Understanding building regs about stoves (Scotland)

[ProDave]

I am starting work on the main living room and need to provision for the installation of a wood burning stove. I am having trouble understanding the seemingly conflicting building regs.

 

The house is being built in Scotland, and is based on the 2013 issue of the Scottish building regs as that is what was in force when out building warrant was submitted.  Stoves and flues come under section 3 of the Scottish regs and you can download it from here http://www.gov.scot/Topics/Built-Environment/Building/Building-standards/publications/pubtech/th2013domcomp

 

Sections  3.19 is what is giving me trouble.

 

It is a timber framed house. I do NOT have a constructional hearth. I could install one, but it would be a LOT of work.

 

Now, what I normally see, is a stone slab superimposed on the floor, and a stone slab up the wall behind the stove. That is what most people seem to do. But I am trying to make that fit with building regs.

 

3.19.1 starts by saying combustible material must be 200mm from a flue.

3.19.2 modifies that by saying a "system chimney" (i.e. insulated twin wall flue) only has to maintain the manufacturers stated distance from flamable material, usually about 50mm.

 

So the flue looks straightforward, un insulated plain flue sections >200mm from flamable material.

 

Now onto the stove itself.

 

3.19.5 talks about the hearth. It usefully gives the minimum size of hearth. Then starts to contradict

 

3.19.5 a A constructional hearth 125mm thick.   OR

3.19.5b b a 12mm thick superimposed hearth IF the manufacturer states the bottom of the stove will not exceed 100 degrees.

 

Then it starts to contradict and later says a superimposed heart must be at least 50mm thick AND positioned on a constructional hearth

 

There seems so be mo mention in building regs about the usual stone slab up the back of the stove.  It seems if I wanted to, I could just have a plasterboard wall and as long as the stove is 150mm away from it, that would be okay according to the regs.  I think in practice it would most certainly NOT be okay. I know the sides and back of a stove can get very hot indeed and I would say some protection IS needed.

 

So the first purpose of this thread is to determine just what I need to do to the floor to put a stove on it?

 

We have a timber floor (JJI joists) covered in OSB. There will then be 25nm battens to create a space for UFH pipes, then solid wood flooring probably 22mm thick.

 

So idea No 1 is leave that empty where the stove is going, lay the floor, then pour a 47mm thick slab of concrete level with the top of the floor. Then lay a superimposed hearth over that, overlapping the floor slightly.  That would be the best we can do, but depending which of the conflicting regs you read, it may or may not meet regs.

 

Any other ideas?

 

I am planning to use the Burley Springdale 3KW stove with ducted air intake. It has the option of an additional 170mm base, which I believe (I need to check) would meet the 100 degree max so would be okay for a superimposed hearth. So it all boils down to interpretation of just how thick a superimposed hearth has to be and upon what it has to sit.

 

 

 

[Nickfromwales]

Do you have a copy of the MI's that you can upload here for the Burley?

[ProDave]

http://burley.co.uk/wp-content/uploads/2016/08/9103-9112-Burley-Wood-Burner-Installation-Instructions-AUGUST-2016.pdf

 

Page 5 says max hearth temperature <100 degrees,  10cm between rear and combustible material, and 35cm from side to combustible material. minimum hearth thickness 12mm

 

So does that overrule what building regs says?

 

The 35cm from the side to combustible might be an issue. The stove will go at the end of a run of kitchen units and that large gap might limit things a bit.

[gravelld]

Do you also have to consider the ventilation requirements - you can get conflicting advice on this too.

[ProDave]

1 minute ago, gravelld said:

Do you also have to consider the ventilation requirements - you can get conflicting advice on this too.

That's not an issue:

 

a) under 5KW and there are no requirements to provide a specific means of air supply to a stove.

b) the Springdale has a ducted air kit, so we will be taking combustion air up from the ventilated void under the floor, which is allowed by building regs.

[joe90]

I think like so many other BRegs it might depend on your inspectors interpretation, can you not ask him to clarify what is ambiguous in print? After all it’s him that signs it off!

[Ian]

@ProDave

Not sure about Scotland but in England & Wales building Inspectors will accept a HETAS certificate as proof of compliance with the Building Regs. Your installer should be HETAS registered so, if the scheme is acceptable to your BCO, you would only need to reach agreement with the installer.

https://www.hetas.co.uk/consumer/certificate-of-compliance/

[joe90]

When I installed a stove for a customer it was much cheaper to do it through building regs and frankly the inspector was not aware of the regs, asked me if I new the regs?, have I done one before? And could I send a copy of all the forms to him. ?.

