Plastering the inside of a fireplace

Discussion in 'Builders' Talk' started by arkenm, Aug 10, 2018.

  1. arkenm

    arkenm New Member


    Currently renovating my home and am about to reinstall the log burner. The previous owners just lined the inside with ordinary plasterboard, I've just bought hardiebacker board as recommended to me. Does anybody have any experience in what sort of plaster I can use to finish this without it cracking?

    There will be fairly large gap compared to some around up to 120mm on each side and 100mm from the back of the log burner to all walls that will be finished.

    Appreciate any advice if somebody has had to tackle something similar.
  2. Wayners

    Wayners Well-Known Member

    I used cement based tile adhesive to stick board on then added a few screws to hold. Brick slips over the hardi backer board with same adhesive. Sand and cement ready mixed (just add water) and pointed brick slips. Was easy to do as brick slips brushed off clean which was a surprise. Guess you could just use a sand and cement render over the board but won't look great. Could use some heat resistant black paint I guess to cover the render and make it black
    Last edited: Aug 10, 2018
  3. Allsorts

    Allsorts Well-Known Member

    Did the previous occupants use the wood burner? And was there any damage to the existing p'board?

    If you actually touched these surfaces with your hand whilst the burner was running, you'd almost certainly find them quite comfortable to touch - ergo at a temp of - ooh - 50oC max?

    The hottest part will be the surface immediately above the burner - the front 'ceiling' of the fire recess where the main heat convects out. Even this is barely going to get to 80o-odd degrees - a bit 'ouchy' to touch, but nothing more.

    There's a constant flow of refreshed air going over these surfaces, and also radiated heat isn't that powerful.

    I built a complete 'decorative' chimney breast for a 5kW multi-fuel burner and simply used a double layer of p'board (just in case...) and a normal plaster skim over it (tho' I did use the 'proper' fireproof board for the back wall). Absolutely no issues at all, and the sides of the recess are barely warm in use. The top edge - where the heat flow comes out - does, of course, become quite 'warm' - you wouldn't want to keep your hand there for too long - but 'hand-hot' is actually only at around 60oC - this is nowhere near causing damage to p'baord or skim.
  4. ramseyman

    ramseyman Active Member

    I’ve found from bitter experience that plastering the inside of recesses for wood burners will inevitably lead to cracking. This occurred in a recess which in fact was a knock through between two rooms with plenty of air flow but the ‘walls’ at the side of that particular burner still got very hot- it had a clearance of about 150mm which is a bit tight. The only way I do this now is put a fire resistant board such as hardibacker and the only thing I stick on that is brick slips, not tiles so agree with Wayners.
    Allsorts likes this.
  5. Allsorts

    Allsorts Well-Known Member

    Fair do's, Ramseyman.

    This was for a small - <5kW - stove, and the surrounding gap was kept quite large. I'm not anticipating any issues as the sides only get warm, and the upper edge just 'hand-hot', which is of no concern to plasterboard or skim.

    The oak sleeper has shrunk around 8mm in thickness, tho'...

    SDC16959 - Copy.JPG
    ramseyman likes this.
  6. ramseyman

    ramseyman Active Member

    Pleased that all seems ok
    Allsorts likes this.
  7. arkenm

    arkenm New Member

    That's almost identical to what I'm doing as far as the oak sleeper too! Thanks for the help, did you ever consider using the heat resistant plaster? I've heard pretty mixed reviews.
  8. stuart44

    stuart44 Active Member

    You can get heat resistant plaster from Vitcas, but it's really expensive. When it comes to render or pointing lime seems to perform better than OPC.
  9. Allsorts

    Allsorts Well-Known Member

    I didn't know there was such a thing :)
  10. fillyboy

    fillyboy Well-Known Member

    Plenty about it on the plasterers forum,
    I've avoided it in the past because of the price and being notoriously difficult to get a good finish, the good finish is not so critical inside the fireplace but the face of the chimney breast is a bit of a focal point. I tend to use sand cement inside the fireplace and gypsum on the front face, but with higher output burners, I have occasionally had problems with cracking.
  11. stuart44

    stuart44 Active Member

    Have you tried a mix with more lime in it, such as 1/2/9 or 1/3/12 OPC/lime/sand? You mmay find the extra flexibility of the lime helps avoid any cracking.
  12. fillyboy

    fillyboy Well-Known Member

    Was going to try that next time.

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