Ply wood delaminated on warm deck roof

[burts]

Hi all, I built a ground floor extension 3 years ago with a warm deck roof. It was 18mm OSB, vapour membrane lapped up at all sides of 100mm rigid insulation on top. Then 18mm ply and a 3 layer felt system.
I did everything bar the felting.

I whilst making a material list for a second storey extension I noticed the the felting was spongy all over the roof.
Started ripping the roof off today and found the top layer of t has completely delaminated all over ….. there was signs of sweating/condensation but don’t think there was a leak.

Vapour membrane dry and so was everything underneath it so the problems was between the ply and felt …..
What has likely caused this to go so quick ?

The 3 layers of felt were stuck together well but it came of the ply easy and there was lots of round circles (is this a breather membrane)

Anyone tell me what’s happened here?

Thanks

 

[omph]

water ingress ?

 

[^woody^]

What grade of ply?

If not the material, then it will be interstitial condensation if the issue has affected the whole roof area.

 

[burts]

water ingress ?

The felt all looks water tight and it was more sweating.

 

[burts]

What grade of ply?

If not the material, then it will be interstitial condensation if the issue has affected the whole roof area.

Hi woody, it wasn’t marine ply but it was definitely an exterior grade ply.

I’m fairly certain it wasn’t water ingress and did appear to sweating/condensation but what would cause that?

The first layer not being put down correctly?
The first layer was that with holes in this supposed to make it easier to rip up when replacement is needed but I had another builder tell me that causes problems?

Will be thankful for your advice.

 

[burts]

What grade of ply?

If not the material, then it will be interstitial condensation if the issue has affected the whole roof area.

I have just read up a little about interstitial condensation. It is over the entire roof so I’m guessing this is what it is.

What can cause this?

Here are the things I think I could have possibly done incorrect.
1. My vapour barrier sheet was in 2 sections due to size and the taping together had failed or was rubbish in general.
2. My insulation was only 100mm.
3. My insulation had some gaps that wasn’t filled.
4. The joins were not foil taped.

Will any or all of the above caused my problems.
First time I’d done anything like that and clearly may not have comprehended the importance of all the above

 

[Deluks]

Moisture trapped within the insulation layer during construction? Did it rain at all when you were carrying out this work?

Any downlights in the room below?

Damaged vapour barrier? When putting screws into polythene vb in the past, I have noticed a tendency for the screw to snatch the polythene and tear it, if firm pressure isn't applied to the board you are screwing through.

You may never find out.

 

[burts]

Moisture trapped within the insulation layer during construction? Did it rain at all when you were carrying out this work?

Any downlights in the room below?

Damaged vapour barrier? When putting screws into polythene vb in the past, I have noticed a tendency for the screw to snatch the polythene and tear it, if firm pressure isn't applied to the board you are screwing through.

You may never find out.

There are lots of down-lights below but they are very rarely all on.

I think it’s poor VP install by me, only used 100mm insulation and gaps wasn’t filled or tapped.

I’m a heating engineer …. Done a self build for the first time. All came out fine except this

 

[Deluks]

Well the rest of it hasn't fallen down, you live and learn!

Whether or not the downlights are on is irrelevant, it's the installation of the lights which is a risk to the vapour barrier above.

 

[burts]

Well the rest of it hasn't fallen down, you live and learn!

Whether or not the downlights are on is irrelevant, it's the installation of the lights which is a risk to the vapour barrier above.

The rest of the build is fine

I fail to see how installation of lights can affect a Vapor barrier that is about 250 above it with firrings and structural deck Inbetween

I assumed you meant heat

 

[Deluks]

Then the downlights won't affect it, if the VB is intact underneath then the moisture could only have got in from the top.

 

[burts]

Then the downlights won't affect it, if the VB is intact underneath then the moisture could only have got in from the top.

As in a leak?

The structural deck is completely fine so maybe through the top?

 

[^woody^]

Air is getting in, or got in through the un sealed spot light cut outs, or via a poorly fitted vapour check layer and condensed on the cold deck in the air voids.

A leak would be localised and noticeable.

 

[burts]

Air is getting in, or got in through the un sealed spot light cut outs, or via a poorly fitted vapour check layer and condensed on the cold deck in the air voids.

A leak would be localised and noticeable.

I agree a leak would be noticeable.

Surely Spotlight cutouts wouldn’t be sealed otherwise you could never change/maintain them.

I would go with i have most likely fitted the Vapor barrier poorly and gaps in the insulation……. Naively not knowing the importance but surely would have damaged the structural deck also.

Or is it that the hot air is rising through the VB then through the insulation gaps and condensing on the ply directly under the felt ?

Some questions moving forward
1. Can I use OSB 3 board on the top deck
2. Do I need to upgrade the 100mm to 125mm or is that not an issue
3. Are they special screws to use to fix top deck to structural deck that self seal is any ok

Thank you

 

[^woody^]

Surely Spotlight cutouts wouldn’t be sealed otherwise you could never change/maintain them.

Or is it that the hot air is rising

Fire rated or bathroom rated units are room sealed, and removable for service.

It's not a case of "hot air" rising, but merely the air from the room which may be warmer and more likely to condense, or more humid with similar outcome.