[ProDave]

Re Heatas.  I hate paying for what I can do.

 

I will install the flue, which is a long run and by far the most time consuming bit, and then see if the supplier will just come and stand the stove on the hearth, connect the flue to it, and give me a heatas certificate for a fair price..  if he won't then I will do that bit a well.

[Crofter]

Could you install the stove after everything is signed off?

 

Btw I've used cement board rather than pb behind my stove as it is rated as non combustible.

[ProDave]

Re cement board.

 

I have been thinking about that. Would it but up to adjacent sheets of plasterboard, and when skimmed be "invisible" or would there be issues with differential expansion and cracking along the joint?.

 

Would the plaster skim be liable to cracking and flaking off if exposed to heat?

 

I know (from the stove we have in the caravan) just how hot the stone slabs behind and alongside the stove get, I really have concerns that plaster on a cement board would withstand that, thought it would appear to meet regs.  That's why I am still thinking of a stone slab behind and to the side of the stove.  And if I do that, then ordinary skimmed plasterboard should be fine with the very much lesser heat from the flue.

 

A cheaper alternative might just be to tile behind and alongside the stove.

[jamieled]

I used cement board to protect the walls in our Caravan from the wee wood burner that we have.  Although it's not quite what your asking, I filled the gaps where the cement boards join with fire cement (fairly rough cut and join) and I can confirm that movement has resulted in the lot flaking off despite them being well screwed to the batons behind.  In retrospect fire cement was probably a bad idea as it needs to be fired to a certain temperature to cure and that was not going to happen in the joins.  Anyway, I'd share your concern that skim plaster wouldn't last long.

 

 

[PeterW]

We have used a cement board with skim and it will be fine up to the temps you are talking. 

 

[Crofter]

I've got the exact same stove as Dave is planning to use, and have a 600mm wide strip of cement board running up the wall behind it- the wall is plasterboard everywhere else. Plaster skim over the whole lot.

There has been some cracking but I put this down to movement in the floor joists, caused by the weight of the hearth. I had to build it up using paving slabs as I couldn't get breeze blocks locally. It's no worse than I have had at the corners of some windows and I'm not too worried about it.

[gravelld]

On 25/04/2018 at 11:41, ProDave said:

a) under 5KW and there are no requirements to provide a specific means of air supply to a stove.

b) the Springdale has a ducted air kit, so we will be taking combustion air up from the ventilated void under the floor, which is allowed by building regs.

 

Sorry - missed (a). But (b) can still be a difficult one to argue *if* you also have a ventilation system and it comes down to who you ask. Last sweep I was talking to about was saying a lot of HETAS peeps aren't happy about signing them off with ventilation systems, although BC tend to be more allowing.

[dpmiller]

Local HETAS installer/dealer flat-out refused to talk to me about anything less than a Dibt-approved unit if MVHR was involved.

[Triassic]

I used 10mm thick slate tiles behind mine,mthese were glued onto 15mm thick battens over plasterboard.

[ProDave]

I already spoke to BC on a previous site visit and they are happy with air drawn up from under the floor, as they were in my last house.

[Temp]

On ‎25‎/‎04‎/‎2018 at 09:25, ProDave said:

Now, what I normally see, is a stone slab superimposed on the floor, and a stone slab up the wall behind the stove. That is what most people seem to do. But I am trying to make that fit with building regs.

I had a quick look at the Scottish regs from your link and they look very similar to the regs in England.

 

As I understand it the hearth issue becomes clear if you start by dividing solid fuel appliances into one of two categories: 

 

1) Those that can raise the temperature of the hearth >100C 

 

If your WBS falls into this category then a constructional hearth is required. The Scottish regs describe various alternative constructional hearths but the key feature is a 125mm concrete slab ( see fig 3.43 and 3.48).

 

2) Those that are incapable of raising the hearth >100C.

