Cavity Insullation wet in new extension top of walls

[arh]

I have recently had a ground extension built using a shared boundary cavity wall. This external wall has now become an internal wall on both properties. The wall extends a metre above my flat roof and it capped with concrete slabs. There appears to be water getting into the top of the internal walls in certain places as the paint has become bubbly. It appears that somehow the cavity insulation is getting wet. Builder appears to be confused about where the water is coming from.

My builder took all the caps off, layed down a DPM and replaced but problem is still present. He has also checked the lead flashing and sealed this but this did not help. Flat roof itself seems to be watertight. I do not understand where the water is coming from? The blockwork is rendered and painted. Could the water be penetrating this? Could the water be coming through the mortar in the end caps?

I've been reading about cavity trays and was wondering if these were installed above the lead flashing would the problem be solved?

Thanks ARH.

 

[^woody^]

A parapet wall should have a DPC beneath the copings, and also one at roof level in the form of a tray to direct water which may enter the cavity towards the outer skin and out via weep holes.

The coping should also have adequate overhangs and a drip to throw the rain away from the wall. The top of the copings should be angled too, and not just flat slabs.

Also there should be no render to the rear face of the parapet wall

 

[JohnD]

when you climb onto the flat roof in heavy rain, do you see any ponding?

 

[martyc]

Also there should be no render to the rear face of the parapet wall

Woody...whats the reason for this?...my parapet wall is a boundary and my neighbour is about to have it rendered!!

Marty

 

[^woody^]

Parapets should only be rendered on one side so as to allow any moisture to evaporate from the 'bare' side.

Render both sides could trap moisture within the wall

 

[arh]

The parapet wall is an existing boundary wall and is rendered on both sides. My extension has been added to this. The cappings are flat slabs and they do not overhang. There are also no cavity trays fitted.

There is no significant ponding on the roof.

I will speak to my builder, however, do both the cappings and the trays need to be fitted? Is there any other solution? Is it difficult to fit the trays to exisitng walls. Will this also prevent the water on my neighbours side? I'm not aware that he has any issues.

Thanks.