Paranoid about spontaneous failure of large skylight

[hendriQ]

About to order a rather large glazing unit that is 3900mm by 820mm in size. It is double glazed with no joins (so each pane of glass will be 3900 by 820). Weight is about 180kg.

Asked about the warranty and the vendor said "well obviously I can't guarantee it won't spontaneously fail". I asked what he meant and he said that all glass can in theory spontaneously fail. On probing further, he tells me the inner pane will be heat treated so that if it fails it will shatter into thousands of pieces, each piece no bigger than 1cm by 1cm.  I asked if heat treatment is in accordance with BS EN 14179-1 and he is going to find out. The outer pane however will just be regular glass. With heat treated glass in accordance with BS EN 14179-1the rate of failure is 1 in 400 tonnes of glass. So for a skylight this weight, there is something like a 1 in 4444 chance of failure of the inner pane (assuming both panes are of equal weight, which I think is likely as they are both are same thickness). So not too likely, though more likely than I'd like. But failure of the outer pane is much more likely and if that fails it won't shatter into tiny little pieces, it will shatter into shards. Surely that would then cause the heat soak tested inner pane to also break? 

This unit is going in an upstand we've built to make a skylight directly above our dining table. Not so enthusiastic about this now.

Window company said laminated glass would be about £800 to £1000 more. Not sure why that's so expensive.

 

Do people worry about this sh!t, or am I being paranoid?

[JohnMo]

Our windows (which are much smaller) had to be approved by the structural engineer due to wind loads, would have thought that size sky light would need someone qualified to approve the design.  1cm X 1cm glass is huge, if it hits you.

 

I would get professional advise.  They have said the risk, so the responsibility could fall on your shoulders if all went wrong.

 

I landed via a ladder fail, on glass side tables, the glass was all over the floor (6m X 6m), the shares as as sharp as a sharp thing.

[hendriQ]

35 minutes ago, JohnMo said:

Our windows (which are much smaller) had to be approved by the structural engineer due to wind loads, would have thought that size sky light would need someone qualified to approve the design.  1cm X 1cm glass is huge, if it hits you.

 

I would get professional advise.  They have said the risk, so the responsibility could fall on your shoulders if all went wrong.

 

I landed via a ladder fail, on glass side tables, the glass was all over the floor (6m X 6m), the shares as as sharp as a sharp thing.

Well the structural engineer has approved the overall design of the house from a structural perspective, so that's something I hope! Not sure wind is an issue as the skylight is effectively flanked by one wall on the long side and another wall on the short side; it's above a ground floor extension, so not very high up either, so I doubt there is much wind there.

When you say your windows were approved by a structural engineer, do you mean rooflights (that are almost flat) or actual windows which tend to be orientated in a plane that is perpendicular or close to perpendicular to the wind?

Edited November 15, 2021 by hendriQ

[Declan52]

Cover it with a clear plastic film. If in the highly unlikely event that it shatters then it won't rain down on what's below.

[Ian]

@hendriQ

the glass spec that would be used on a typical commercial project with overhead glazing or for balustrading would be for the lower pane of glass to be toughened laminated (IE: 2 panes of toughened glass laminated together). The upper pane would be typically be toughened. (All toughened glass to be heat-soaked)

 

with toughened laminated glass, if one of the 2 panes breaks the plastic interlayer and the other intact pane holds everything in place.

[hendriQ]

10 minutes ago, Ian said:

@hendriQ

the glass spec that would be used on a typical commercial project with overhead glazing or for balustrading would be for the lower pane of glass to be toughened laminated (IE: 2 panes of toughened glass laminated together). The upper pane would be typically be toughened. (All toughened glass to be heat-soaked)

 

with toughened laminated glass, if one of the 2 panes breaks the plastic interlayer and the other intact pane holds everything in place.

Okay, but this isn't a commercial project. It's my house. But maybe that's all the more reason to spend more to get a safer product.

What have forum users tended to specify for their overhead glazing?

[Adsibob]

1 hour ago, Declan52 said:

Cover it with a clear plastic film. 

What sort of product do you have in mind? 

[MJNewton]

5 hours ago, hendriQ said:

The outer pane however will just be regular glass

 

Howcome? I don't recall ever seeing double glazed units that require toughened safety glass to only have it in one pane. Even aside from what any regs my say (I don't know what they do say) it doesn't make sense to me one pane would be left untreated. I certainly wouldn't have overhead glazing without safety glass - I'm surprised it's even available.

[Adsibob]

52 minutes ago, MJNewton said:

 

Howcome? I don't recall ever seeing double glazed units that require toughened safety glass to only have it in one pane. Even aside from what any regs my say (I don't know what they do say) it doesn't make sense to me one pane would be left untreated. I certainly wouldn't have overhead glazing without safety glass - I'm surprised it's even available.

Yes, I think you are right. This article suggests that for any rooflight which is less than 5m above the ground, both panes need to be either toughened or laminated: https://www.buildingproducts.co.uk/overhead-glazing-options

[Ian]

12 hours ago, hendriQ said:

Okay, but this isn't a commercial project. It's my house. But maybe that's all the more reason to spend more to get a safer product.

What have forum users tended to specify for their overhead glazing?

Yes, you're correct. Commercial projects almost always carry higher risks than domestic so it will depend on your attitude to risk whether or not you wish to exceed the minimum requirements of the Building Regs.

