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Thechnical issue regarding plaster.

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Samsaranda22/03/2018 14:59:18
1408 forum posts
5 photos

My enquiry may seem rather strange on a model engineering website but it is probably no more strange than other subjects that I have encountered on this forum. My problem relates to the wall plaster on my lounge chimney breast above an inset log burning stove, we had the stove installed about 18 months ago, after a few weeks of use we noticed that the plaster above the stove had cracked, obviously heat had caused this. The stove is not a particularly high output, rated at 5 kw max. Last summer we engaged a plasterer to repair the damaged area, he stripped the affected area back to bare brick, built up with appropriate undercoat and then skimmed the top surface with finishing plaster but incorporated lengths of scrim tape within it to reinforce and stabilise it. We didn’t use the fire for a number of months which would have allowed the plaster to thoroughly dry out, when we did start to use the fire the cracking reappeared, only more extensive than before. As you will appreciate I am under pressure on the domestic front to resolve the situation with smooth plaster which doesn’t crack. Is there somewhere a magic plaster that can be used in these circumstances to remain intact in spite of the thermal stresses, perhaps someone on this forum has experienced the same and found a solution and would like to share the remedy. I have heard mention of “ heat proof plaster” but there doesn’t appear to be any information about it or anywhere that it is available, if it exists. Can anyone help please?

Dave W

Steve Skelton 122/03/2018 15:19:08
140 forum posts
6 photos

Hi Dave, how big is this area ?- If it is small enough then just buy an 8 x 4 sheet of plasterboard and carefully cut it to the size and fix on dabs with a suitable filler around the perimeter.

Cheers Steve

Clive Brown 122/03/2018 15:23:38
807 forum posts
39 photos

I'm no expert but I'd be surprised if it's heat. Our last house had 2 fireplaces with roaring coal fires at times, the plastered brick chimney-breasts getting distinctly warm, but showing no sign of distress. Is there any sign of movement in the underlying brickwork?

Could you consider dry-lining with some type of plasterboard?

richardandtracy22/03/2018 15:34:04
943 forum posts
10 photos

We have a cast steel repro Victorian fireplace **LINK** in our living room. It's set in the plaster, and because it's steel, the thing expands more than standard cast iron and gets hot all over. The net result is cracks, as described, in the chimney breast. After getting a plasterer in to fix it, and the same thing happening again, we bought a big picture to go above the mantelpiece. Cheaper than a second go with the plasterer. A dot & dab plasterboard sheet is a good alternative.



not done it yet22/03/2018 15:39:45
6749 forum posts
20 photos

I would be starting with the wood burner installation, not just assuming it is a plaster fault. Was a SS insulated flue liner part of the installation?

Samsaranda22/03/2018 16:55:29
1408 forum posts
5 photos

NDIY, installation carried out by registered HEATAS engineer and SS flue liner fitted, all appears to conform to required regulations.

Richard, with the installation that we have the body of the fire fits into the brickwork of the chimney and is therefore not in contact with the plaster although heat does eventually get conducted to the plaster. It would appear that heat is the problem as all the cracking happens above the fire, I.e. the area which is hottest.

Steve, plasterboard may be a solution but not sure about flammable substance that close to heat source, I seem to remember that there are specific regs about plasterboard in close proximity with a heat source such as a wood burner, will have to research that.

Dave W

Bazyle22/03/2018 17:24:24
6301 forum posts
222 photos

You can get versions of plasterboard that does not have the paper, more like a cement board, for proximity to fire. It may be the bricks are moving underneath, perhaps due to a steel reinforcing rod buried within.

James Alford22/03/2018 17:34:01
463 forum posts
79 photos
Not sure whether it helps, but when we had our log burner installed, the fitter used a lime based plaster to avoid cracking caused by heat. He explained why it helps, but I cannot recall what it was.

James Alford22/03/2018 17:37:31
463 forum posts
79 photos
Deleted. Duplicate posr.

Edited By James Alford on 22/03/2018 17:38:30

Clive Foster22/03/2018 17:37:48
3105 forum posts
107 photos

Various fireproof versions of plasterboard around. When I had my attached garage re-built with rooms above the building inspector said we needed to use some pink fire resistant stuff for the garage ceiling. Possibly this :- **LINK** which can be got in a smaller sheet than the standard 8 x 4. Jewson, and presumably other builders merchants, sell various other types :- **LINK** .

Even ordinary plasterboard has some degree of heat and fire resistance.

Maybe space the plasterboard away from the chimney breast a bit with thin battens. Even 1/2" should be enough to break the bond between chimney breast and plasterboard sufficiently to stop the cracks. Double layer of plasterboard should work about as well.


