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Damp on new plaster inside of chimney breast

Discussion in 'Plastering and Rendering' started by Tom.C, 30 Aug 2020.

  1. Tom.C

    Tom.C

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    Hi all,

    New to this forum, so I'm hoping this is the correct place for this subject.

    I approaching the end of a kitchen extension project, and I am having some damp issues inside the chimney breast which I was hoping to get a bit of advice on. The house is victorian, and I don't believe that this particular chimney has been in use as a working fireplace for many years. The chimney is located on a party wall which we share with our neighbour.

    During the build, the chimney breast has been opened as our cooker will be recessed inside. I was not aware of any of the original brickwork having any damp issues when this was opened up. The inside sides of the chimney breast were rendered by my builder and the rear side of the chimney breast had a course of brickwork installed to bring it forward slightly. My plasterer did a bonding coat onto the brickwork at the rear and the render on the sides, and then a plaster skim coat over the bonding.

    IMG_20200830_181539.jpg

    The skim coat was completed 6 weeks ago and the vast majority dried out fairly quickly but the bottom corners are still damp. There is also a crystallised salt build up on the surface where it is still damp.

    The plan is to tile the inside of the chimney breast, but this can obviously not be done until the wall is dry but I'm not sure if these damp areas are ever going to dry out.

    If anyone is able to provide any advice on what I can do then that would be much appreciated.
     
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  3. bennymultifinish

    bennymultifinish

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    difficult to say what’s happening without seeing the brickwork. looks like rising damp which is common in victorian fireplaces. i’m suprised your builder committed to it though without tanking and /or rendering with waterproofer.
    there’s likely no dpc in the brickwork so you’ll need to get it off and do it again.
    maybe think of installing a chemical dpc.
    i’d never use bonding coat in or around a fireplace as is pulls moisture in like a sponge.
     
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  4. ted456

    ted456

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    Tom.C,
    The wrong plaster has been used - it should have been rendered with 4:1 or 3:1 sand & lime render and a Limelite skim for all remedial work.
    What do you mean by "render"?

    What you show could be a mixture of rising damp and chemical reactions to soot - Hygroscopic salts attracting moisture.

    Please, Stand back & take a pic of the whole chimney breast and the back wall its on?


    Perhaps you've plastered up to a Stove steel Register plate in the flue - in which case you'd be well off hacking off back to brick all the plaster you show. Hack off up to the Register plate or flue opening/cbreast lintel. Then post another pic.
    Sooty brickwork may have been left in-situ, all sooty remains need wire brushing off.
    Old, soot contaminated bricks might have been used to "bring the back wall forward"?
    And maybe the flue was left unswept?
    Perhaps you'll need to hack off to 1m around the outside of the c/breast?

    Traditional c/b's often didn't have DPC's.

    What kind of board is on the back hearth "floor"?

    fwiw: the gas pipe needs clipping.

    fwiw: all chimney flues need sweeping and smoke testing - including blocked off flues that need opening up and then ventilating.
     
  5. JohnD

    JohnD

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    is there a concrete hearth beneath that floorboard? Underneath is likely to be builders rubble, which easily carries damp upwards. as Ted says, chimneybreasts often lack a dpc, which didn't matter much when there was a fire in the fireplace, and a chimney sucking air away and ventilating it.

    is the rest of the floor in that room concrete, or wooden boards with a ventilated void beneath?
     
  6. Tom.C

    Tom.C

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    Thanks for your responses, and sorry for my late reply.

    Here is a photo of the whole chimney breast.

    IMG_20200907_191504.jpg

    I've tried to find some earlier photos of the chimney whilst the works were taking place but don't have many decent ones unfortunately. I've included a couple below for reference.

    IMG_20200518_091704.jpg
    IMG_20200517_184052.jpg

    Regarding what is below the finished floor, there is a 50mm thick concrete screed slab which had a damp proof membrane installed prior to being poured (see above photo). Below the concrete slab for the rest of the floor is a suspended beam and block floor, but inside the chimney breast my builder just threw down a bit of concrete to bring the level up to the same as the top of the beam and block. From memory, there were concrete blocks below this.

    On the above photos you can see where my builder rendered on the sides of the chimney breast with a sand cement mix. The plasterer applied a bonding coat to the back prior to a skim coat. Does this bonding coat need to be removed up to where the damp is occurring?
     
  7. bennymultifinish

    bennymultifinish

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    chop the back off . inject a chemical dpc cream in the lowest mortar bed , tank the wall then render with sand cement with sbr.
    cant for the life understand why a builder would render the sides and not the back.
    bonding coat is totally the wrong option.
    or
    chop the back (bottom) off and leave it breathe.
    push your cooker in and forget about it.
     
  8. Tom.C

    Tom.C

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    Would the entire rear wall and inside side walls need to be tanked (i.e. all the way to the top)?

    If I just remove the plaster and bonding where it is still damp could I finish these gaps with a breathable plaster?

    Ultimately we want to tile the inside of the chimney breast. What should we do in this instance?

    Cheers
     
  9. bennymultifinish

    bennymultifinish

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    there are several bagged lime based plasters available . I don’t think its the answer if you intend tiling over it and sealing the moisture in behind the tile anyway.
    is the new existing render all the way up?
     
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  11. Tom.C

    Tom.C

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    I can't remember how far up the new render went to be honest. Does that affect how far up it would need to be tanked?
     
  12. JohnD

    JohnD

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    is it still looking just as wet? Or has it dried out at all?

    I suppose the water is creeping up the concrete and rubble under the old hearth position. Might there be an old water pipe under there?

    What's on the other side of the wall with the chimneybreast?
     
  13. Tom.C

    Tom.C

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    The other side of the rear wall is my neighbour's side.

    There is no water pipe under there.

    Doesn't look like it has dried out at all. If it has, then very minimal drying.
     
  14. Mr Chibs

    Mr Chibs

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    What make is your kitchen? is that a bamboo or oak worktop... looks good.
     
  15. Tom.C

    Tom.C

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    Thanks Mr Chibs! The worktop is bamboo. The units are from a company called Handmade Kitchens of Christchurch.
     
  16. Tom.C

    Tom.C

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    So I've just chopped off the plaster and bonding back to the brickwork where it is damp. See photos below.

    IMG_20200924_134803.jpg
    IMG_20200924_134822.jpg
    IMG_20200924_134830.jpg
     
  17. bennymultifinish

    bennymultifinish

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    you need to take it off 1 mtr up from the floor and chip and wire brush off the plaster on the sides up to 1 mtr also.
    inject a dpc cream into the lowest mortar bed. then fill all the loose brickwork and missing pointing in with sand and cement . tank the back wall and sides , render the back wall with sbr in your mix and skim it all in.
     
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