MortarThePoint Posted August 21, 2021 Share Posted August 21, 2021 We've two fireplaces to receive stoves (WBS). Screed going in very soon and finalising UFH in the rooms. Am I right in assuming Cemfloor will be fine here? There will be an additional hearth material laid on top (e.g. 75mm stone).. I have standard expansion perimeter strip elsewhere, but what should I use in the fireplace? It needs to be noncombustible for around the fireplace. Link to comment Share on other sites More sharing options...
Temp Posted August 21, 2021 Share Posted August 21, 2021 (edited) The Approved Document describes two types of stove, those that are/aren't certified that they won't raise the hearth over 100c. This determines the total thickness of the hearth needed. From memory it's 12mm or 225mm. So check that first. Edited August 21, 2021 by Temp Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 5 minutes ago, Temp said: The Approved Document describes two types of stove, those that are/aren't certified that they won't raise the hearth over 100c. This determines the total thickness of the hearth needed. From memory it's 12mm or 225mm. So check that first. The depth should be fine, it's round the edge between wall and floor I'm concerned about. I've seen some links that say 125mm depth for constructional hearth and I'll have that. One WBS will definitely be 12mm compatible but not certain of the other. Link to comment Share on other sites More sharing options...
Temp Posted August 21, 2021 Share Posted August 21, 2021 I would suggest no perimeter insulation on the one that needs the Constructional Hearth. Should be ok on the other one. We have regular wood skirting around the inside of our recess. Think I would put perimeter insulation around before screeding, trim it back flush and cover with the stone. Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 16 minutes ago, Temp said: I would suggest no perimeter insulation on the one that needs the Constructional Hearth. I think I need something there for expansion Link to comment Share on other sites More sharing options...
Temp Posted August 21, 2021 Share Posted August 21, 2021 1 minute ago, MortarThePoint said: I think I need something there for expansion Air gap? Link to comment Share on other sites More sharing options...
PeterW Posted August 21, 2021 Share Posted August 21, 2021 9 minutes ago, MortarThePoint said: I think I need something there for expansion Why..?? Coefficient of expansion on concrete is around 10x10-6m/°C so if you heated up a 1m concrete slab uniformly from 15°C to 80°C then it would expand around 0.5mm. Expansion foam is there for a number of reasons, usually to de-bond the slab from the structure and also so as it dries and contracts you don’t get gaps. A 15mm expansion foam will also be pretty much covered when you have boarded and skimmed the wall behind - beware that ordinary plasterboard is not suitable for behind a stove - and the remainder of any foam will be below your hearth anyway so is not subject to any sort of temperature that would cause issues. Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 37 minutes ago, PeterW said: Why..?? Coefficient of expansion on concrete is around 10x10-6m/°C so if you heated up a 1m concrete slab uniformly from 15°C to 80°C then it would expand around 0.5mm. Expansion foam is there for a number of reasons, usually to de-bond the slab from the structure and also so as it dries and contracts you don’t get gaps. A 15mm expansion foam will also be pretty much covered when you have boarded and skimmed the wall behind - beware that ordinary plasterboard is not suitable for behind a stove - and the remainder of any foam will be below your hearth anyway so is not subject to any sort of temperature that would cause issues. Back wall is brick so not getting plasterboard. I'm supposed to have something between the screed and wall to cope with 'expansion'. Sure it doesn't need to be thick, but should be noncombustible which the foam isn't. Is there something thin and compressible that is? 52 minutes ago, Temp said: Air gap? I thought of trying to creat this with wood sheet. 'oilboard' could be an option as has a smooth face, but I have to be certain I can get it out afterwards so I'd rather something noncombustible I can leave in. Link to comment Share on other sites More sharing options...
PeterW Posted August 21, 2021 Share Posted August 21, 2021 7 minutes ago, MortarThePoint said: but should be noncombustible which the foam isn't. Is there something thin and compressible that is? why ..?? Point me at the BRegs where it states that a below slab expansion material behind a structural slab required to be non-combustible and I’ll find you the product. Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 2 minutes ago, PeterW said: why ..?? Point me at the BRegs where it states that a below slab expansion material behind a structural slab required to be non-combustible and I’ll find you the product. It's not fully below slab, but comes to the surface. It will then have a noncombustible 75mm over the top. Looks like Knauf do mineral wool perimeter insulation so that could work but it expect the screed would soak in. Link to comment Share on other sites More sharing options...
PeterW Posted August 21, 2021 Share Posted August 21, 2021 6 minutes ago, MortarThePoint said: It's not fully below slab, but comes to the surface. It will then have a noncombustible 75mm over the top. Looks like Knauf do mineral wool perimeter insulation so that could work but it expect the screed would soak in. So it doesn’t need to be non-combustible, you’ve answered your own question. Use the stuff that everyone else uses, move on .. Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 1 hour ago, PeterW said: So it doesn’t need to be non-combustible, you’ve answered your own question. Use the stuff that everyone else uses, move on .. 75mm < required 125mm minimum noncombustible depth Link to comment Share on other sites More sharing options...
PeterW Posted August 21, 2021 Share Posted August 21, 2021 ..??? This isn’t a constructional hearth ..?? Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 7 minutes ago, PeterW said: ..??? This isn’t a constructional hearth ..?? I'm trying to make it one. The other WBS can have 12mm but one unsure so want constructional hearth Link to comment Share on other sites More sharing options...
dpmiller Posted August 21, 2021 Share Posted August 21, 2021 I went the constructional hearth route, but built off the subfloor with thermal blocks to minimise thermal bridging. The edge foam for the screed went around this. Link to comment Share on other sites More sharing options...
MortarThePoint Posted August 21, 2021 Author Share Posted August 21, 2021 (edited) 40 minutes ago, dpmiller said: I went the constructional hearth route, but built off the subfloor with thermal blocks to minimise thermal bridging. The edge foam for the screed went around this. So the edge foam was outside the constructional hearth area? I have a inglenook so think anything inside that counts as the constructional hearth area Edited August 21, 2021 by MortarThePoint Link to comment Share on other sites More sharing options...
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