Ask a Tradesman
How can i tell if lime scratch coat really does contain lime
We are renovating an old house that back in 1904 would have been lime harled but was cement harled in the 70's. Where the harling is peeling off badly rather than patching with cement we are picking the whole face and then lime harling to allow the property to breath. A slater stonemason did the picking and scratch coat in November 2012 but didn't finish the job because winter was upon us (which is fair enough). I'm now getting it finished but have had several comments that the scratch coat "doesn't look like a lime scratch coat" - as it looks very grey and is quite hard / brittle. It is supposed to be a lime cement scratch coat rather than pure lime scratch coat. But how can I tell that lime was included? Is there a test that could be performed?
lime and cement do not go in the same sentence, their is no cement in lime ,mortar,render. the use of cement in any of the above will only lead to failure .
the fact it has turned grey tells me it is to strong for the wall and will not allow it to breathe.
suggest you find someone who is used to working with these materials.
while most builders merchants will sell lime it is hydrated lime not the type of lime required for rendering or pointing as it will not set without the aid of cement, it is generally used as a training product in bricklaying and stone-masonry schools as sand can be added and it can be re used again and again once cement is added it will harden to hard to allow your wall to breathe thus defeating the purpose it is required for.
the lime used in mortar and render is hydraulic lime,or non hydraulic lime which can only be purchased from specialist company this is mixed with sand and aggregate for render or pointing, non hydraulic lime is the usual lime used as it sets slowly, where as hydraulic lime will set a lot faster and is usually used for brick building.
Answered 13th May 2013
Hi, if the scratch coat is grey and brittle then it sounds like cement has been used. To allow the walls to breathe, only lime and sand should be used, not cement.
I don't where your property is, but any building merchant who sells lime will be able to send out a rep to confirm if cement has been used in the mix.
Hope this helps, Malcolm
Answered 18th May 2013
If it's grey it's more than likely a sand/ cement mortar. Possibly with a waterproofing additive, In a 1904 house chances are there is no cavity either, A lime render with sharp sand would be nearly white in colour. Take the scratch coat back to the brick and redo in lime. It sounds like your in Scotland by the way you describe the work. I'm not sure how exposed this work will be. Be careful using Hydraulic lime as an NHL 5 can set very hard, NHL 2 or a lime putty, based render with fibre reinforcement and a pozzolan would be better suited. Using a putty based render would take longer than a Hydraulic render to carbonate but better in my opinion
Answered 18th Mar 2017