Well, just on the off chance someone knows about this, as I am in a bit of a quandry...

New fireplace is one of the impending jobs stopping me from spending time on the car...

Problem is the new hearth is larger than th existing concrete slab that the old hearth (originally tiled) sat on.

I've heard the if I just plonk the new and expensive marble hearth over it and the floorboards that it won't be supported properly and the cold air getting underneath with warm air on top might crack the marble.

I gather the best thing to do is bed it onto some cement/mortar type mix to try and seal the underside and some dollops to support it in the middle.

Problem is I'm not sure how to go about it.

Should I hack up the floor boards and pout cement/concrete down the sides to make the existing base bigger, sohuld I lay a thin concrete base over the top of the floorboards and hope to loose the extra inch with the carpet or try and seal the cracks in the floor boards with silicon/no nails or something else and do the best I can bedding it into cement to try and seal it/support it.

Anyone a fireplace fitter / done this before / got experience?
I'm buggered if I'll pay someone a silly amount of money to do it for me as I know it'll be a straight forward enough job when I know what I'm doing with it. But don't want to bugger the marble as it wasn't cheap (and the first one cracked before I even tried to install it, so it's already been replaced once [foc])

here's a pic to show what i'm on about (not to scale)

Image deleted by owner

Thanks,

Ned.

you might have to chisel?knock the original bed out , and from there see whats happening if you dont want it too high .

Laying a bed on floor boards will be ok ,as long as it isnt 3 inch high or something , if you can get it to sit flat then a highly adhesive sealant could be used , but dont kick yourself if it comes loose in years to come .

Im sure my old man fixes them down with something - its been a while since i was a lowly plumbers labourer !

Ned

Probably no help but what I did in a similar position (no concrete base in my case) was to lift the wooden floor stiffen between the joists (my old joists are flexible at the best of times) and lay concrete paving slabs directly on top of the joists. I tiled these and have had no problems what so ever.

Heat/cold shouldn't be a problem as there should be no great heat on the hearth surface (not near the edge anyway) my hearth never gets anything like hot to the touch.

If you were to just lift the boards and lay narrow pieces of flag around the edge of the existing concrete then set your marble on a thick bed of mortar I would have thought it would be OK. A thin paving flag should only leave around 10mm protruding.

Cheers

Phil

Hi Ned
Make a shutter about 1 inch less than the marble hearth. Some simple pieces of timber the same depth as the existing concrete will do.Mix some concrete or cement motar and fill to the same hight as the existing concrete hearth and leave to set. remove shutter after 24 hours or so. The 1inch less all round allows your carpet to go under the leading edge and is neater than having it on show. You can bed the new marble on tile adhesive spead evenly on the concrete.NOT IN BLOBS AS THIS WOULD LEAVE GAPS UNDER THE MARBLE WHICH WOULD BE THE WEAK POINT IF KIDS STAND ON IT.
Hope this helps. regards Kipper.

id silicone it. But then, id silicone most things...