Steve Skelton 122/03/2018 17:39:00
140 forum posts
6 photos

Dave, you can use the pink Gyproc FireLine board designed specifically for this sort of thing - to quote BG

"Gypsum plasterboard with glass fibre and additional additives to increase the fire protection characteristics with pink paper lining for easy recognition. The additional fire protection properties above standard board products enable the plasterboard to be used in partition, ceiling and steel encasement systems where more stringent fire performance is required such as in domestic separating walls, corridors, garages and steel encasement. "


NJH22/03/2018 17:40:58
2314 forum posts
139 photos

I had a flue liner ( flexible metal tube) fitted in my chimney then, via a closure plate, into the back of the woodburner. That seems to work well ....... but then we fitted an Air Source Heat Pump ( to replace the existing oil fired C/ H) and since then we’ve only had a couple of fires. Still have a shed full of logs though - out in the sticks here so power cuts can be more frequent ( not recently thankfully!) . Logs are thus a nice insurance policy! The only downside is that, with all the insulation etc. , the wood burner soon makes it VERY warm and we end up in hot summer clothing in the middle of winter! ( bit of a shock if more logs need fetching from the wood shed!)

So Dave I think the answer to your problem is to fit a flue liner.


Edited By NJH on 22/03/2018 17:45:52

Brian Sweeting22/03/2018 18:24:05
453 forum posts
1 photos

I would recall the installers of the stove.

Get the plate at the bottom of the chimney removed for a visual inspection of the installation.

Is it a freestanding stove fitted into the structural opening or a stove that is built into the brickwork?


Edited By Brian Sweeting on 22/03/2018 18:27:17

John Paton 122/03/2018 19:02:17
324 forum posts
20 photos

You really shouldn't need to use a board product for this unless there os a problem behind that you don't want to fix.

It is not easy to diagnose the problem remotely but two things I would want to eliminate form enquiries as a starting point.

1. Is the undercoat plaster also cracked and what pattern is the cracking? If it is and if the cracking is a straight line or zig zag. If so then suspect that the masonry behind the plaster is cracked and able to move with changes in temperature. If it is the real answer is either to cut out the cracked bricks and replace (ideally) or maybe to cut lines across the crack and insert stainless steel reinforcement fabric bonded into the masonry to pin the two sides together. If there is structural cracking behind even a board product is likely to show cracking on at least one edge.

If you use a flexible mesh lathing and lime based mortar there might be sufficient flexibilty to accommodate the background movement.

2. If the cracking does not go through the plaster undercoat and maybe if it is cracking with a crazed pattern it would suggest an inherent problem with the finishing coat plaster which has failed to fully bond to the undercoat (can be a number of reasons for this and the inclusion of scrim could well be part of it as it can reduce the actual thickness of the finish plaster and also cause excessively fast drying. Once the plaster is dis-bonded, the minor expansion and contraction with heat will cause it to reveal in the warmer parts first. The test for this disbonding is to tap the surface with a hard item like a coin edge (a bit like John Cleese did in Fawlty Towers!)

It should be borne in mind that woodburners operate very differently from open fires in that the flue temperature is very much hotter when the burner is running hard. this is because you have much less air mixed in to dilute and cool the flue gases. It is not uncommon for the first 1.5metres of flue to reach red heat if the fire is really roared up! This concentrated heat could be reaching the chimney breast if the flue liner is not properly centred within the flue and this can occur where the flue goes round the chimney throat above the old fireplace (the flue would have been designed to restrict and change angle at that point to help induce 'venturi' type draw in the open fire. That change in direction of the flue can cause the flue liner to go from one side of the flue to the other as it needs an 'easy' bend and cannot exactly follow angular changes in the masonry flue.

You don't say the age of the building and its history, but buildings in which sulphur rich coal (or oil) have been burned and where there has also been dampness at some time (often where the chimney flue crosses sideways within a wall as rainwater dropping down the chimney lands there), can cause sulphur attack of the masonry mortar which becomes soft and swells when damp. The crystals which form and cause this will change size when they dry out and so movement occurs when the chimney is brought into use (becoming warm and dry) and again as it cools down and becomes damper in summer months. This effect does not usually manifest at low levels in buildings except in very old buildings - usually associated also with brown staining coming through the masonry.

The fact that your plasterer chose to incorporate scrim makes me think he knew there was some sort of problem with the background masonry and did his best to overcome the problem with a cheap solution (rathe than suck his teeth and say 'you will need to get a bricklayer in before I can replaster that chimney'

So, I recommend a bit of detective work to see just what is going on under the plaster!

Sam Longley 122/03/2018 19:08:57
939 forum posts
34 photos

If your wood burner projects from the fireplace a bit then the problem could simply be the heat from hot air rising up the face of the wall. Plasterboard with a skim coat of plaster is no good as the skim coat will just crack. Plaster on brickwork will do the same even if it is bonded to stainless steel lathing.

There are a couple of options.

One is to remove a 4ft wide section above the fire ( assuming that is the width of cracking) & place Fermacell board on some fire resisting expanding foam. Support it in position until the foam goes off then plug & screw the board to the wall. the board, being 2400 * 1200 * 9mm th will be storey height & should not expand & contract too much. Feather the vertical edges & tape & joint this to the surrounding plaster . You could cover the joint with a bead like a vertical dado but this would look a bit "pants". Emulsion over the board after sealing it with an approved sealer first.

