Paint flaking off rendering & damp in cavity wall - help

Hi there - wondering if I could call upon your collective advice.

In September last year, we painted our extension, which is a 1980s flat-roof breeze block addition to an Edwardian brick property (NOT our doing!), with a reputable masonry paint. Prior to painting, the surface was brushed down and a stabilising solution applied (also manufactured by a reputable company).

In January of this year, we started to notice that the paint was bubbling and starting to fall off the wall. Additionally, there were little spots where rendering had been blown away (about size of 20 pence piece). There was also evidence of damp on rendering around the bottom of the back door frame, and on the brickwork at the base of the extension. This is at the point where the soil pipe enters the ground.

To add to our joy, internally, salt deposits began building up on the newly painted wall across the length of the lintel above the back door.

As our boiler had been slowly depressurising after an expansion vessel problem had been fixed - and because the bathroom is directly above the back door - I cut out a section of ceiling to see if there was a leak. No evidence. However, I was able to see that where a hole had been made to allow an extractor fan pipe to go through the cavity walls (directly above the lintel), water droplets were clearly visible on the external cavity wall (as were salt deposits).

Further investigation with a screwdriver stuck into the masonry outside revealed that a 3m wide (approx) section of the rendering from the very top of the extension all the way down the the brickwork base was damp and, in places, had the consistency of putty. However, outside of this band the rendering is bone dry (even in places where the paint is falling off.

Needless to say, we are extremely concerned and puzzled as to the cause. Suggestions have been made ranging from moisture build-up due to the use of the stabiliser, leaking soil pipe, through to a problem with the flat roof. In terms of the latter, I have checked this and there is no visible signs of cracking/damage, although I am not aware of it having been repaired or replaced in recent years.

Any thoughts from you good people out there as to the potential or most likely cause?

Appreciate any help you may be able to give - many thanks for your consideration.

 

It could be any of those really. Just do what you are doing and observe. Whichever it is it's pretty minor stuff and nothing to lose sleep over.

 

Can't add much apart from the use of stabiliser. If the surface was sound and not dusty and you apply stabiliser this will be like painting on glass and is a common cause of flaking/ bubbling masonry paint.. though i would'nt think it would blow render.

regards john

 

I've had an issue with blown rendering on exterior walls (and it wasn't too long applied), but I'm not a wall expert so can't advise you... but I used a quick question form on a page about rendering from www.avantcoatings.co.uk . They were pretty fast at replying and seemed to know their stuff.

Hope this helps... Good Luck

Bob

 

I've heard they are cowboys that spam forums.

 

Ahh, guess you've got a history with those guys then?

 

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