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Plaster salt problem - wont dry

Discussion in 'Plastering and Rendering' started by FabricatingStuff, 2 Dec 2018.

  1. FabricatingStuff

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    I would really appreciate some advice about a problem with a re-skim.
    1930's house, there was a brick wall that had a small chimney (kitchen stove) on the other side of it. When the chimney was taken down I could see all of the tar and creosote
    on the bricks. It stunk too. I chipped it all off, even used a grinder to remove every last bit. This side of the wall is plaster boarded and I have no issue with it.
    The side that did not have the chimney was due a re-skim, some of the old plaster had come away at the top of the wall (in line with chimney) and I could smell the tar again. Stupidly, I didnt do anything about this, hoping that when it was patched and skimmed it would be fine. It has now been 3 weeks now and this bit of plaster just wont dry out. I had the whole hall stairs and landing done and everywhere else dried within 4 days. This patch is at the top of stairs, and now when we cook, (boil vegetables, rice etc) this patch gets more damp. Then it goes back to being a bit damp and never dries. (due to renovations I have no kitchen door at the moment).

    I decided to write this post, because a lot of other posts get confused by damp issues. This wall is an internal wall, and even though I knew it was not leaking, I have been in the loft, checked either side of the top of the wall, and it is bone dry. A lot of very dry dust can be disturbed when lifting insulation, no way there is a leak.

    I am pretty sure the plaster has got contaminated, I can also see oily blotches in a few places. What are my options please, If I stop cooking for a few weeks, will it dry for good? Does it need to be hacked off? What do I do to stop a repeat problem.
     

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  2. bobasd

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    so you hacked off an plasterboarded an now no problems?
    on the other side you left the old plster untouched but now salts are showing?

    sounds like the flue or flues in that chimney breast need sweepin an venting from bottom to top - i dont see any vent in the photo.
    what state is the chimney stack on the roof an the flaunching and terminals/pots?

    typicly, you will have to hack off back to brick an then render in a 3:1 sand an lime mix.
     
  3. FabricatingStuff

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    A few years ago the house was extended. The chimney was completely removed from top of roof, all the way to the down stairs room. There is no chimney, or breast. It used to be on what was an external wall. During my the extension this became an interior wall and has now been plaster boarded after I removed all of the tar my self. (if you are wondering it was a very small chimney about 14 inches square that came from the kitchen, so I only had to grind off the tar from a 14" wide strip up the wall). The wall is an internal wall now, it stops at the ceiling and I can go in the loft and look at the top of the wall. there is no damp and no leaks.

    I am asking about the other side of the wall....

    It was the interior of the same wall that I think was a render rather than plaster. I had the wall re-skimmed. There was a bit of render that had fallen off - we were back to bricks where it fell off, in line with the chimney. The bare bricks were patched with Bonding and then skimmed. The damp patch marks in the photo are where this happened (around the spot so to speak). I think the tar has leached through the mortar, and in the photo you can see a 'S' shape which is a mortar joint.

    Interestingly, the damp diagonal line in the photo on the very left is not inline with the chimney, but was also back to bare bricks and had bonding at the same time, surprised that this is contaminated too. I do know that a long time ago, when the chimney was there there was a leak so I expect that the water ingress has carried the chemicals through the mortar and render?


    Please can someone explain to me what I need to do to put this right?

    If it was hacked off, is there some kind of sealer that could be applied to the bricks so it can be plastered, or does it need this lime mix?
     
  4. cdbe

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    I had exactly the same issue when I removed my chimney breast. It's tar. Fortunately I knew it would happen so I did a few thin layers of SBR/cement slurry and render (I think it was 3) until it was dry, then skimmed it. If you can't do that then hack off the plaster and stick some foil backed plasterboard over it and skim. The photo is of the tar showing through the first coat. From this I've learned that the tar soaks more into the joints so worth raking them out a bit as well.
    IMG_20180425_095705696.jpg
     
  5. bobasd

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    OP, your not certain you only think it was render?
    you might also have used a gypsum skim which is wrong. so is bonding. i suspect you used all gypsum plaster?
    you should render with two coats of lime render an a remedial finish eg limlite an use plastic beads.

    all soot an tar an signs of contamination has to be removed.you've obviusly not done enough.
    i cant understand the bend - is that the backwall of your old flue?
    why not post photos of that whole wall an the same position from the other side of the wall?
     
  6. bobasd

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    cdbe,
    cant understand what you did, here's how it should be done:

    totally sweep flue then wire brush clean an grind off all soot an tar, if v. bad hack out all beds an perps.
    blind the brickwork with two coats of SBR, second one tackty.
    then two 10mm coats of 3:1 sand an lime render, an a limelite skim finish.
     
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