Sweating roof

[marsaday]

I have recently put in a velux window. The artificial ceiling is close to the velux and when i went to do the boarding up this week i found the ceiling very damp (the plasterboard basically broke away. So i hacked out all the damp stuff and then went up through the hatch to have a look at the roof space.

Well the timbers next to the chimney breast were very wet. I dont think the chimney is to blame her, but rather the method of roofing the house. It is slate, but underneath they have used polythene (for concrete floors) and then the space between the rafters has been filled with glass insulation.

All i have done is to remove the insulation from between these wet timbers and cut out the polythene sheeting. There is now a good airflow around this chimney area.

Will this be enough ?

 

[marsaday]

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In the attic space, you can see rotten lathe.

 

[marsaday]

The velux in relation to the wet roof which i have cut out.

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The roof void above the artificial ceiling. This main area is not sweating as much as around the chimney breast area because there is more air flow i think in this main area.

View media item 73381

 

[xr4x4]

I have no idea what you've done but by the look of those pictures, you haven't done it right.

 

[marsaday]

I have installed a velux and discovered the previous owner has roofed the house with slate, then plastic membrane, and insulated between the rafters at the same time with glass fibre insulation (not rigid board).

The roof is now sweating. The worst area is close to the new velux installed (above it in fact). so i have cut out the plastic under the slates here to let the roof breath.

Does anyone have any experience with a roof sweating like this.

 

[shane12345]

I've re-roofed so many that were done in the 70s/80s with that plastic/rubber felt.
Most were very condensated to the point it was making the rafters turn white.

Can't see much more of an answer than ripping it off.

 

[loply]

So what's happening here? is the condensation passing up through the ceiling/loft insulation and condensing on the inside of the roof, because the roof is covered in a non-breathable membrane?

And now the interior of said roof has been boarded and plastered?

Would insulating the inside of the roof help as it would make the surface warm and thus less liable to condense?

 

[marsaday]

Yes the air is passing up to the roof, which is original lathe and plaster, but a false ceiling has been put in which is insulated.

Interesting that a lot of these roofs were done like this. The rest of the roof is ok because of the design and so is breathing much better.

I am hoping that if i cut out the plastic in the danger area this will allow air flow to take place and so help with the problem.

I suppose it is wait and see and cross fingers.