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Design Considerations

In usual environmental conditions, we recommend annual inspection of the patent glazing system and any incorporated components by those responsible for the building’s maintenance. Please refer to the Maintenance Check List at the end of this document which should be photocopied and used as a record of inspections.

If it is known that the environment may be aggressive, such as a chemical factory, marine environment, polluted or similar, the intervals should be increased to quarterly or six monthly inspections depending upon the severity of the problem.

Access to vertical or sloping patent glazing to both the inside and outside will be required periodically for inspection, cleaning and maintenance purposes. It is vital persons carrying out this work use suitable access equipment and necessary personal protection such as gloves, hats & goggles to ensure their own safety and the safety of others who may be in danger from falling debris, tools or other materials. We recommend the method of access be agreed with a specialist provider of access systems and a method statement issued before commencement of any inspection.

Temporary access may be required for inspections and small maintenance jobs and should be removed as soon as possible after use. Ladders and crawling boards should be used to spread the load over the patent glazing system. Neither the glass or the patent glazing system should ever be directly walked on. Boards and ladders should be supported at purlins or other structural members and carefully secured to ensure they do not move during use.

Temporary access can also be provided by mobile platforms such as scissor lifts and cranes. Scissor lifts, a form of powered platform are particularly suitable for access from inside the building. The manufacturers instructions must be carefully followed and we recommend this type of equipment only be operated by experienced personnel.

Permanent access systems such as that shown in the above photograph are particularly suitable for large and or high areas of roof glazing. These may take the form of fixed or movable ladders on tracks supported off the structure of the building and should not be attached to or transfer load to any part of the patent glazing system.

Cleaning should be carried out to remove debris and grime from the roof to maintain good light transmission. The frequency of cleaning will be determined by the location of the building and how dirty the roof gets. We recommend the use of a mild detergent solution and soft cloths. We do not recommend the use of abrasive cleaning pads or scourers.

It is important to observe any recommendations from the applicator/manufacturer of any architectural finish such as polyester powder paint or similar to avoid damaging the surface finish, but generally their advice is in line with that given above. The use of hosepipes or jet washers are not recommended as it is likely gaskets and or seals will be disturbed resulting in permanent damage and leaks not just during the cleaning process, but by rain in the future.

Cleaning & Maintenance Manual Download

The performance of patent glazing roofs subjected to smoke temperatures – By J. Colvin​

Pilkington Glass Consultants

Sponsored by the Patent Glazing Contractors Association,
191 Cirencester Road, Charlton Kings, Cheltenham, Glos GL53 8DF
Tel: 01242 578278, Fax 01242 578283


Illustration and diagrams

FIGURE 1 General arrangements showing the Specimen over the furnace

The use of large areas of glazing in roofs of atria has led to fire officers to question the integrity of the roof glazing when it is subjected to hot smoke from fires. Atria are often part of the escape routes and also access routes for firemen, so the collapse of glass panes, or larger areas of roof glazing, is a hazard to be avoided.

When subjected to smoke temperatures, the materials in a glazed roof will attain temperatures roughly half way between the smoke temperature and the external ambient temperature. On this basis, up to smoke temperatures around 300°C, it can be predicted that aluminium and steel glazing bars will remain intact and also that wired glass, laminated glass and toughened glass would remain in position without collapse. This prediction is supported by tests conducted in Norway (1), where all three types of glass, glazed in aluminium frames, remained intact for 30 minutes when subjected to furnace temperature of 300°C.

