Stove constructional hearths, PIR insulation and weight limits

[readiescards]

The building regs for a log burner hearth J, page 37, section 2.24b:

Constructional hearths should be made of solid, non-combustible material such as concrete or masonry, at least 125mm thick, including the thickness of af any non-combustible floor and/or decorative surface.

 

Is PIR foam 'solid, non-combustible' ?

 

 Sure I know it is not like concrete but my stove is stated as not heating the hearth beyond 100degC so the manual states a 12mm only hearth is needed (though I fear that is for a non-recessed fireplace according to BR) and I'll have a 55mm screed and a 70mm decorative slate hearth above the PIR foam anyway.  

 

Otherwise I'll need to remove 100mm of the 200mm sub-floor PIR foam, add in a 100mm constructional hearth, top that with 55mm screed and a Xmm decorative hearth with a 90kg (empty) stove on top - that is quite a lot of weight on 100mm of PIR foam - will 100mm PIR foam cope with all weight that? 

 

Paul

 

[ProDave]

As I understand it, if the stove manufacturer states it only needs a 12mm hearth, that is all you need to provide.  I see plenty installed on just a slab of stone sitting on the wooden floor. 
 

[ADLIan]

Pur/pir foams are NOT classed as non combustible.

[Crofter]

Bearing capacity for the foam should be available somewhere.

 

Incidentally, I was pondering something similar myself today. I'm going to make my hearth using cheap and cheerful slate floor tile- a single 60x60 on top, and then to build up the height I'll cut up some smaller cheaper ones and mortar them into a little U-shaped upstand. That leaves me wondering what to put in the middle, and I was actually thinking PIR would be just the job, easy to work with and it would get rid of my offcuts. This is to go above a chipboard suspended floor, so I don't want to just fill it with concrete. Fireproofing not an issue really as, like the OP, my stove can go on a 12mm hearth.

[Tennentslager]

I used a slate paving stone from B&Q, £6 each and about 30/40mm thick.

[Temp]

The regs say if the stove is certified "not to raise the hearth more than 100C" then you only need something 12mm thick but that 12mm must project above the surrounding floor, it cannot be flush with it. The idea is to stop rugs migrating too close. 

 

If using stone would make it thicker than 12mm (or well supported) because it will get stood on and it would be a shame to crack it. 

 

Our Clearview Solution 400 is sitting on a slab of black granite. I went to local monumental stone mason and he had a suitable offcut of kitchen worktop. He put it through his honing machine to take off the gloss. That's sitting on top of engineered wood, insulation and a beam & block floor.

 

 

 

 

 

 

Edited February 6, 2017 by Temp

[ProDave]

14 minutes ago, Temp said:

The regs say if the stove is certified "not to raise the hearth more than 100C" then you only need something 12mm thick but that 12mm must project above the surrounding floor, it cannot be flush with it. The idea is to stop rugs migrating too close.

 

Interesting.

 

Our present house has a constructional hearth (it didn't need it) and one of my design "features" was I didn't want a raised hearth, so the finished, tiled surface of the hearth is dead flush with the surrounding wooden floor.  BC never raised any issue with that.
 

[dpmiller]

does a constructional hearth need to be a poured slab, or could it be built up from the subfloor in block? Thinking about thermal block in particular, as it's rising through the floor insulation.

[Temp]

There are diagrams of constructional hearth in the Approved Documents. If you want to do it differently I would discuss with your BCO. 

[dpmiller]

just wondering if the collective have any thoughts on the implied meaning of the term "masonry" per the OP.

[Cpd]

10 hours ago, dpmiller said:

does a constructional hearth need to be a poured slab, or could it be built up from the subfloor in block? Thinking about thermal block in particular, as it's rising through the floor insulation.

I don’t know the regs but I expect you could justify using thermal block and then a slab of stone  on top of that. Seems a shame to not have the underside / sub layer as well insulated as possible. Just finished rebuilding two fireplaces and went with 150mm pir, lining the walls of the old fire place and then 100-150mm of block in front of that, was a shame I did not have the time to rip up the old hearth and rebuild it with some added insulation..... next time. 

[cwr]

On 07/06/2019 at 07:44, dpmiller said:

does a constructional hearth need to be a poured slab, or could it be built up from the subfloor in block? Thinking about thermal block in particular, as it's rising through the floor insulation.

How about foamglas, would that be acceptable?