems can be quantified.
Heat strengthened glass may perform as annealed glass or more nearly like thermally toughened glass. This depends on the degree of heat strengthening.
Thermally toughened glass shatters into fragments. A vertical pane may remain in its frame, but sloping panes are likely to fall out when they break. The greatest risk with toughened glass is that the fragments may clump together and fall en masse.
Heat soaked toughened glass is used to reduce the risk of failure in service - the toughened glass is heated for several hours to encourage reversion of the impurities to the low temperature state. Panes of glass with critical inclusions should shatter during this process, but this depends on the time and temperature used for heat soaking
Laminated glass may include one or more panes of toughened glass. If all panes are of toughened glass then the broken glazing has reduced structural integrity and may pull itself free from the opening, depending upon the fixing method.
Plastics glazing materials
Poly-methyl-meth-acrylate (PMMA, acrylic) and poly-carbonate (PC) are available as semi-rigid sheets. These materials are usually better at resisting impact, and when broken tend to remain in place. However, they also deflect more under load. Designers have to ensure that rebate depths are sufficient so that the deflected glazing cannot pull free from the framing system. BS 5516 bases its design charts for the selection of plastics glazing materials on a standard edge cover of 15 mm, compared to a minimum edge cover of 7 mm for glass.
Ethylene-tetra-fluoro-ethylene (ETFE) is used in the form of thin membranes.
Single glazings
Annealed glass has been extensively used in glasshouses with few reported incidents. However, where the general public is permitted access, for example at retail garden centres, then other types of glazing can be used. For commercial and domestic glasshouses not normally accessible to the general public there is not normally any restriction on the type of glazing used.
The CWCT Standard for slope glazing requires that for all other types of construction, unless risk analysis has shown otherwise, single glazings shall be either:
For areas of glazing up to 5 m above lowest floor level:a) laminated glass, or | These glazings offer the least risk to passers-by should breakage occur. Note that in this zone there may be additional requirements for impact resistance, and the glazing shall also be selected in accordance with Approved Document N of the Building Regulations (see safety glazing Section 08.07) |
For areas of glazing over 5 m and up to 13 m above lowest floor level:a) laminated glass, or | Toughened glass breaks into fragments when it fails, whether due to nickel sulphide inclusion, accidental impact or vandalism. When toughened glass breaks it has no load bearing capacity for self-weight or live loads; in a sloped orientation the glass will fall from its framework. Toughened glass of more than 6 mm thickness breaks into unacceptably large fragments (the largest dimension is at least the thickness of the glass). Note that a 3 m2 area of 6 mm thick glass weighs 45 kg - this represents a significant weight of falling glass fragments. |
For areas of glazing over 13 m above lowest floor level:a) laminated glass, or | Fragments of toughened glass will reach significant velocities when falling from greater than 13 m, and so toughened glass shall not be used - the preference is for glazing which will remain in place until repairs can be effected. |
Guidance from the USA indicates that when toughened glass is used for single glazing then a mesh positioned immediately below the glazing ensures that falling clumps of glass are either separated into individual fragments or prevented from falling
Multiple glazings
Where multiple glazings are used the designer has to consider the consequences of the upper pane(s) being broken some time prior to the lower pane breaking. The designer also has to consider the extra loading that will occur if water leaks through a cracked upper pane into the unit. Note that a cracked upper pane will be less visible from below, and regular inspection of slope glazing systems is necessary. The use of multiple glazings generally implies regular use of the floor beneath the slope glazing. Safety issues are therefore of more concern, and the choice of glass for use in multiple glazings is limited.
The CWCT Standard for slope glazing requires that:
| Where multiple glazings are used the lower pane shall be either a) laminated glass, or b) heat soaked toughened glass, of not more than 6 mm thickness, with a maximum pane size of 3 m2and no more than 13m above lowest floor level, or c) wired glass. | The use of multiple glazings suggests a requirement for higher levels of comfort, and thus implies the frequent presence of people beneath the glazing. These types of glazing minimise the risk to passers-by, should the glazing break. Note that plastics glazing materials shall not be used in sealed units, as they are not vapour tight. |
Where the lower pane or panes are of heat soaked toughened glass then the next lowermost pane should be suitable for use as a single glazing in the event of failure of the toughened glass. Otherwise any type of glazing may be used for the upper panes of a multiple glazing unit, subject to the results of a proper safety risk assessment.
The safest cost-effective combination is considered to be toughened glass over laminated. However, for any asymmetric combination of panes care must be taken that the glazing unit is properly marked and installed the correct way up. Where laminated or toughened glasses are used then each pane should be clearly marked to identify its type.