 

The relevant guidance is in BS 5516-2 and it takes a risk based approach. Below are some extracts relevant to your build:

 

 

[SuperPav]

I personally wouldn't want any roof lights that weren't laminated on the inner pane, having had a large pane of toughened glass shatter (from impact), while the risk of serious injury from toughened glass shards is very small, it is still highly unpleasant..

 

If you got one that is toughened on the inner pane, then as someone else suggested, I would get a clear film and stick it onto the inside surface - this will stop shards flying down if it does spontaneously shatter. 

[hendriQ]

Are the films easy to apply DIY? It's a rather large skylight, so might be tricky.

[Mr Punter]

20 minutes ago, hendriQ said:

Are the films easy to apply DIY? It's a rather large skylight, so might be tricky.

 

I doubt it will be much more expensive for the inner pane toughened and laminated and the outer pane toughened.

[Iceverge]

Have you run this past your SE re snow loading? 

[hendriQ]

6 minutes ago, Mr Punter said:

 

I doubt it will be much more expensive for the inner pane toughened and laminated and the outer pane toughened.

Quote I have for both panes to be toughened and heat soak tested is £3100. Quote for lamination as well on the bottom pane adds £850. Waiting for another quote from a different company, but this one includes installation which is handy because this skylight is a little awkward to fit given it weighs 170kg and is so long.

[markc]

7 minutes ago, hendriQ said:

Quote I have for both panes to be toughened and heat soak tested is £3100. Quote for lamination as well on the bottom pane adds £850. Waiting for another quote from a different company, but this one includes installation which is handy because this skylight is a little awkward to fit given it weighs 170kg and is so long.

850 extra seems expensive, bottom panels are often film coated as opposed to truly laminated (2x thinner panes with film sandwich). If they are just film covering single bottom pane then it’s a ripoff.

Edited November 17, 2021 by markc

[PeterW]

What thickness glass is this ..? Have you considered having this as 3 panels with slim glazing bars as it would be safer, cheaper and easier than one big panel. You’ll be picking the unit up with suction lifters so it will need a crane or handler to move this as a unit nearly 4m long will bow substantially. 

[Makeitstop]

That does sound ultra pricey tbf. 

 

Have to admit, I was kind of curious to know what this type of thing costs in total.

 

I'm not sure I'd feel there was a benefit in a single panel of glass at the size above, which is why.....  

 

I am looking at using Velux Modular skylights in a kitchen extension due to begin (relatively) shortly. I like the idea of the bank of glass at approx 3 metres wide and approx 1.2 metres or so deep, and also like the option of having an electric controlled vent for airflow when required. The separating bars on these is very slim and to my mind, doesn't really spoil the aesthetic at all.

 

I can't remotely imagine trying to install this thing at 170kg. Yikes...

Edited November 17, 2021 by Makeitstop

[JohnMo]

2 hours ago, PeterW said:

What thickness glass is this ..?

From the weight and area of glass, it should be two sheets of 10mm

[hendriQ]

7 hours ago, JohnMo said:

From the weight and area of glass, it should be two sheets of 10mm

It’s actually a bit lighter than I thought, because it’s made from 6mm glass. 

[tonyshouse]

When I did skylights I used laminated bottom pane, then two toughened panes, top one over-sailed the low edge by75mm to form a drip 

 

no failures apart from one on installation, laminated cracks very easily 

 

watch out for Newton’s rings and puddling on the glass if too flat 

[hendriQ]

On 15/11/2021 at 18:39, Declan52 said:

Cover it with a clear plastic film. If in the highly unlikely event that it shatters then it won't rain down on what's below.

Thinking this through a bit further, why is this the case? If I cover the inside of the glazing unit with film, the film will be stuck to the underside of the skylight. I can't see how that film is going to keep the failed glass from falling down. The film is just very thin plastic. Even a sheet that is about 2m²  is not going to weigh more than a couple of kilos. Compare that to the gravitational potential energy exerted by 140kg of glass, this film might actually make things worse. Instead of the glass falling as particles or 1cm cubes, all those pieces would be kept together by the film resulting in a heavy weight that would doubtless cause serious injury.

The skylight is about 2m above head height. So not a huge height to let it accumulate speed under the force of gravity, but enough that I would prefer it to fall as particles than as one large sheet.

Am I missing something here?

[dpmiller]

the very thin plastic is VERY strong.

[ToughButterCup]

On 15/11/2021 at 17:46, hendriQ said:

...

Do people worry about this sh!t, or am I being paranoid?

 

Yes. No.

Our large sheets of glass were the largest single item by value in the whole build. I wanted to fit and forget. I did fit them, and did forget them until I used the strimmer for the first time. Loads of loose MoT1 lying about.

 

Yes, it did throw up some stones, and

Yes, they did hit the window (2m by 3m).

Yes, I did stop. Sharpish.

Yes, I did check our buildings insurance.

But I can't remember what the outcome was. Man I wish I was 30 years younger.

 

Loads of strimming left undone. ?

[SteamyTea]

On 18/11/2021 at 06:15, tonyshouse said:

When I did skylights I used laminated bottom pane, then two toughened panes, top one over-sailed the low edge by75mm to form a drip 

The way I understand it is that toughened glass can take a greater impact before failure, then the laminated catches the pieces.