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Ask an Expert - Glass - General

Please note: Most of the answers we feature here are from 1999 - early 2002. We endeavour to keep all links etc up to date, however if you spot any errors please let our webmaster know at It should also be noted that some replies may change in light of changes to legislation especially with regards to Planning Permission and Building Regulations. To submit a new question or to query an existing question visit

Question submitted by Chris

I need a glass porthole of 1.5m diameter in the floor of an internet cafe, in Paddington, London, allowing customers to see into the basement below when standing on it. Can anyone recommend an affordable construction company that could install such a supporting glass floor? Or suggest a starting point for finding one. Is this likely to be a complex project?

This Question answered by Brendan Bermingham of - Everything is possible but this project would need a specialist contractor. (Usually a Structural Glazing company) The glass would need to be two leaves of 19mm/25mm laminated together, split up into 750 x 750 modules, and would need a SUBSTANTIAL sub-frame with glass edge cover of 45mm. The job is more than possible but I would allow £5 to £10 thousand pounds as a budget.

Question submitted by Mr McFarlane

Please could you tell me whether a Georgian style internal glass door (i.e. consisting of several panes of glass) has to have specially toughened glass to confirm to safety standards? Please could you also let me know what the current safety standards are with regard to toughened glass on internal doors/windows.

This Question answered by Brendan Bermingham of - If the glass is less than 250mm wide and not more than 0.50 m2 in area then it can be 6mm "ordinary" glass, otherwise it must be toughened or laminated safety glass, and it must be clearly marked as such. Areas where you must fit toughened glass inside or outside the home are windows that are less than 800mm from the internal floor and windows that are within 300mm proximity to a door, ie a kitchen window next to a back door.

You will also find this link useful

Question submitted by Steve

The rear of my house faces due south and incorporates two large 'picture' windows to gain full benefit of the sun and the views. However, this is causing a great deal of bleaching to furniture and carpets. I do not want to fit blinds or tinted glass. One replacement window company has suggested Pilkington K glass as the coating filters the radiation responsible for bleaching. Another suggests using any laminated glass. Are either suggestions correct?

This Question answered by Brendan Bermingham of - It is ultra violet radiation that causes the fadding. Pikington K will have very little effect in this regards, however laminated glass will cut out most of the uv rays, which will reduce the fadding a good bit.

Question submitted by Nick

I want double (or triple) glazed windows put in purely for sound insulation (heat insulation is a bonus) and am wanting to know the most effective type of combination to effect this. My idea at the moment is to have two 8mm and 6mm laminated panes with a max air gap of 16 or 20 mm. Are there any companies that actually build double glazed windows more for sound insulation?

This Question answered with help from Brendan Bermingham of - Having varying thickness of glass helps with sound insulation. (better than two pieces of glass at same thickness) We would suggest you would be better with one leaf 10mm instead of 8mm. Most sealed unit manufacturers can make a sealed unit similar to that specified above. You may also like to enquire about Pilkington Acoustic Laminate. This is a laminated glass produced with a special acoustic cast-in-place (CIP) interlayer. It is available in thickness of 7, 9, 11 and 16 mm.

Question submitted by Gurnam

I have recently brought 2 pieces toughened glass from a local supplier. Both glass seems to be slightly "currled" at the corners; and hence does not sit flat on my dinning table. I have tried turning upside down, but still no joy. The table is brand new, and flat. I have been told that when glass is toughened this can happen & can not be avoided. Is this true? I was expecting a reasonably flat piece of glass. Should I have a dispute with the supplier?

This Question answered by Brendan Bermingham of - Without seeing the actual panes of glass it is very difficult to pass comment, however I have been involved in the glass trade for over thirty years and I have never seen toughened safety glass rejected by a customer because it was "curled" at the corners. It is only when you get very long panes, 2 to 3 meters in length, that you will get a bow in the glass.

I suggest that you ask the supplier to get the toughening companies representative to inspect the glass.

Question submitted by TEB

What can I use to replace the glass in the roof of my lean-to greenhouse (12ftx8ft) and where could I get whatever you recommend. I would like to use sheets of material 2ft x 8ft to get rid of joins, the roof has only got about a 16" slope. I would like to point out that the rear of my house faces South and gets the sun all day. Hope you can help me with this.

This Question answered by Brendan Bermingham of - Depending on the the depth of the rebate there are three products that would fit the bill in this instance.

You could use 4mm toughened safety glass, you could us 4mm "glass clear" solid polycarbonate or if you have sufficient rebate depth you could upgrade to 10mm twinwall polycarbonate. All of these products are available from my company. Telephone Number 01325 310520

Now a serious word of warning - it sounds as if your greenhouse in glazed in 3mm horticultural glass. If this is the case you should either pull it down or re-glaze it completely with a safe glazing material. A greenhouse in a garden is a very dangerous thing - especially if you have children and especially if you are going to use it as a conservatory.

