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New render with cracks

We have offered on a house - 1920's semi - which has been significantly extended in the last 6 months and has been fully rendered. However, the (new build) end wall - approx 10m by 5m - has a fine (1mm) crack running at approx 70 degrees from the eaves to a 1st floor window, from the bottom of that window to a top corner of a ground floor window and then to the footings. The wall also has very visible "patching" in the render where other cracks have been filled. The "tap" test reveals areas in the render which appear to have popped from the underlying blocks. There is also a similar crack below a window on the rear of the property - fine, 1mm only. The builder is insistent that this is only a cosmetic problem, is common in new render and that no remedial work is necessary.
The Building Inspector has indicated that the foundations are correct and substantial, (Good news!) and that such cracks are most probably due to expansion/contraction of the blockwork.
Any thoughts about the significance of such cracks or is this really to be expected in new render?

2 Answers from MyBuilder Plasterers

Best Answer

They probably used thermalite (grey) or Durox (white) blocks in the construction which in my opinion are useless to render over, I always insist in using expanding metal lath when rendering over these as they suck the life out of the mortar on contact which results in no adhesion,,if you can knock it and it sounds hollow thats the problem, render has divorced form the blockwork.....once it rains and water penetrates through the cracks it will then freeze and blow,,,get a plasterer to give you some sound advise and a quote to do it correctly and adjust your offer, do show the seller the quote and include redecoration.

The other possibility is the builder only used one coat of render rather than the correct two coat system, but even so by what you have described it wont be to long before it cracks in other places and starts falling off, I would say 99% if you started to hack it off it would come off in large sheets rather than small pieces.

I have seen this happen so many times, especially on new builds, in particular on new housing estates...

Hope the above helps.


Answered 7th Nov 2012

It is not to late yet when you see hairline cracks only it can be repair without hacking whole render off.let fix 20mm polystyrene boards in to adhesive and mechanical plate plugs which will keep render strong over existing render straight to bricks over boards fibre glass mesh and finish of with silicone coloured render.Job guaranteed .no more cracks and also save energy hide thermal bridges,so heat no escape through poor render.


Answered 19th Dec 2012

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