However, smoke temperatures can rise as high as 600°C, and at temperatures this high it is no longer easy to predict the performance of glazing bars or glass, since the material properties of aluminium, laminated glass and toughened glass start to become critical at around 300°C. An increasing volume of enquiries about the performance of glazed roofs at smoke temperatures above 300°C, together with the publication of BS 7346:Part3: 1991, which suggests that smoke curtains should be tested at a smoke temperature of 600°C, led the Patent Glazing Contractors Association (PGCA) to have an ad hoc test performed at Warrington Fire Research Centre (2).​

Smoke Temperature Test

In order to obtain the maximum possible amount of information from the test, it was decided to attempt the following temperature regime:

  • 300°C furnace temperature for 30 minutes
  • 400°C furnace temperature for 15 minutes
  • 500°C furnace temperature for 15 minutes
  • 600°C furnace temperature for 30 minutes

The patent glazing was designed as a monopitch roof of area 4045mm long x 3400mm down the slope, the roof pitch being 15°. The patent glazing bars spanned 3300mm between supports and the single glazed glass panes were fully supported only on two opposite longitudinal edges by the patent glazing bars. The large roof area allowed six generic types of patent glazing bar and four types of glass to be installed. Figure 1 shows the general arrangement of the patent glazing roof.​

The patent glazing bars, enumerated in figure 1, were:

  1. Inverted, thermally-broken aluminium bar with aluminium cap.
  2. Traditional lead-sheathed steel bar.
  3. Traditional plastic-sheathed steel bar.
  4. Inverted aluminium bar with aluminium cap.
  5. Traditional aluminium bar with aluminium wings.
  6. Traditional aluminium bar with aluminium cap.
  7. Inverted aluminium bar with aluminium cap.

The glass panes, enumerated in figure 1, were:

  1. 7mm thick Georgian Wired Cast glass.
  2. 7mm thick Georgian Wired Cast glass.
  3. 6mm thick Georgian Polished Wired glass.
  4. 6mm thick Clear Toughened glass.
  5. 7mm thick Georgian Wired Cast glass.
  6. 6.4mm thick Clear Laminated glass.


​During the first ten minutes of the test, all the wired glass panes and the laminated glass pane cracked, but all the panes remained in position.

By the end of the first period of the test, the interlayer material (PVB – polyvinylbutyral) in the laminated glass had started to degrade. During the second period of the test (furnace temperature 400°C) the interlayer began to turn brown, but the laminated glass remained intact. Apart from further cracks in the wired glass panes, none of the patent glazing bars or the other glass panes was showing any signs of distress.

When the furnace temperature was increased to 500°C, small pieces of glass quickly became detached from the laminated glass and the interlayer material began to glow in places and degrade very quickly. Five minutes after the furnace temperature was raised to 600°C, the laminated glass pane collapsed, necessitating the termination of the test.

At this stage in the test, all the patent glazing bars were intact and showing no signs of distress, although some of the gasket materials, plastic sheathing and lead sheathing had melted. All the wired glass panes were still in position, although cracked, and the toughened glass pane was unaffected. Subsequently, a destructive test on the toughened glass pane indicated that it was still fully toughened.


​The results of the test have shown that sloping patent glazing, conforming to BS 5516, glazed with panes of wired glass, toughened glass or laminated glass, at least 6mm thick, will remain integral up to smoke temperatures of 400°C. If smoke temperatures above 400°C over escape or access routes are predicted from fire analysis studies, then it would be advisable to not use laminated glass.

Although the duration of testing at a furnace temperature of 600°C was short, the behaviour of all six types of patent glazing bar, the four wired glass panes and the toughened glass pane indicated that these were not in any distress. None of the bars was showing any significant distortion or bow, which might indicate imminent collapse. The wired glass had cracked in the early stages of the test, but was stable. The toughened glass looked undamaged and the subsequent ad hoc breakage test indicated that it was still fully toughened. It is very likely that sustained exposure for 30 minutes at this temperature would have caused no further deterioration of the integrity of the system.

Two other inferences may be drawn from the results of this test:

1. Patent glazing bars are lightweight supporting members and, in this test, were spanning a longer distance than they would normally be expected to. Heavier supporting members of aluminium or steel should perform equally well.

