Floor construction in fireplace

[Andrew Gabriel]

I just unbricked a bricked-up fireplace, and intend to turn it into an alcove which could take a shelf or cupboard door on the front. The floor of the fireplace was just tiles on earth. Now I want to run whatever flooring (carpet or wood) I eventually use into this alcove, so I'm thinking of excavating this enough to pour in a solid floor, preferably with a dampproof layer and possibly with thermal insulation too (although we're only talking of an area of 2-3 square feet). One other factor is that I don't want to rule out the possibility of using it as a fireplace sometime in the future, so I guess a layer of expanded polystyrene under the screed is ruled out. Is there anything similar suitable for high temperatures?

Any suggestions on construction methods, including suggestions for the mix and depth of the floor?

[The Natural Philosopher]

You can use poly if you go down far enough. My fireplaces are laid on full plastic DPM all round and that hasn't failed yet.

I would guesstimate that going down about 6-9" and then rubble/concrete/DPM/insulation/concrete plus final surface is good enough.

I jhave notice with our open fires that heat travels upwards: The bricks at the fireback did get red hot, but the hearths are relatively cool, even when hot ashs falls on them. I'd say 3-4" of tiles and conctrete is fine with teh plastuic insulation.

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[Andy Wade]

How about a vermiculite mortar (as used for filling in behind firebacks)? You can still buy 'micafil' for this purpose, or at least you could in '96 when I did my fireplace. The mix is about 1:6 OPC:micafil (no sand), with just enough water to make it workable. I'd guess about a 150mm thickness would be OK, with (say) a 40-50mm sand & cement screed on top to get a smooth surface.