One for the renderer's

I don’t get to ask questions that often but here we go . I’m fitting a cassette multi-fuel stove in a stripped out open fireplace. All the finish plaster was blown for 6-8 feet around the opening so most has been stripped off. Most of the original render is still OK & intact up to around a foot around the fire opening (been on there for 40 years!) & I went ahead & repaired the area up to the opening with a conventional sand/lime/cement render base which I though would be OK. I know that gypsum plasters are only good up to 50 degrees & so intended to use heat proof plaster on the area immediately around the opening but I’m now being told I should have used a heat proof render as well.

Now I could chisel it all off & start again but do any of you renderer’s know what the realistic maximum temperature a conventional sand/lime/cement render base is? Is it likely to cope with the wall temperatures around a cassette stove or is it likely to fall off? Apart from the inconvenience of pulling it off & starting again, the heat proof stuff seems very expensive & I’ve no idea where I can get it locally at the mo. The dealer I spoke to also said it was a real pig to use so I asked if his guys could actually plaster but he didn’t answer that one; my diplomacy obviously failed me once again!

I wouldn't like to guess the maximum temperature, that render can withstand around a multi fuel stove Richard. It might be worth getting in touch with companies that install these burners, and get some info from them. Let us know how you get on.

Roughcaster.

Scud it scratch it render it ...you wont have any bother.

roughcaster said:

It might be worth getting in touch with companies that install these burners, and get some info from them. Let us know how you get on.

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That’s’ exactly who I was talking to about this heat resistant stuff & then asked could his guy actually plaster ! I just got this feeling that he was trying to push me to something I didn’t really need, he didn’t say much more about it after he realised I wasn’t “the usual punter”, but he's got me worried about the suitability of render.

legs-akimbo said:

Scud it scratch it render it ...you wont have any bother.

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Didn’t scud it but did give the bricks 2 good doses of PVA, applied a fairly wet initial scratch coat let it firm off & then floated out to depth over that, do you reckon it’ll be OK to leave as it is? Seems to have bonded very well.

Richard, you should not use pva under render on brick or block there is no need, you should just damp them down,
why people seal up a good surface with a skin of adhesive is beyond me as the render will stick far better to just a damp brick / block.
rant over.
The render will take a great deal of heat without blowing off the wall most likely more than that fire can chuck out,
but the harder the render the less it will take, what happens is it will expand and crack, this can be very shocking for the home owner
as in the case of some 30 odd years ago I rended the chimney breast over a parkray fire that was flush mounted in the chimney breast for my cousin, (2nd year learning my trade) I used a 4&1 mix, about 4 months later one winter evening he was roaring the fire up while sitting just
in front of it nodding in his chair when the render cracked, there was only one crack about the thickness of a hair appeared but when it crack
by his account it when off like a shotgun scaring the living daylights out of him, ( we still laugh about it).
The render hadn't blown lose from the wall just that hair line crack across it, the finish was thistle board finish and most probably had a large
handful of lime in it.
So not knowing your mix I would still say leave it it won't come off unless that pva peels out of all them little pore's that the render would
normally key to.

marshman said:

Richard, you should not use pva under render on brick or block there is no need, you should just damp them down,
why people seal up a good surface with a skin of adhesive is beyond me as the render will stick far better to just a damp brick / block. rant over.

----- it won't come off unless that pva peels out of all them little pore's that the render would normally key to.

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I only use water on new block/brick work but this was an old, dusty brick wall where the original render had been removed. I was always under the impression it’s best to PVA in this case as “all those little pores” as you put it are blocked with the remains of the cement from the original render mix! Still reakon this is a bad idea

The mix I used was 5:1:1 but did knock up the initial scratch coat fairly wet as well so it won’t be as strong as the float coat.

I’ve also now found out the stove manufacturer recommends using a high temperature screed but I can’t possibly see how it warrant’s one capable of withstanding 1300 degrees, that’s a bleedin furnace & I can't imagine they used anything special on there back in 1968; still undecided, based on the above any further thoughts?

Still reckon this is a bad idea ?.
Yes, just damping down with water, the more you splash on the better, that will take care of the dust and prime the surface
ready for the new render, and your right to use a wet / very soft scratch coat more so if you are in a building that you don't
have the chance to damp down, (I like to scratch coat using a waterprooffer, as this stops or slows down the suction which
lets the float coat cure dry rather than get sucked dry).

Might be a little hard but it won't come off, I've been in old places where the old render around and in side the chimney
where old stoves have been have been bloody hard home brew mixes of something like 2&1 and the brick have shattered
trying to chop it off; it will most likely crack as it expands but unless your using lime putty mix or something like 9.2&1
then it might not crack with the heat but more likely craze over the years as it dries.
I don't know the best mix. I've done fire places in 4&1 / 6&1 / 7.2&1 / 9.2&1 and others, none have come off or I have never had any
come back, well just the one but what I've seen is the gypsum finish have cracked, more so now that lime is no longer added
by the plasterer or his labourer to the finish.
multi or board finish are much finer now being not so course grain, the set times are slower and the way some people over trowel
or polish their walls, (I have a little laugh at the term polish, this was something you did to pi55 off the decorator ).
so the surface is so hard and smooth that paint barely sticks is far from how I was taught.

I’ve also now found out the stove manufacturer recommends using a high temperature screed.

Screed ! sounds more like you need fire bricks, well I've screeded many open hearth with 4&1 sharp sand or grit and not known
it to break up, also a stranded 6&1 concrete mix and I've not seen them break up and what was the old 50 / 60's tiled surrounds
fire places made of ?.

Thanks Marshman, very helpful

I also have a little laugh when some guys seem so proud of the mirror like finish they have managed to achieve.