2. It is likely that, glazed in a vertical position in a lightweight frame, toughened glass would conform to the test requirements for smoke curtains in. BS 7346: Part 3.​


  1. Both aluminium and steel patent glazing bars should perform adequately at smoke temperatures up to 600°C.
  2. Sloping patent glazing, conforming to BS 5516, incorporating panes of wired glass or toughened glass, at least 6mm thick, should remain integral at smoke temperatures up to 600°C.
  3. At smoke temperatures below 400°C, sloping patent glazing, conforming to BS 5516, incorporating panes of wired glass, toughened glass or laminated glass, at least 6mm thick, should remain integral.
  4. Aluminium or steel supporting members of a heavier section than patent glazing bars should also withstand similar smoke temperatures.
  5. Toughened glass, at least 6mm thick, in lightweight frames should be suitable for smoke curtains conforming to BS 7346: Part 3.


  1. Per Peterson: “Overhead Glazing Recommendations in Norway Based on Fire Tests”, Glass Digest, August 15, 1988, pp 87-88
  2. WARRES No. 51333:”Ad-hoc Fire Test on a Patent Glazing Roof Structure”, copies available form the Patent Glazing Contractors Associates, 13, Upper High Street, Epsom, Surrey. KT17 4QY
  3. John Colvin is Technical Services Manager with Pilkington Glass Consultants and is a member of the Technical Committee of the PGCA.

​Condensation occurs when the surface temperature is below the dew point in the atmosphere. Designers should be aware that wherever the patent glazing system is at a lower temperature than the surrounding atmosphere and there is sufficient humidity, condensation is likely to occur. External factors such as frost, cold rain and low air temperature will increase the risk of condensation and similarly internal conditions where humidity is increased in kitchens, bathrooms, bedrooms and swimming pools for example, will also run a higher risk of problems from condensation.

Ventilation will help to reduce these problems together with the use of double glazed units with low U values and ThermGard, our thermally broken roof glazing system.

Patent Glazing has proven to have an excellent safety record. However, it is important to ensure you specify the correct glass type in relation to the overall height of the glazing above the floor level. The following recommendations apply:-

Glazing Height above floor level Suitable types for Single Glazing Suitable types for Double Glazing
Upto 5 Metres Toughened, Laminated or Wired Glass The inner pane must be Toughened or Laminated. The upper pane should be Toughened in either instance.

5 to 13 Metres Toughened glass not more than 6mm thick and panes not larger than 3sq/m.
Laminated or Wired Glass

The inner pane must be Toughened or Laminated. The upper pane should Toughened in either instance.

Above 13 Metres Laminated or Wired Glass The lower pane must be Laminated

From time to time, it is desirable to use Wired Glass in double glazed combinations. However, there is a high risk of breakage due to thermal stress and advice should be sort from the sealed unit manufacturer.

We do not recommend the use of Toughened Glass in single glazing or to the inner pane of double glazed units over swimming pools, food & beverage preparation units or any areas where the small broken pieces characteristic to this glass type, could cause subsequent injury due to contamination. In these situations laminated glass or

The most common types of glass suitable for over head glazing are detailed below. However, other infills can be accommodated including Polycarbonate, GRP & insulated panels:

Laminated Glass
Laminated glass consists of two sheets of glass bonded together with an interlayer. Commonly used are two sheets of annealed glass 3mm thick joined using a 0.4mm interlayer providing a finished pane 6.4mm thick. Alternatively, an interlayer 0.8mm thick is commonly available, offering a higher safety rating (see below). The glass is still breakable and is a little stronger than in its original form. However, the glass simply cracks and remains in place due to the bonded interlayer. This type of glass is also commonly used in the motor industry for car windscreens. Toughened glass may also be laminated to form a stronger pane when required. Laminated glass is currently the most commonly used type of single glazing for canopies, walkways, railway platform glazing and other unheated spaces. It is also suitable to use as the inner pane to double glazed units with Toughened on the outer pane.

6.4mm Laminated Glass obtains BS6206 Class B Safety Rating
6.8mm Laminated Glass obtains BS6206 Class A Safety Rating
ALL laminated glass with a PVB interlayer minimum 0.8mm thick obtains BS6206 Glass A Safety Rating

We do not recommend the use of Toughened Glass in single glazing or to the inner pane of double glazed units over swimming pools, food & beverage preparation units or any areas where the small broken pieces characteristic to this glass type, could cause subsequent injury due to contamination. In these situations laminated glass or polycarbonate are preferable.

