Thin coats of Hydraulic lime in chimney for a woodburner

[Saycheese5]

Hi,
I want to cover the brickwork on my fireplace recess and also on the face of the fireplace prior to installing a woodburning stove that will give off convected heat from both sides and the rear, radiant heat from the front. I am involving Building control who will check the work done. The problem I have is the width of the recess is currently 580mm and the width of the woodburner is 510mm. As gypsum plaster would crack over a temperature of 50 celcius, I am thinking of using hydraulic lime plaster but would like to know if I can get away by putting two thin scratch coats followed by a thin finishing coat on each side of the recess with a total thickness of 10mm (around 3mm thickness for each coat)? Or maybe two coats only, say 7mm for the scratch and 3mm for the finishing coat? Once installed, the woodburner will come forward, leaving a gap of around 250mm at the back but 25mm on each side. 25mm is a bit narrow but I’m trying to avoid making the coats thicker to reduce the gap even further. I’m not contemplating widening the recess, that would be too much work as it is solid brick. I would greatly appreciate your thoughts.
Many thanks.

 

[RedHerring2]

25mm gap is very small and less than BR allow. I think 150mm either side is minimum required.
Sorry, I don't know the effects of lime plaster in those confined spaces.
But if what I say is accurate, about 150mm clearance each side, I can't BCO accepting that much less.

Maybe Richard C will be along later to add his encyclopedia type links.

 

[Saycheese5]

Before my planned project I have spoken to the Building inspector who told me there needs to be a 150mm gap away from combustible materials, if not combustible then to follow the manufacturer's instructions. When I rung the manufacturer they recommend 80mm to allow enough air to go around the stove. However, due to the gap I will have being quite narrow, around 25mm, I will bring the woodburner forward by quite a bit allowing plenty of air to circulate at the back. In reality the 25mm gap on each side between woodburner and the sides of the wall recess will only be partial because most of the woodburner will be coming forward into the lounge. Thanks.

 

[RedHerring2]

OK, I have to accept what you say, but my information says 450mm sideways clearance for combustible materials and 150mm recommended sideways clearance for non-combustible surfaces.

If you're going for 25mm clearance I would expect that you're going to have to think long and hard about the composition of that surface.

 

[Saycheese5]

Hi RedHerring2 and thanks for your reply.

I have looked at the Building Regulation part J 2010 and section 2.28 reads "Combustible material placed on or beside a constructional hearth should not extend under a superimposed hearth by more than 25mm or to closer than 150mm measured horizontally to the appliance". Unless I'm missing something, my understanding is that this applies to the hearth. Diagrams 24 & 26 show that the gap between that appliance and the wall could be smaller than 150 mm, however the sides of the hearth between the appliance and the floor should be 150mm minimum. I'm also thinking about Fireplace inserts (or cassettes) where there is no gap at all (but of course a hearth in front). I know Vitcas sell some Heat proof render and plaster but it's really expensive.

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADJ_2010.pdf

 

[RedHerring2]

Diagrams 24 & 26 show that the gap between that appliance and the wall could be smaller than 150 mm,

Your query was about the space between the appliance and the side wall.
The hearth is under the fire so not relevant.

I agree that Diagram 26, top right corner allows for space less than 150mm " to a suitably heat resistant wall".

Paragraph 2.27 also states that dimension can be reduced to manufacturer's recommendations, so that's your answer.
Go by your manufacturer's recommendations, or ensure that you have a suitably heat resistant wall.

 

[Saycheese5]

My main problem is to find an adequate and reasoble material to cover the brickwork. I took the gypsum plaster off the fireplace recess and the face of the chimney breast because it's not heat resistant. I could apply sand and cement render but I think it would be more prone to cracking (hairlines) than hydraulic lime. I would like to try a hydraulic lime render since I heard it can expand and contract without cracking but I need to find out if I can apply 2 or 3 coats with a total thickness of 10mm? I have contacted twice a lime supplier who told me 10mm thickness isn’t really achievable but to apply 3 coats totalling 26mm, but I still did not get an answer as to why it has to be so thick when the internal plaster I took off my internal walls was only 10mm thick. Would someone please be able to give me an answer? Many thanks.