Glazing thickness
Glazing panes in slope glazing systems are frequently triangular or trapezoidal in shape. For monolithic panes (not hollow section plastic glazing materials) the equivalent area of the panes can be calculated from the following formulae, and conventional glazing selection charts in BS5516 may then be used:
These formula are taken from the AAMA publication’ Glass design for sloped glazing’.
Performance in use
There is a large number of different types of slope glazing system. Some of these are only intended to provide a basic level of performance (for example horticultural glasshouses).
The following is a general description of the performance issues which the designer must consider in more depth for slope glazing systems than for vertical glazing systems:
Water management
With slope glazing systems water is usually allowed to drain down the slope into a suitable gutter. However, water run-off from adjacent walls and roofs onto slope glazing systems also needs to be considered. Run-off water from conventional walling materials may contain dissolved chemicals which can attack materials used in slope glazing systems. Systems which have unsealed joints between overlapping panes of glass are more likely to leak if water is driven back up the slope, and through the joints, by the wind. Unsealed slope glazing systems with poorly-lapped joints would be unsuited to being used at low angles of slope (less than 15o from horizontal); face-sealed slope glazing systems are more likely to remain watertight, but the designer then needs to address issues such as ponding.
Drainage is critical in slope glazing systems. Two lines of defence against water leakage are usually required - an outer layer of seals to prevent water ingress and an inner layer of properly designed drainage channels to remove water which passes the first line of defence; the volume of water intercepted by a slope glazing system is greater than for a vertical facade, and internal drainage channels are more likely to be blocked by sedimentation if poorly designed.
Lateral deflection of framing members may also prevent proper drainage, and poorly positioned setting and location blocks can inadvertently block drainage channels. Suction generated by drainage flows may draw more water into the system through poorly sealed joints.
Ponding of water on slope glazing systems is undesirable, as it both increases the loading on the system and can lead to etching of the glazing. A slope is desirable to assist water run-off, and details such as cover-plates should be shaped to allow water to flow past. A canopy or a barrel-vault roof may have zero slope at the apex. It is generally better to have a framing member along the ridge with sloping glazing either side than to have a level pane at the apex on which water may collect.
Deflections in service shall be allowed for - the slope shall be sufficient to avoid ponding under normal load conditions. Deflection limits may need to be reduced for systems with near-zero slope - it is always possible to calculate the deflection at which a part of the slope glazing system will reach zero slope. Sagging of large span glazings can lead to negative slopes within the length of a pane, and sealed multiple glazing units bow in response to changes in temperature and barometric pressure.
Condensation has to be considered by the designer. During night-time conditions the effect of clear night sky radiative cooling may reduce the temperature of a slope glazing system significantly below that expected for a vertical facade, thereby increasing the risk of condensation. Proper allowance has to be made for the collection and drainage of condensate run-off from any slope glazing system.
For angles of slope greater than 30o from horizontal condensation will run down a clean glass surface and can be collected in a suitable drainage channel. Condensation may not run as freely from some plastics surfaces, nor from dirty surfaces.
Airtightness
Airtightness may or may not be required of an overhead glazing scheme. If a roof is located over an arcade or railway station which is open at one or both ends then airtightness is not a particular concern. Ventilation through an open-jointed system may help to reduce wind-loading on the roof and to prevent the build-up of heat. A similar criterion may be applied to horticultural glasshouses, where ventilation may be required to limit temperatures. The presence of unsealed joints in a slope glazing system will reduce the level of airtightness that can be achieved. However, for sealed slope glazing systems a similar level of airtightness can be achieved as for the highest performance vertical curtain walls, if this is appropriate to the application.
Thermal conditions
The effects of solar radiation on facades and roofs and typical surface temperatures resulting from solar radiation are given in Section 05.04. Depending on its orientation slope glazing may be subject to more extreme temperatures as it intercepts more solar radiation than a vertical facade.
For a slope glazing that is facing away from the sun (north facing in the UK) the air on the underside of the slope glazing can generally be assumed to be warmer than the slope glazing, and the normal upward movement of heat is undisturbed. For a sun-facing slope glazing system the glazing may absorb sufficient solar radiation to raise its temperature above that of the air beneath the glazing. In this situation the normal process of convective heat transfer is greatly reduced, and higher temperatures will be achieved.
Temperatures may be modified by shading systems whether positioned above, within or below the slope glazing system. Shading systems may include the use of white-out paints or shade netting in horticultural glasshouses.
Ventilation from directly beneath the slope glazing, whether intentional or incidental will also affect temperatures.