I recently installed a Stovax Studio 1 Woodburner and after reading various posts and articles, rendered the chimney breast with a 5/1/1 mix and finished with a layer of Vitcas. Unfortunately cracks are appearing, including one which starts at the middle of the top edge of the frameles edge, which came with the fire, to the top of the chimney breast. These are only hairline cracks which tend to open up when the fire is hot but become virtually invisible when all is cool. I would suggest that they never exceed 0.2 mm in width. Can anyone suggest a solution?
Cassette woodburner and cracking plaster
- Thread starter yorkieboy
- Start date
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Will a big picture cover it? You need the advice of RichardC. I hope he's still about.
Sometimes you can still get random hairline cracks forming even with Vitcas plaster, the main thing is the plaster doesn’t actually fall of the wall as gypsum will. I’ve got two or three persistent hairline cracks in a random crazing pattern in the immediate area over the top of my own Stovax Riva cassette but they are barely visible close up & not at all from any distance; these can be repaired using ordinary polyester filler. I completely ballsed it up with my own stove back in 2007 & within 6 months, the gypsum base/finish I initially used had totally disintegrated & fell off the wall around the stove, I had to strip it all back & redo it with lime render & Vitcas; I’ve done a few more since but so far not had any major problems or complaints.
If the cracks expand/contract with the heat & extend way beyond the hottest part of the wall around the fire, it would suggest the render itself has cracked beneath the plaster; did you render in two coats, the second (float) coat slightly weaker than the first (scratch) coat? How did you prep before rendering? I assume you took it back to bare brick before rendering & nothing was left on there? How thick is the Vitcas plaster? How long did you leave it all before firing up the stove? A couple of pics may help. You may have damage to substrate itself, did you examine the mortar joints/bricks carefully for any cracks before you rendered? This needs more extensive remedial work & strengthening with metal lath, no amount of render or plaster over the top will stop the cracks repapering.
If the cracks expand/contract with the heat & extend way beyond the hottest part of the wall around the fire, it would suggest the render itself has cracked beneath the plaster; did you render in two coats, the second (float) coat slightly weaker than the first (scratch) coat? How did you prep before rendering? I assume you took it back to bare brick before rendering & nothing was left on there? How thick is the Vitcas plaster? How long did you leave it all before firing up the stove? A couple of pics may help. You may have damage to substrate itself, did you examine the mortar joints/bricks carefully for any cracks before you rendered? This needs more extensive remedial work & strengthening with metal lath, no amount of render or plaster over the top will stop the cracks repapering.
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thats what i was thinking rich mabey the render was not fully cured before the fire was lit thus drying it (rather than cureing) and ruining the bond of the render.
see rich you cnt leave i listen to ya, proberly would have a few fire places to re-do by now if i haddent.
see rich you cnt leave i listen to ya, proberly would have a few fire places to re-do by now if i haddent.
Thanks Richard for picking up this problem. The chimney breast above the lintel over the top of the fire was blocked in with toplite blocks and rendered with two coats as you described. This was left for a fortnight before the vitcas was applied, a thickness of about 4 or 5 mm. The fire was not ignited for 7 or 8 days. I note that polyester filler can be used for the hairline cracks, but the crack right up to the ceiling does worry me.
So is this a new chimney & flue installation? I’m venturing outside my comfort zone a bit here but my biggest concern is that you have used lightweight blocks for the chimney breast/flue case. I’ve not done a new chimney install only renovations but all the ones I’ve done to date have had a brick constructed fire opening, flue throat & chimney breast right up through to the pot; they only go to dog toothed blocks outside the immediate flue/heat zone. I’ve had a look at the spec sheet for Toplight blocks & they are class O but I’m concerned that this spec may be similar to BG plasterboard (class 1). The classification may only apply to a one off exposure to fire &, in the case of BG, PB (even Fireline) is not suitable for continuous/fluctuating exposure to temperatures much over 50C. Unfortunately I can’t find any info on the Tarmac spec sheet relating to continuous temperature rating/exposure but did you check with Tarmac they are actually suitable for high heat installations such as a chimney breast & around a flue?
You were also a little impatient lighting the fire IMO; a conventional two coat render base of around 12-14mm with 3-4mm of multi will take around 4 weeks to fully cure & I would have left it at least that; in fact your far better doing this sort of work at the end of the season (now), it will then have around 6 month to dry out slowly & naturally.
A full height crack that opens/closes with heat worries me a bit; I would cut away at least a couple of sections of the Vitcas plaster & render either side of the crack to see how far down it goes. You may just have cracked the render but check the blocks as I’ve an uncanny feeling they may have cracked due to thermal shock. Even if they haven’t, the thermal expansion/contraction properties of the blocks when exposed to high temperatures may be outside the scope of even sand/cement/lime render which means you will never cure it.