The other option is probably the best & was my solution as I found that the heat from my wood burner actually started to blister the paint on the dado & severely crack the skim coat on the plasterboard above.

I found a stainless steel mirror & bent it & fixed it to the wall as a deflector, I promised the wife 12 years ago that I fully intended to finish the job & form a "P" shape to form a stainless steel mantlepiece. Somehow I do not seem to get round to it . It is removable for the summer & every winter when she fits it back( Notice the "she fits" I get an ear wigging !!!!

see picture

Now I am sure that your other half would not let you get away with that & if she did you would not want the grief i get but you could try a nice mantlepiece that projected a good 9 inches with bracketed ends. This could have a stainless reflector under it for the winter if you fabricated a couple of clips. You can get stainless steel mirrors online which are nice & polished. It just needs something to reflect the rising heat away from the wall. The dado & wall above is quite cold & the heat does not pass through the reflector at all. The wallto the right hand side (can be seen in the picture) has recently started to crack & will be sorted when the house is decorated this April. No cracks whatsoever above



If you are not sure then it is worth just filling the cracks for now & installing a plate deflector like i have & see if it works. It is a cheap option before you start blowing cash on major repairs that may not be needed.

But do not leave it 12 years like I have !!!!!



Edited By Sam Longley 1 on 22/03/2018 19:31:03

alan-lloyd22/03/2018 19:25:31
176 forum posts

the chimney breast should be skimmed with a sand cement mix before plastering, have a look in your loft, smoke can leech through brickwork

Jeff Dayman22/03/2018 19:35:18
2223 forum posts
47 photos

Maybe some sort of decorative ceramic or porcelain tile on a mortared metal lath would work for your wall. Just an idea.

John Paton 122/03/2018 19:36:10
324 forum posts
20 photos

Yes good point Sam your woodburner looks to be set rather close to the wall and maybe that is a factor with Dave's installation also - I would normally look to keep them a minimum of 400mm from the wall or back within the old fireplace. Anything within 600mm will be prone to heat damage which is why traditionally fire surrounds are of material unaffected by heat (brick or stone)

I can well imagine that this winter many homes will have roared their woodburners up much harder than normal, but some people do this from time to time to burn the soot out of the flue. My friend who has been involved with woodburners for 40 years has always advised us to have solid flue for the first 1.8 m as flexible liners do not like such intense heat. This might be erring a bit on the safe side and allowing for unreasonable use (I would think 1.5m sufficient) but it is based upon his real life experience and gives a clue about flue temperatures!

I have a very small woodburner in my workshop and keep an oil filled radiator 400mm to one side of it as a heat baffle and another 300mm between that and my nearest machine tool (Cutter Grinder). Even with this the grinder gets quite warm despite the fact that the woodburner only gets used lightly - mainly to use up wood offcuts from the workshop which I save up for harsh weather.

Samsaranda22/03/2018 19:50:34
1408 forum posts
5 photos

Many thanks to all who have contributed to the above, in respect of some of the points raised the chimney is straight with no throat, that had previously been removed, and of course was lined with a stainless steel liner, the house was built in 1965 and is conventional brick construction. The chimney breast is conventional brick with sand and cement undercoat and gypsum topcoat plaster, the brickwork was exposed under the suspect area and was intact with no cracks apparent, the undercoat was re applied and skimmed with gypsum based plaster. It was suggested that lime based plaster could be a solution as it tolerates heat much better but no local plasterers that I contacted was happy to use it, the excuse was that final finish probably wouldn’t be acceptable, I think the real reason was that it was an old traditional skill which had fallen out of favour and they hadn’t used it before. My research to date, today, reveals the existence of non combustible plasterboard and I have ascertained that regulations require non combustible plasterboard must be used if within 400 mm of the sides of the installation and 900 mm above the fire so if boarding is used it would need to be non combustible. I have also been informed that heatproof plaster is available and should be used with its own heatproof plaster undercoat, it is suggested that in the case of my inset fire then all the plaster on the front face of our chimney breast should be removed in order to make a good finish all over and that the heatproof undercoat should be used with the plaster topcoat and no gypsum should come into contact with it, even to the extent of ensuring that any tools used are scrupulously clean with no traces of gypsum, obviously reacts in some way with the gypsum. The heatproof plaster system is apparently good for up to 650 degrees C, so it seems that is probably the way to go, definitely a job for the summer and hopefully next winter when we use the fire no cracks, it would certainly earn me some brownie points, so far all the cracks have been down to me, you know how it goes with the domestic department. Once again thanks to all who have responded to my enquiry.

Dave W

Samsaranda22/03/2018 19:56:30
1408 forum posts
5 photos

John, thanks for your response but my wood burner is an inset one which is mounted directly in the brickwork and is flush across the front face with no gap around it, I am envious of you having a wood burner in the workshop would love one in mine, I make do with an oil filled radiator, again thanks for your response.

Dave W

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