Toughened Glass
Toughened safety glass is manufactured by heating common annealed glass to a temperature in excess of 600ºC causing it to soften. The surfaces of the glass cool quickly creating high compression resulting in up to five times increased strength. In the event of breakage, the pane shatters into relatively harmless small pieces of glass called ‘dice’. Whilst these dice may cause small cuts, it is unlikely serious injury will occur, so it is deemed to be ‘safety’ glass. It is important any cutting, polishing of edges or drilling be done before the process as once ‘toughened’ the glass cannot be worked on and is simply likely to shatter. Toughened glass is most suitable for the upper pane of double glazed units or in single glazing where the overall height above floor level does not exceed 5 metres.

All Toughened Glass has BS 6206 Clas A Safety Rating.

Wired Safety Glass
It is important not to confuse Wired Safety Glass with Wired Glass. The former has thicker wire which enables classification to BS6206 Class C Safety Rating, while the latter is now no longer suitable. Wired Glass has been used since the end of the 19th century for overhead glazing as the wires hold together any broken pieces of glass providing a similar, but not as efficient function as the PVB interlayer on Laminated Glass. Wired Safety Glass provides fire resistance, though care must be taken to ensure a suitable glazing system is also utilised to provide the required half hour or one hour protection.​

Glazing Bar Centres
Traditionally patent glazing was always installed at 241/2” centre bar to centre bar spacings as a 24” wide pane of glass was easy to handle and can fit under-arm enabling relatively long pieces of glass to be man-handled into position by one fixer. Another reason was that there was not the great variety of glass types as there are today and simple wired glass was manufactured in stock sheets 48” wide – cut one in half and you had no waste! Today patent glazing bars tend to be installed at ‘nominal 600mm centres’. At this width handling remains relatively simple and hence the cost of installation remains as low as possible. Wider spacings can be achieved, although at Lonsdale we tend not to recommend the use of two-edge support systems beyond 750mm as a rule of thumb. In some circumstances, spacings can be increased upto 900-1200mm, but this requires special consideration of all the site conditions. Wider glass, particularly double glazed units are more difficult to handle and it is important installation aspects be considered as due to weight and size, additional labour and equipment may be required thus adding to the cost. Please speak with Technical Support for advice.​

Glazing Bar Spans
Lonsdale provide a wide range of systems suitable for both single and double glazing. Generally, glazing bars are available up to maximum spans of 4 Metres for double glazing and 5 Metres for single glazing.

Sometimes these spans can be increased subject to the design wind, snow and dead loads. Glass manufacturing and handling limitations means that single panes or double glazed units may require to be joined using cames. View diagram.

Alternatively, where long down slope dimensions are required, the glazing can be built up in a series of tiers subject to the glazing bar profiles maximum span capabilities. View diagram.

Please refer to our Product Pages where tables showing the relevant glazing bar spans for our various systems can be viewed. These are for guidance only, please refer to Technical Support for advice. Lonsdale also offer several glazing bars suitable for fixing on top of timber or steel rafters.

​Structural considerations
Lonsdale products are designed to withstand some of the toughest combination of loads likely to occur including dead load (self weight of bars & glass), design wind, snow and maintenance when required. Please contact Technical Support for advice on how best patent glazing can be attached to the structure. Our systems may feature expansion joints where necessary to accommodate any movement in the building.

​Structural Tolerances
Not all patent glazing systems are as inherently flexible as Lonsdale products. However, the dimensions and placing accuracy of the supporting structure remain vital to a successful installation. Slight variation can be absorbed by the systems of approx 1.5mm per pane width, but tolerances over this become difficult and may lead to the integrity of the system being compromised.