 

[mointainwalker]

I'm not sure it is the answer but could be the sand.

I use sand which is sold as 0-2 mm grain size but have found that the spec allows a significant percentage of grains larger than that. Can be 5-6 mm which obviously becomes your minimum coating thickness.

For a small area like you want, you could sieve the sand to ensure uniform particle size.

Why do want three coats ? It is not going to be very visible, so why not do one ?

 

[Saycheese5]

The hydraulic lime I would like to get is premixed with sand and lime and does not contain cement. I just need to add water. I haven't bought any yet. What i'm trying to understand is why the supplier told me I need three coats but could not give an explanation as to why it needs to be thick. So if I can apply two coats, I will. One for the base and the second as the finishing coat to look smooth especially on the face of the chimney breast which will be quite visible. Hydraulic lime seems to expand and contract more easily than sand and cement, therefore less cracks. I need to make sure I'm doing the right thing before going ahead, which is to have a total thickness of about 10mm which is the same as the plaster on my walls.

 

[stuart45]

The supplier has told you that because it was traditionally that thick to get the walls dead level.
For what you are doing, you could use non hydraulic hydrated lime/sand for the base and lime/silver sand for the finish and keep it to about 10mm.

 

[masona]

In reality the 25mm gap on each side between woodburner and the sides of the wall recess will only be partial because most of the woodburner will be coming forward into the lounge. Thanks.

Not sure if you can get away with 25mm gap with the heat against the walls but I don't know how much of the 25mm section will be. My feeling is that there will be a hot walls causing problems later? Not sure if the BCO will signed it off, best to check it first or how to overcome it

 

[Saycheese5]

Thanks for your replies.

Isn't hydrated lime more difficult to work with than hydraulic lime and takes longer to set (the advantage being a lot cheaper)? Could I apply two coats instead of three to make a total thickness of 10mm?

The gap of 25mm between the woodburner and the sides of the recess of the fireplace will go for 120mm, after that there will be more freedom with the remainder of 250mm coming forward in the lounge. The Building inspector seems hapy with that as long as the recess is not made of gypsum plaster (it will crack), he didn't advise me what to get, but I presume hydated lime/sand should do the job...

 

[Richard C]

Gypsum plaster is not suitable for high heat as you’ve established. You can use Vitcas heat proof render as a base coat but a sand/cement/lime render is just as good; apply in 2 coats, the final slightly weaker than the first. For finishing use Vitcas heat proof plaster in the area immediately around the fire.

Have a read of these links which should reveal all;//www.diynot.com/forums/viewtopic.php?t=183614
//www.diynot.com/forums/viewtopic.php?t=211524
//www.diynot.com/forums/viewtopic.php?t=242738
//www.diynot.com/forums/viewtopic.php?t=224751

 

[Saycheese5]

Thanks for those links Richard and Stuart 45 for the brief info about lime. Last time I looked at the Vitcas website for heat resistant plaster the price was a reduced 50 quid just for 20 kg (excluding delivery)! I've also heard that lime is relatively heat resistant (I hope) and I'm curious about trying rendering with lime/sand and finish with a lime/sand plaster finishing coat(probably with hydrated lime). I know it requires more work but I'll be interested to hear if any of you have done a similar project on a fireplace (I've tried to look at archives on this site). Could I also apply one base and one finishing coat (2 coats) like plastering as I'm trying to avoid applying also a scratch coat which I understand in more for external rendering? I'm trying to avoid learning the hard way

 

[stuart45]

I've also heard that lime is relatively heat resistant (I hope)
IT IS

Could I also apply one base and one finishing coat (2 coats) like plastering
YES YOU CAN

Not shouting at you, just want to make sure you see it.