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Hygroscopic salts - replastering advice please

Discussion in 'Plastering and Rendering' started by masmith79, 25 Sep 2007.

  1. masmith79

    masmith79

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    Hi,

    I'm after some advice regarding the replastering of damp areas due to hygroscopic salts.

    Background:

    About a year ago my girlfriend's house (Victorian - 1900ish) started showing damp patches on the internal walls. We contacted the council, who (after a number of builder's visiting us) advised us that we had raising damp in ALL of our walls and the walls needed to be injected and replastered.

    A few weeks ago, we decided not to wait for the council any more (we were applying for a grant to get the job done) and got a damp proof specialist in. He walked in the front door, smiled and told us instantly that we didn't have rising damp. He examined the walls and told us that damp proofing had already been done on the walls (not by him) and it was the hygroscopic salts penetrating the "new" plaster which was causing the damp.

    He advised us the best thing to do would be to remove the plaster at the damp places and re-plaster with a sand/cement/waterproofer mix. He did not charge for the visit!

    Advice needed:

    Ok, due to our relief (the cost wasn't going to be thousands!) and our anger (the council allowing contractors to rip them, and us, off) I never actually asked some simple questions: Simply put:

    1. What type of sand? (Building sand, sharp sand, etc?)
    2. Should we allow the wall to dry once the old plaster is removed before replastering?
    3. How long do we need to wait before putting paper on the wall again? (he said we could paint after a month)

    Also, any other advice would be helpful, thanks.
     
  2. noseall

    noseall

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    you will need to use an s.b.r. slurry mix application first.
     
  3. masmith79

    masmith79

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    Noseall, thankyou I will look into that further.

    After doing some research I have come up with the following process:

    1. Preperation
    Remove plaster up to a height 30cm (1ft) above visible dampness.
    Rake out mortar joints to 12mm (1/2") deep.

    2. First coat
    Mix 3 parts Grade 'M' (sharp, loam free) sand to one part cement with waterproofer (at suggested mix) using no more than 1litre of water for 6.25kg of dry mix.
    Compact into raked out joints and render to a thickness of 12mm (1/2").
    Key render.

    3. Second coat
    Repeat mix for first coat excluding waterproofer.
    Render to an additional thickness of 12mm (1/2").
    Key Render.

    4. Third coat
    Use a porous finishing plaster to finish the surface.

    Between each coat the render should be allowed to set a bit.

    Does this sound good, or am I missing something important? (ie the sbr slurry mix)
     
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