Question submitted by M Ronson

My Daughter has just purchased a new build property adjacent to a railway line. We do not believe that the double glazing units installed by the developer comply with the specification supplied to us prior to purchase, as the noise transmission is quite high. The spec for the living room is 10-12-6 and the bedroom is deep void. What do these specs mean? And how can we establish whether these have been fitted.

This question answered by the Windows Today editorial team - 10-12-6 stands for a 10 mm pane of glass separated by a 12 mm cavity (air gap) from a 6 mm pane of glass. Most normal double-glazing uses two panes of 4mm thick glass. The thicker the glass used - the better the sound and heat insulation. A larger air gap will also improve sound insulation. At 28 mm overall thickness the above type of sealed unit is one of the largest sizes of sealed unit available.

There are electronic tools available which will measure the overall thickness of sealed units and also the thickness of glass. A specialist glass merchant or building surveyor specialising in glass and glazing would have such tools. Another way, which may not be so easy for you - is to remove the glass and measure the overall thickness of glass and glass thickness' that way. These are not easy solutions for most people - perhaps you should get your original supplier to return to site and prove that they have supplied what was specified. The specification sounds quite good to us and if used would certainly be better than "normal" double glazing.

As a last resort you could consider also installing secondary double glazing on the inside of the replacement double glazing. Because of the much larger air gap created by secondary glazing the sound insulation would be improved even more.

Question submitted by Ann

I have just had some Thermal Break Aluminium Windows installed in part of our house. We used the existing wooden frame because it is substantial and is a feature of the house. On the exterior there is a gap between the bottom of the Aluminium Frame and the existing wooden frame. The gap is about 5MM. Is this normal?. The installer says it's there for drainage. The Interior seems quite OK.

This question answered by the Windows Today editorial team - A small gap for drainage is quite normal. Amount of gap varies from system to system but in our experience 5 mm is not unusual. With some systems its possible to have "front drainage" - i.e the water does not drain out at bottom of frame - but at the front of the frame. Its our guess that for some technical reason this was not possible with your window. It may also be that your timber frame is rebated and has a timber stop or upstand on the inside. Its unlikely you have anything to worry about - especially if the interior seems OK. 

Drainage is essential on all windows - in order to ensure any small amounts of moisture / water which enter between the gasket and glass have a method of escape. If the moisture could not "escape" you would find that the sealed unit would eventually "break down".

Question submitted by David

Can you please tell what the law says regarding the replacement of windows as regards if toughened glass should be fitted. I have been told that as I am replacing windows I need not fit toughened glass even if they are of a size and area that requires toughened glass to be fitted if I was building a new structure.

This question answered by the Windows Today editorial team - You should find the information at useful:

In particular Diagram 1. The rules are quite clear on the use of safety glass - no matter if its "replacement" or "new build".

Remember toughened glass is just one form of safety glass.

Safety glass includes:

* Toughened Glass (also called tempered) categorised as Class A

* Laminated Glass available in Class A, B or C

* Wired Glass (also called Pyroshield safetyclear/textured) categorised as Class C

Certain internal and external areas are considered 'critical locations' in terms of the safety of vertical glazing, as they are at risk from accidental human impact. The critical locations defined by the standard are similar to the Approved Document N of the Building Regulations 1991.

The 'critical locations' in any internal or external domestic area are:

a) Doors - Any glazing or part of that glazing in a door, which is between the finished floor level and a height of 1500mm above the floor level, is in a 'critical location'.

b) Side Panels to Doors - Any glazing or part of that glazing, which is within 300mm of either side of a door edge and which is between the finished floor level and a height of 1500mm above the floor level, is in a 'critical location'.

c) Windows, partitions, and walls - Any glazing or part of that glazing, which is between the finished floor level and a height of 800mm above the floor level, is in a 'critical location'.

Question submitted by Joe

What type of glazing would you use to prevent heat from the outside being transmitted into the building i.e. to reduce internal heat to building?

This question answered by the Windows Today editorial team - As you live in Australia we suggest you have a look at the Pilkington - Australia web site at Click on the Residential links for details of their range of Pilkington SMART GLASST One possible product to consider is - Pilkington ComforTone® toned glass. This is an ideal way to simply and cost effectively minimise the extremes of an Australian summer. The colour (tone) of the glass acts like sunglasses, helping to 'shade' the inside of your home by reducing the amount of the sun's heat entering through the windows. This glass can be combined with Pilkington Insulight® Insulating Glass Units1 - a type of Low E glass if you wish.

Visitors from the UK could check out Pilkington SuncoolT - this glass gives high light transmission combined with low heat transmission.

Question submitted by Simon

Why is it when I've seen some new windows being fitted the glass looks all funny - like it is wobbly and then with some other firms they look different.

This question answered by Cavan Sullivan of Welsh Window Systems - The reason for what you say is "wobbly glass" is probably that the glass is toughened. During the toughening process the glass is heated up and then cooled rapidly. This process is carried out several times.

