Is it ok to render 25mm thick over existing render?

Discussion in 'Plastering and Rendering' started by Ratty80, 15 Aug 2011.

  1. Ratty80

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    Dear all
    I would really appreciate some advice on plastering my chimney breast.

    I am fitting a built in cassette wood burning stove but my chimney is too shallow so I need to pack out 25mm on the front of the chimney breast.

    I am intending to use a sand cement and lime mix of the ratio of 5:1:1

    I am worried about weather this will stick to the front of the chimney breast as it is painted in emulsion. Currently there is a fire retarent render and skim on the chimney breast, which has been painted

    I was intending to scuff it up using a stanly blade and then coat in pva. Is this going to be enough to make it stick?

    The finished total Plaster on the chimney breast will be 40mm..is this going to be a problem?

    If no to the above then I'm guessing I need to apply the 25mm in two coats?

    Any advice at all would be a massive help:)
     
  2. roughcaster

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    I wouldn't chance putting 25mm of new render onto an existing painted, render and skim. You would need more than a stanley knife and pva to make an adequate key. You'd need to rough it up big time, either with a cutting disc, or go to town on it with a hammer and chisel,,, the rougher the better. It defininitely needs a mechanical key. Another way to provide that key if you dont want the mess, would be to fix EML to the chimney breast, then scratch coat,, (maybe twice),, and then render top coat. Either way will provide a good key.
     
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  3. Ratty80

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    Thanks roughcaster. I think I'll get the sds chisel on it.

    Just one other question how would u recommend finishing the edges...both corners of the chimney breast and the edges round the stove?

    I was thinking of screwing 2 pieces of wood on both sides of the chimney breast so they stick out 25mm. Then using a level or straight edge to shave of an excess.

    If I use the stop bead could I get this 25mm thick? And do I screw and plug. It in position?

    Thank u kindly for taking time to answer:)
     
  4. roughcaster

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    Are you rendering all of the chimney breast, (front and sides), as well as the internal opening where the woodburner will go?
     
  5. Ratty80

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    No just the front.

    It is a built in wood burner that sits inside an opening of 755x435 by 355 deep (currently the depth I have is 330, 25mm to shallow) so I'm not even doing the sides of the opening and the edge of the plaster around the opening will be covered by the flange of the wood burner.

    Thanks for your advice, hopefully makes sense if not I can send pics.
     
  6. roughcaster

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    I understand what you mean, but a pic would be useful Ratty..
     
  7. Ratty80

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  8. Ratty80

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    I'm thinking that maybe I don't need any beeds at all? Just the strips of wood at the sides to work to. I guess I can just brake off any excess plaster around opening as it won't show anyway?
     
  9. roughcaster

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    Cheers Ratty.
    If it was me, i'd chisel off some of the existing render on each corner, and fit a deep coat bead to each corner. I'd fit/dab on, each one so that it protruded 25mm from the front breast surface of the old render. I would dab them on with dabs of render. The bead would be set "flush" with the render surface on "either side" of the chimney breast. When the render dabs have hardened, you will be able to work off of the corner beads. Scratch coat the wall first, keeping below the surface of the beading. When you render top coat, you can render right out to the bead, gently using them as your guide. You will need to scratch/patch the render around each corner, using the spine of the bead and the original face of the render as your guide. This way will give you a much neater job in the long run, as you have set a corner bead "into" the render. I'd still advise to bring 25mm of render out in 3 coats of 7 or 8 mm, rather than 2 coats of 12mm or so,, (2 scratch and one top coat).

    I'm not sure whether you can get a 25mm stop bead,, although i know you can get a 20mm one,,, or you could go with the batten down each corner, then scratch and render as you said. With both of these ways, you'll have some work to do on each corner to disguise/feather the two "outside edges". I hope you can understand what i mean with this, if not, ask, and i'll explain some more.
     
  10. Ratty80

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    Thanks that is incredibly helpful!

    Just to clarify is that a deep cut stop bead I should use or r they different things? Do you think this will provide a better result than with battens? Just my only experience of using corner beads I found it really difficult to get to stick and get level.

    Also just one other thing if u wouldn't mind what lime do I need to use? I have bought 25kg bag of hydrated lime...though have just been reading posts referring to lime putty...will hydrated lime be ok?
     
  11. hotrod

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    Hate to pee on your bonfire ratty but I would check those clearances in the MIs (I've just done one and they needed app + 50mm). Of course if you have - apologises. If you decide to use gyplyner / supalux to frame out I'll add advice / pics.

    hth
     
  12. Ratty80

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    Thanks hotrod. The builders opening is according to manufacturer manual, it allows about 5mm all round but then design of it is to have an inner and outer box so I guess this must be an alternative system to the gap all round.

    I was very interested in doing with Superlux though reading through advice and looking up on google it appeared that this is gypsum based and was not really suitable for the continual extreme heat.

    Also I was struggling to think of a way of fixing it? Apparently dot n dab isn't good enough as the dabs can't withstand the heat. Battens was out of the question as wood is combustable material too close to the fire.

    I would be very interested to here the method as I'm slightly worried my rendering won't be up to scratch.
     
  13. joe-90

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    But was the pun intended? :p :p :p :p :p :p
     
  14. hotrod

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    Supalux isn't gypsum based, it's a calcium silicate board and ideal for applications where heat is a concern. I have used it in conjunction with the metal stud BG gyplyner system (Lafarge call theirs GTEC) so that gets round the combustibility / instability angle of timber. There's an album called 'Fire' on my profile with loads of pics of this system at each stage of construction. It's set to private so I've sent you a friends request so you can view.

    hth
     
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  15. Ratty80

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    Thanks very much hotrod. I think that is probably going to be the best way for me to do it.

    Did you sauce the materials locally? Is there a supplier you would recommend.

    Also what was the total packing out? 12mm for the board then how much for the steel studs?
     

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