Constructional hearth on beam and block

[Dave Anderson]

Has anyone built a constructional hearth on a beam and block floor? I'm struggling with this a bit so any pointers much appreciated. I'm finalising the construction drawings - works starting hopefully mid- March - it's a new build bungalow. We're wanting a wood burner in a recess (ie in a chimney breast).

My reading of the bdg regs is that the 12mm tiles option (if the appliance won't heat the hearth above 100 degrees) only applies if the appliance is not in a recess. Therefore I need a 125mm constructional hearth.

My floor is 65mm screed on 120mm PIR. I could I suppose build the hearth straight off the beam and block, but surely that would be a massive cold bridge. 

What do people do?

[nod]

Build directly onto the screed The BB will easily support a wood burner and hearth 

[Conor]

You could put down a sheet of 70mm compact foam on the beam floor, then ~115mm screed locally, then tiled finish. That's basically what I did as a base for my 300L HWC.

Edited February 13 by Conor

[Dave Anderson]

30 minutes ago, nod said:

Build directly onto the screed The BB will easily support a wood burner and hearth 

Thanks nod, but if the PIR is still there then the 125mm hearth has to have a 50mm air space below it, which would put the hearth surface 110mm above the floor.  We're looking for something lower than that.

32 minutes ago, Conor said:

You could put down a sheet of 70mm compact foam on the beam floor, then ~115mm screed locally, then tiled finish. That's basically what I did as a base for my 300L HWC.

Thanks Conor, what do you mean by compact foam? Is that a non-combustable material?

[Conor]

17 minutes ago, Dave Anderson said:

Thanks nod, but if the PIR is still there then the 125mm hearth has to have a 50mm air space below it, which would put the hearth surface 110mm above the floor.  We're looking for something lower than that.

Thanks Conor, what do you mean by compact foam? Is that a non-combustable material?

No, just a high density insualtion that will give you decent U value for the thickness and allow you to put loading directly on it. More used for door blocks etc. I'm not aware of the exact detail you need to build but sounds like under the 125mm build up needs to have an air gap?

[Dave Anderson]

Yes Conor, the top of a constructional hearth has to be at least 250mm above any combustible material, unless there is a 50mm air space between them. Seems to be very difficult to achieve where insulation is above the structural floor, unless the hearth is raised significantly above the floor level.

[Mr Punter]

You could use Rockwool floor insulation.  Not great insulation values but non combustible.

[dpmiller]

I built our hearths straight off the subfloor in thermal block.

[Dave Anderson]

13 minutes ago, Mr Punter said:

You could use Rockwool floor insulation.  Not great insulation values but non combustible.

Thanks for the idea. I suppose I'd be worried about rockwool providing stable support to the hearth and stove.

6 minutes ago, dpmiller said:

I built our hearths straight off the subfloor in thermal block.

Thanks, sounds like a good solution. I might have to check with my SAP assessor (or maybe best not to?)