 

If your WBS falls into the second category then you can get away with a 12mm thick layer/sheet of something non-combustible like glass, slate stone etc). This must be the right size and shape and form a 12mm step up to discourage rugs being put too close.  In the Scottish Regs this appears to be detailed in section 3.19.5b

 

Quote

3.19.5 Relationship of solid fuel appliance to combustible material 

a) a constructional hearth at least 125mm thick and with plan dimensions in accordance with the following sketches or

b) a free-standing, solid, non-combustible hearth at least 840 x 840mm minimum plan area and at least 12mm thick, provided the appliance will not cause the temperature of the top surface of the hearth on which it stands to be more than 100ºC.

 

 

See also figure 3.41 and 3.42 which specify the size of the hearth in relation to the foot print of the stove. eg it must extend 300mm out the front in case embers fall out when the door is opened. 

 

Edit: My stove has legs so I assumed it was incapable of raising the hearth over 100C. I used a 20mm sheet of granite from a stone mason on top of wood flooring over PIR insulation between joists. I went for 20mm because my stove is heavy and I was worried about the weight cracking it.

 

Edited May 2, 2018 by Temp

[Temp]

More..

 

I understand your confusion with figure 3.43 but my understanding is that only applies to stoves that can heat the hearth over 100c.

 

It looks like the regs were written at a time when everything needed a constructional hearth so all the sketches applied. Then some time later when stoves became popular they added paragraph b).

 

 

 

[Crofter]

On 25/04/2018 at 18:02, Crofter said:

I've got the exact same stove as Dave is planning to use, and have a 600mm wide strip of cement board running up the wall behind it- the wall is plasterboard everywhere else. Plaster skim over the whole lot.

There has been some cracking but I put this down to movement in the floor joists, caused by the weight of the hearth. I had to build it up using paving slabs as I couldn't get breeze blocks locally. It's no worse than I have had at the corners of some windows and I'm not too worried about it.

 

Just to follow this up:

I had a closer look at the wall behind the WBS yesterday, and there are hairline cracks following the join lines of the cement board immediately behind the stove. However these are actually in the paint, not the plaster itself. So now I'm wondering if it was to do with different drying rates on the various materials? It could be that the cement board and PB have different thermal expansion properties, but the whole thing is skimmed with plaster which has not itself cracked, so it's a bit of a puzzle. Anyway, it's hardly noticeable and not the end of the world to run a paintbrush over it. I'll see how long it takes for the cracks to reappear, if indeed they do.

[ProDave]

Another question for @Crofter or anyone else that has used the Burly Springdale stove with the ducted air intake.

 

I am worrying about the height to set the air intake.  It says 140mm above the floor. But then you have to add the as yet unknown thickness of the superimposed hearth.

 

Looking at the instructions, there seems to be a 40mm diameter spigot on the back of the plenum box and they supply an 80mm diameter galvanised air intake tube.

 

That raises 2 issues, one good, one bad.  The good issue being an 80mm tube fitting over a 40mm spigot will have a lot of wriggle room so my height does not need to be so precise.  The bad issue is how do they seal an 80mm pipe onto a 40mm spigot?

[Crofter]

The spigot on the back of mine was definitely an 80mm, same as the galv pipe. But mine is the through-the-wall kit, and I think yours is the through-the-floor?

[ProDave]

I will be using the through wall kit as well. I have built a bit of flat 100mm ducting recessed partly into the wall that terminates in the wall with a flat to round adaptor. I will have to make my own 100 to 80mm reducing plug for that.  My logic was it was better to do this and get (any) stone behind it drilled, than trying to get a hole drilled in the hearth slab, then lifting that down over the pipe.

 

Still not decided whether to use a big slab of stone, or tiles.  Tiles will no doubt be way way cheaper.

[ProDave]

Well we have a stove now.

 

Not what I expected, I was convinced we were going to buy the Springdale and yesterdays tour of all the stove shops was just to prove it was the only option. Not so.

 

SWMBO did not like the Springdale, it was "too traditional" She did like the look of the firebox, which is basically a Springdale wrapped in a more squared off box. But the trouble with that model is although they do a version on a base, it is a solid base with no log storage.

 

However we did find this:

 

 

It's a Mendip Sroves Churchill Log store version.  At a shade over 4Kw it's a larger stove than the Springdale but it does have the room sealed air intake at the bottom already built in as an 80mm diameter hose entry without having to buy a separate plenum box.  Unusually for the ducted air intake stoves it will also burn smokeless coal.

 

It's list price was £1067 and I would normally have rejected that on price, but this one was being offered as an ex display at a discount that made it not so much more than I was expecting to pay for the Springdale.

 

It will be some time before I actually install it.