The purpose of a scratch coat

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jack88

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wondering what the different views are on this subject as i've heard so many different reasons...firstly what are the purposes and necessities of a scratch coat....secondly what are the advantages of having the 2 coats over just 1 if the wall is straight, has key and has suction??
 
Whatever the situation renders go on 15 mm plus you wouldn't be able to do this in one pass unless you're spraying it so we scratch and float
 
For conventional sand and cement rendering you will have too much suction to control getting it flat. A scratch coat you can add, take off and scrape to get it flat and true. Scratch to create a mechanical key. When you apply the top coat you can have controlled suction and usually only have to apply an even thickness render coat.

The one coat renders generally go on thicker, probably have water retainers and go on dense backgrounds like 7.5 newton blocks. Walls with higher suction tend to need a pre-rend.
 
and just to add to Mr Rigsby's post (who is a very knowledgeable spread) that a robust render needs to be 15-18mm thick and this can not be achieved in one coat by traditional means, because of the reasons John mentioned, but also because the weight of the render cannot be applied in one coat to be able to withstand the shear and pull out forces acting upon it , therefore a scratch coat is applied and allowed to cure thus allowing the top coat or coats be applied without the risk of failure
 
from what I know about sand and cement render, the 2 coats are different strengths - they have different amounts of sand in them. The top coat has far more sand in it so as to avoid cracks, it's more flexible if you like.
 
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