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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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General: Sealing fireplace hearth (cob/concrete)

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    Hi,
    We've recently excavated an old stone/cob fireplace in our cottage which was hiding behind a thick 1960s concrete eyesore.. As it's an old cottage it doesn't have any kind of DPC and the construction is granite/earth on the lower half and cob on the upper half.

    Hence, the backs/sides of the fireplace are granite/earth, which I have lime rendered. The hearth base was just earth/stone, which I have (perhaps not wisely) laid a 2-3inch concrete bed on.

    We are now looking to lay some slate on this concrete but the concrete is damp and there seems to be dampness creeping up the plaster at the back/sides, up to 4-5inches high. I guess this is because basically its just earth underneath..

    I'm wondering what I can do about this?!

    Could I bitumen or damp proof the concrete base somehow, before laying the slates. Which may resolve the problem of a damp base, but if so, would i need to cut into the base of the sides to insert the DPC or something in order to stop the dampness rising up the sides?
    • CommentAuthortony
    • CommentTimeSep 5th 2007
     
    You could replace the concrete with limecrete?

    The slate will form a damp barrier itself

    In a fire place you should not need much in the way of damp proofing -- in any case do not use bitumenous stuff it will stink and may catch fire ,it also contains carcinogens.
  3.  
    Don't use a DPC with cob. The material is designed to be breathable on all sides, and sealing it is asking for trouble. The concrete will force damp through the cob so, as tony suggests, limecrete would have been a far better solution - or even a cob or lime-ash base with the slate simply bedded on top.
  4.  
    ok thanks for the replies guys. I think the best solution would be to replace the concrete base with limecrete, but I'm not sure how this would stop the damp soaking up the sides (I guess the answer to this is it doesn't matter that damp is soaking up into these, thats the point of it being breathable!)

    We're planning to put a large piece of Delabole slate in over the base, so maybe I can just get away without replacing with limecrete and just plonking the slate in and forgetting about it.. I guess it'll dry out when the woodburner is lit

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