You need to check with Tarmac technical about their blocks but please post any feedback you get from them as it’s always very useful.
You were also a little impatient lighting the fire IMO; a conventional two coat render base of around 12-14mm with 3-4mm of multi will take around 4 weeks to fully cure & I would have left it at least that; in fact your far better doing this sort of work at the end of the season (now), it will then have around 6 month to dry out slowly & naturally.
A full height crack that opens/closes with heat worries me a bit; I would cut away at least a couple of sections of the Vitcas plaster & render either side of the crack to see how far down it goes. You may just have cracked the render but check the blocks as I’ve an uncanny feeling they may have cracked due to thermal shock. Even if they haven’t, the thermal expansion/contraction properties of the blocks when exposed to high temperatures may be outside the scope of even sand/cement/lime render which means you will never cure it.
You need to check with Tarmac technical about their blocks but please post any feedback you get from them as it’s always very useful.
Yes, this was a new build. I engaged a local woodburner centre to fit the fire and they visited the site on 3 occasions before actually fitting the fire to liaise with my builder. They insisted on about 600mm clearance above the fire so that they could use an adaptor to connect to a 9 inch round class 1 flue. The void was lined with a scamales blue board ( think this is the term ) and filled with vermiculite before my builder blocked up the void. They tried to persuade me to fit a 8 inch granite surround to the fire and then conventional plasterboard and were not willing to use vitcas, saying it was horrible to use and would crack anyway. I wanted the clean, minimalist appearance produced by the edgeless frame kit with the Studio 1 and, after reading various posts on the topic, went for the 5/1/1/render and vitcas finish.
I notice that you ( RichardC ) are in Norfolk. If I need professional help are you interested? I'm North Walsham way.
I notice that you ( RichardC ) are in Norfolk. If I need professional help are you interested? I'm North Walsham way.
Tarmac technical services tell me that the Toplite blocks have no problem in handling heat so it looks like thermal expansion of the fire frame or the supporting lintel have caused the problem. I will report when all is solved.
Thanks for getting back.
Another thing to consider/check is that even if the lightweight blocks themselves can withstand thermal expansion/contraction OK, the thermal expansion rate of the blocks is probably greater than a sand/cement/lime render mix/Vitcas can tolerate = large cracks!
Traditional construction methods in high heat areas around fire openings is high density brick which will have a much lower thermal expansion rate for any given temperature.
Another thing to consider/check is that even if the lightweight blocks themselves can withstand thermal expansion/contraction OK, the thermal expansion rate of the blocks is probably greater than a sand/cement/lime render mix/Vitcas can tolerate = large cracks!
Traditional construction methods in high heat areas around fire openings is high density brick which will have a much lower thermal expansion rate for any given temperature.
Hi folks, I seem to have a similar problem to yorkieboy (and I'm also in Norfolk).
We're refurbishing our ground floor and putting a Morso S80-90 inset woodburner into the existing fireplace. We will surround the stove with vermiculite and the fireplace will then be closed up around the woodburner using concrete blocks. This is all as specified by the woodburner centre who sold me the stove. Image attached shows where we've got to.
What I would like to achieve is the same appearance as the brochure, i.e. a contemporary look where the woodburner looks a bit like a wall hung tv.
The stove sits inside an insert which has a front flange designed to sit flush with the wall (according to the instructions). The fitter though has other ideas and says that the plaster will all crack and fall off if we don't put tiles or slate or something else behind and around the flange. This will change the clean lines of the stove.
So I am thinking we should chipp back the existing render around the fireplace and re-render with 6:1:1 lime mix, then use Vitcas plaster. However again the fitter simply says he's seen this done before and it all fell off.
Can anyone offer some guidance?
Thank you.
We're refurbishing our ground floor and putting a Morso S80-90 inset woodburner into the existing fireplace. We will surround the stove with vermiculite and the fireplace will then be closed up around the woodburner using concrete blocks. This is all as specified by the woodburner centre who sold me the stove. Image attached shows where we've got to.
What I would like to achieve is the same appearance as the brochure, i.e. a contemporary look where the woodburner looks a bit like a wall hung tv.
The stove sits inside an insert which has a front flange designed to sit flush with the wall (according to the instructions). The fitter though has other ideas and says that the plaster will all crack and fall off if we don't put tiles or slate or something else behind and around the flange. This will change the clean lines of the stove.
So I am thinking we should chipp back the existing render around the fireplace and re-render with 6:1:1 lime mix, then use Vitcas plaster. However again the fitter simply says he's seen this done before and it all fell off.
Can anyone offer some guidance?
Thank you.
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