Buildings with 15-20% rooflight area are more energy efficient
Incorporating rooflights into a building will cut energy use and reduce CO2 emissions. The benefits of natural daylight to the well-being of school children, care home residents, hospital patients and in the work place are well documented. The De Montfort University’s Institute of Energy & Sustainable Development is a respected authority on energy use and considerable research has shown that rooflights, combined with automatic lighting control, considerably reduces a building’s energy requirement thus lowering CO2 emissions. Simplified Building Energy Model (SBEM software) used to calculate the energy efficiency of buildings, shows that carbon emissions increase when the rooflight area is reduced below 20% of the overall roof. Combine this with the fact that studies show 93% of aluminium used in construction is recycled, rooflights should be a feature of any ‘green’ project.​

Buildig Regulations Part L
A brief outline of the regulations is given here. However, we recommend you read our more thorough report here.

New Build Dwellings & Non-Dwellings
Part L states that rooflights should have a minimum insulation performance of 2.0W/m2.k. This is easily achievable with the Lonsdale ThermGard series glazing bars and low-e, double glazed units. Please see our QuikSpecs for typical examples of glass types. Note that rooflights with inferior performance can be used, but should be no worse than 3.3W/m2.K. However, steps must be taken to ‘trade off’ and improve other areas of the building in order to comply using the whole building method

Replacement or Additional Rooflights To Dwellings & Non-Dwellings
Rooflights being replaced or new rooflights added to existing buildings are required to have a minimum insulation value of 1.8W/m2.K

Lonsdale ThermGard Patent Glazing System can achieve excellent insulation values when combined with high performance low-e glass. We are able to provide Uvalue Data Sheets specific to your project taking into account the bar lengths, spacings & glass type enabling you to more easily gain approval by Building Control. You can view an example here, please contact our Technical Department for advice.​

Air Leakage, Heat Loss & Solar Heat Gain

Air Leakage
Part L requires that the building envelope’s air permeability should not be worse than 10m3 per hr/m2 at an applied pressure of 50pa The Lonsdale ThermGard system has been previously tested and found to compliant with latest regulations. Therefore it is suitable to form part of the buildings ‘air-barrier’ provided special attention is paid to the abutments with the building and additional air-tight components incorporated in accordance with our standard air-tight details. Please contact our Technical department to request our Air-Tight Binder.

Heat Loss
Generally DGUs featuring standard glass types are 1.6W/m2.K, but this can be improved by argon filling the cavity and using high performance, soft coat low-e glass to 1.2W/m2.K. The efficiency of the unit can be improved further be using thermally improved spacer bars.

Solar Heat Gain
The inside of the building can heat up during daylight hours due to the sun. This is termed as solar heat gain. To reduce this effect, solar control glass can be adopted to reflect heat and reduce glare from the sun’s rays. This lessens the burden on air-conditioning systems thus reducing CO2 emissions. In simple form, this may be body tinted glass in blue, green or bronze or more sophisticated, coated clear glass that allows maximum light transmission, but at the same time substantially reduces heat gain.

​Aluminium Patent Glazing systems are tried and tested. Many installations installed forty to fifty years ago are still giving adequate service, although now are likely to require refurbishment. Railway stations are good examples. We suggest a 35 year design life is realistic for aluminium patent glazing and single glazing. For double glazed situations, advice should be sort from the sealed unit manufacturer as many double glazed units are only warranted for 5 years. Whilst the weather tightness & integrity of the glass roof is likely to remain viable for 35 years, it is likely the thermal performance of the double glazed units will decrease. Please refer to our Cleaning & Maintenance Manual for further information. Life cycle costs are exceedingly favourable due to the low-maintenance nature of Lonsdale systems. However, care should be taken at the design stage to consider access arrangements to avoid costly scaffolding costs that may be required to ensure maintenance may be carried out safely.

​We are frequently asked about what can be done to prevent injury from glass fragments to people in the event of terrorist attack in public buildings and spaces. Not only has the likelihood of attack increased, but the type of threat and devices have changed. At Lonsdale, we take very seriously our role in helping reduce injuries and save lives should such an event occur. Due to the sensitive nature of this information and our discussions with British Transport Police, it is not suitable to publish further information on our website. However, if bomb-blast mitigation is an issue for your project then please contact Technical Support for advice.

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