New Render Cracked And Blown Off Fibolite Blocks

15 Jul 2008
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United Kingdom
Our new extension was constructed from Plasmor Fibolite's and rendered 1:1:6 (cement : lime : plastering sand and waterproofer). The scratch coat was applied two weeks prior to the top coat which was applied last Friday.

48 hours later, approximately 1/3rd of the total area has blown with long cracks showing up the worst blown areas.

What could have caused such a large failure? Is it the heat over the weekend? I thought Fibolite's were as good if not better than concrete blocks as a render substrate?

Now, should we hack off only those areas which have cracked and are obviously blown to the eye? OR should we also hack of patches of render which sound blown (i.e. echo when tapped with a hammer) amongst render which is solid? OR do we really need to take the lot off :eek:?
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Could be a number of things the blocks are lightweight which means if they wasent prepped properly they would have sucked the life out of the render also inconsistent mix could cause it and yes the heat may have caused it, not sure who told you lightweights are better than dense blocks but miles and miles of dense blocks are rendered with hard wall and render on interior and render on exterior with no problems I rendered dense blocks last year no probs what so ever still up not one crack I Used waterproofer for scratch 4-1 and 1-1-5 no waterproofer for topcoat
What's the best remedy for this. Are the currently 'sound' areas of render at risk of eventually debonding from the blocks and therefore everything needs to come off? Or can we just tap the wall and knock off those areas which sound hollow or have already cracked?

If there is little point in removing everything, will it be possible to blend any new patches of render to the 'old' in such a way that it will not be visible when painted?
Choice is yours the sound areas don't have to come off but you will have patches if you want a uniform job take the lot off
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Well things have got worse with more cracks appearing and some previously sound areas now also blowing. Looks like we need to start from scratch.

What is the correct 'prep' work prior to the scratch coat being applied and after the scratch coat but before the top coat?
Is not your plasterer/renderer going to redo it? Before scratch coat give the wall a thorough soaking if you want to be methodical rake the bed joints out by 10 mm if I were you before they lay the top coat on I would check that the scratch coat is nice and sound
Is there any need for PVA'ing the walls first to help control the suction?

How thick should the scratch coat be and should it be rock hard, just like the top coat? (i.e. the original scratch coat was VERY sandy, but that may be due to the 1:6 mix)
you could sbr it i suppose i would just soak it never use pva outside do a 4/1 mix for scratch, a scratch coat dosent have to be thick its used to dub out and provide a key

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