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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorrevor
    • CommentTimeFeb 28th 2018
     
    What is the best undercoat / base coat for plaster finish for 7N concrete blocks. Sand cement render or undercoat plaster.
    • CommentAuthordelprado
    • CommentTimeFeb 28th 2018
     
    Having used so much lime plaster on my renovation, i wouldnt dream of using sand and cement. Its just brittle and not very nice at the end of the day. You an persuade modern plasterers to use lime based materials by either buying some sort of traditional pre mix by a company like cornish lime, or slightly more modern like lime green or lime and cement mixes that baumit do.
    • CommentAuthortony
    • CommentTimeFeb 28th 2018
     
    Sand and cement for me with lime in it.
  1.  
    I don't know much about 7N blocks. If they 'suck' a lot you may have to watch the wetting-down. +1 for Baumit. Note that not all Baumit plasters are lime/cement mixes, AFAIK. I am pretty certain that the ones I use most often - RK 38 and RK70 - are pure lime, and they are *lovely* to use. The data sheets refer to 'additives to improve workability and adhesion' but don't state what they are. On the other hand MC55 (external render) does refer to a mix of white cement and lime, plus additives as above.
    • CommentAuthorMike1
    • CommentTimeFeb 28th 2018
     
    Personal preference, unless there are other factors such as cost involved - in which case sand and cement will be cheaper.
    • CommentAuthorrevor
    • CommentTimeMar 1st 2018
     
    Thank you. Lime plaster is not an option . Part of the renovation / extension is a stone farmhouse or what is left of it. Part way through lime plastering this bit, 5 visits lime putty coat still to go on in a few weeks time. Will have to wait about 6 months or more before walls can be painted. In total about 30 man days £1200 of materials and at £70 per sq M total is not cheap. Discussing the standard plastering job with my specialist lime plasterer, about which he admits to knowing very little, he suggested keeping to undercoat plaster as this would be more consistent than a cement render coat and that the render coat may not be at the correct strength for the blocks. The conventional plasterers do not mind what undercoat they use, choice is mine. The mix they would use is 1 cement 4 building sand 1 sharp sand which as Mike states is the cheaper option. .
  2.  
    Posted By: revorThe mix they would use is 1 cement 4 building sand 1 sharp sand which as Mike states is the cheaper option. .

    Over here they use lime putty in with the sand/cement render as a plasticiser.
    • CommentAuthorrevor
    • CommentTimeMar 1st 2018
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite>Over here they use lime putty in with the sand/cement render as a plasticiser</blockquote>
    That is interesting. will ask the lime plasterer about lime putty as a plasticiser. How much do you add?
    In Hungary do you do a considerable amount of rendering?. When I had the outside of the blockwork done with Alumasc render the contractor used 2 Hungarian plasterers for the finish coat. He claimed they were a lot better than the locals and he had used several of them on a big job lasting about 6 months. There seems to be a group of them based in the Manchester area and travel from there to all parts of the UK.
  3.  
    How much lime putty to use - it's a bit like cook until done - it depends upon the type of sand but generally it is about a bit more lime than cement as measured by the shovel into the cement mixer.

    There is no plastering done here - it is all render. The render is put on by the builder and the decorator then puts on a filler coat (much thinner than a skim coat) which is scraped on to fill out any granularity in the render surface to give a paint ready surface.

    Plaster board is taped and filled at the joints so no skim coat there either.
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