The end result is that glass can sometimes look a little distorted. In practice this is not usually noticeable. If it does happen - it's usually in larger pieces of glass.

If you are particularly worried about this and are installing large picture windows or patio doors you could use 6mm toughened glass, which has far less distortion. (Normally toughened glass is 4mm) You could also consider using laminated glass, which does not have any distortion, offers a good degree of security and also will help to stop carpets and furnishings from fading.

Both 6mm toughened glass and laminated glass would be more expensive.

Cavan's web site is found at

Question submitted by Anne-Marie

We have had double glazing fitted in April this year and have leaded light strips on the front of the house. The problem is that they are still going through the "Oxidisation process" 5 months on and they look terrible! The lead is all mottled and look as thought they are covered in salt, what do you recommend or is this a fault?

This question answered by the Windows Today editorial team - When lead is newly exposed to the elements it will go through a "staining" process where white carbonate forms on the lead and you may see white "streaks" on the glass. In time this will end as the lead "ages". One way to prevent this - is to apply Patination Oil to the lead. This process provides a more pleasing finish.

You may find out more about lead, Patination Oil and a new product called LEADSHIELD (a solvent free replacement to Patination Oil) by visiting the British Lead Mills web site at

It would be unfair to say who if anybody is to blame. Most suppliers do not provide "aged" lead as a standard option. Its something that happens - which will eventually go away. We are not sure but it may be too late to apply Patination oil - we believe it should be applied as soon as lead is exposed to elements. We suggest you check with your supplier and/or specialist lead supplier.

Question submitted by Peter

Have you ever heard of a problem with Pilkington K glass units breaking when being installed in a conservatory roof or side frames? Our conservatory - 15 x 15 is nearing completion and four units have broken as being installed. The person responsible for the project tells me that the manufacturers of the glass units - Darby West of Bristol - have said that they have had a problem with the glue that they have been using but they denied this when I contacted them? They have said that maybe the units were slightly oversize so that are now making the replacements one slightly smaller all round? (This question edited).

This question answered by the Windows Today editorial team - I'm familiar with Darby Glass - they are one of the UK's largest producers of toughened glass and a company of excellent reputation.

The problem you are experiencing is not unique to Pilkington K Glass units but can apply to all sealed units. One of the reasons that a lot of conservatory suppliers do not in particular favour glass roofs is that it can be more "problematical" to install. Although the glass is toughened it is in fact quite easy to break during installation just by putting too much pressure especially on the edges. Quite often I find that people assume toughened glass is some form of EXTRA STRONG glass - perhaps a little like "bullet proof glass". However in my opinion "break safely glass" would be a better description of toughened glass. Sure it is quite difficult to break - but not impossible. When it does break it will break into very small sections. These small sections may if you are unlucky give you some scratches or minor cuts but will not pose the danger created by large glass shreds when "normal" float glass breaks. Toughened glass is the most widely specified safety glass in conservatory construction.

Despite the potential installation problems and the fact that installers need to be even more "diligent" in their installation efforts I still favour glass roofs. Their looks/appearance makes up for the possible "problems" on installation especially as your supplier should take responsibility for breakage's etc.

I have known installers to go for many installations without any unit breakage's - then their "luck" runs out and they experience breakage's several times in a row. There is often no logical explanation. For what its worth I think its more likely that if there is a fault it lies somewhere on the installation side - it could be that the units where slightly oversize or perhaps hitting some obstruction on the rafters. I would not be too hard on the installers themselves - this may not be a common problem - but it does happen. I think you (and the installers) have just been unlucky. What matters are that your supplier remakes the units at no additional cost to you. I don't really think you can expect more than that.

Question submitted by Julie

What is the best option - toughened glass or laminate. What is the difference. I have a child and am I right in saying that toughened glass is more dangerous if smashed. Is one more secure than the other. Thank you.

This question answered by Tina Dunlop @ Almost Impartial Guides - Both Toughened and Laminated glasses are forms of "safety" glass. Quite often I find that people assume toughened glass is some form of EXTRA STRONG glass - perhaps a little like "bullet proof glass". However in my opinion "break safely glass" would be a better description of toughened glass. Sure it is quite difficult to break - but not impossible. When it does break it will break into very small sections. These small sections may if you are unlucky give you some scratches or minor cuts but will not pose the danger created by large glass shreds when "normal" float glass breaks. Toughened glass is the most widely specified safety glass in conservatory construction. Laminated glass will, when hit with force "crack". However it is unlikely to smash. Being very difficult to break means that it can be dangerous in any situation where it would be likely you need to break the glass in order to escape. (Such as a fire) This is off course an advantage if security is a major consideration. Laminated glass is also "thicker" - usually 6.4 mm and as such will offer better insulation. However this thicker glass is also "heavier" which will in turn mean greater "wear and tear" on opening windows / doors. It is also more expensive than toughened glass.


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