Replastering Chimney Questions

[PULLMYFINGER]

Hi,

I was hoping one of you professional plasteres could help me with a couple of questions.

I,m about to replaster my recently stripped chimney breast as my wife with her ever changing mind has decided she no longer likes the bare brick look anymore despite the fact it took me ages with a wire brush to get back to the state you see in the photo's.

However i'm a DIY plasterer and i'm quite happy with my level of skill when it comes to plastering but there is so much i dont know and am always willing to learn new stuff.

1) What is the best way to replaster the chimney with regards of the material to use as it will have a woodburning stove installed

2) The internal back of the chimney is damp not soaking wet but the bricks are darker and when you really close they smell a bit musty(it's about 4 bricks above hearth level and has been like this since it was opened up about 6 months ago with no real signs of drying out i assume it could be penetrating damp as we live on a hill and next door on the other side of my chimney are higher up so my finished floor would be under their floor level if you see what i mean as their house is stepped higher than mine, or it could be rising damp (but after reading a long winded post on here about rising damp i dont even know it that exists).

Is it o.k to sand cement render with water proofer additive on the inside of a chimney and will this stop the damp or is there a better method as like i said before it's not really damp.

3) What is the best way to maintain the curve of the arch of the opening when it comes to replastering the face of the chimney.

Untitled

• PULLMYFINGER
• 27 Jan 2010
• 1

Untitled

• PULLMYFINGER
• 27 Jan 2010
• 1

Thanks

 

[Richard C]

None of my business really but I think it’s a real shame you’re going to cover that up & it would be sacrilege if it’s an original feature of an old property; but that’s your choice.

1) What is the best way to replaster the chimney with regards of the material to use as it will have a woodburning stove installed

Wood burners, multi-fuel & open fires generate a lot of heat in the surrounding wall & it’s important that you do not use ANY Gypsum based plaster, either base or finish, in the enclosure or on the wall area within a minimum of 500mm either side & 800mm above the stove. Gypsum will only withstand a temperature of around 50 degrees C & regular exposure to much above that will cause it to blow, crack & eventually fall off.
You need special heat proof products, something like this;
http://shop.vitcas.com/vitcas-heat-resistant-plaster-16-p.asp
A sand/cement/lime based render will be just as good for the base coat & a whole lot cheaper but don’t be tempted to use Multi over it; you will need a heat resistant finish plaster unless you settle for the coarser render finish.

You can get heat resistant finish boards (again quiet expensive) which may be a better option as it will only cover the bricks rather than obliterating them behind render but I’ve never used them. I wouldn’t use Fireline board either as it’s still Gypsum based & is only intended to provide a one off barrier in the event of fire not as a decorative heat proof finish.

2) The internal back of the chimney is damp not soaking wet but the bricks are darker and when you really close they smell a bit musty(it's about 4 bricks above hearth level and has been like this since it was opened up about 6 months ago with no real signs of drying out i assume it could be penetrating damp as we live on a hill and next door on the other side of my chimney are higher up so my finished floor would be under their floor level if you see what i mean as their house is stepped higher than mine, or it could be rising damp (but after reading a long winded post on here about rising damp i dont even know it that exists). Is it o.k to sand cement render with water proofer additive on the inside of a chimney and will this stop the damp or is there a better method as like i said before it's not really damp.

It’s either damp or it isn’t & a test with a meter will tell you; either hire one or call someone in to test & advise. If there is damp, whatever the cause, you must sort that out first; you will have problems if you just render over it & plaster won’t stand damp at all.

3) What is the best way to maintain the curve of the arch of the opening when it comes to replastering the face of the chimney.

Maintaining the curve should not be difficult & I would use temporary formers & render/plaster to those rather than a fixing a steel beading which could cause problems with cracking from expansion & contraction due to the heat.

Finally; be aware that installing a new stove or recomissioning an open fire is subject to several Building Regulations & will require a Building Notice submission unless you use a HETAS registered installer. The regulations cover the flue, hearth (constructional & decorative), ventilation, stove installation & a small notice stating the fire/flue rating which is usually located next to your consumer unit. You will need certificate of compliance for the above & will be required to produce it in the event of a sale.

 

[trowelmonkey1]

what do you think about using a lime putty based render Rich? better for expansion and breathability?

 

[PULLMYFINGER]

Thanks for the quick response Richard C and trowelmonkey1


Could you tell me were i would buy the fire resistant boards?

Also how do i go about making a lime/sand/cement render could you possibly give me ratio's to mix. Where do i get the lime from and do i just use platering sand?

It is damp in the chimney we had it checked and all the damp company was going to was inject a DPC (now even i can do that) if it's penetrating damp do you know how can i stop the damp without tanking it with bitumen or cavity drain membrane as anything used i pressume would melt with the heat.

Soz about all the questions just trying to get it right 1st time

Cheers

 

[Richard C]

what do you think about using a lime putty based render Rich? better for expansion and breathability?

I know that lime based mortar & plaster mixes were standard materials on (very) old buildings in the days of yore & that they are breathable & very tolerant of movement. Essential when renovating old solid walled buildings &, in many cases, all you will be allowed to use if it’s a listed.

I have to admit I’ve no experience with using them as I’ve not yet done any work on a building old enough to warrant it. I have been using sand/cement/lime based renders for some time which are excellent for renovation work & work just as well for high heat applications so that’s what I’ve stuck to. I assume a lime render will withstand the high heat? (I’ve actually measured in excess of 90 degrees in the wall above a cassette multi-fuel stove); would it not be a little soft? & what about the finish, is it fine enough to leave on its own, can you cover it? Outside my experience as yet & things I’ve yet to discover. It’s also seems to be a tad expensive !

 

[davey5678]

I would look at fire resistant foil backed plaster board. It will protect the brickwork (I agree it is good looking - but Ive been there and it is very sold even after pointing) and be relatively easy to install even for non plasterers. I did an extinct chimney in normal plaster board and it looks great and was a much easier job.

You guys will tell me Im not a "proper" plasterer now - yeah well your right.

 

[trowelmonkey1]

fireboard not suitable for this application as rich said earlier.

if you used a hydraulic lime instead of putty it would set hard, NHL 2 would be fine for brickwork or even NHL 3.5, mixed at a ratio of 3:1 with washed sharp sand for the scratch coat, you can make a lime finish using putty mixed with silver sand at a ratio of 1 sand to 1.5 putty and it makes a lovely finish plaster which leaves a nicer finish than multi.

probably not the cheapest or easiest solution tho

 

[Richard C]

If you do a Google on fire resistant boards it will bring up some choices; Supalux, Masterboard, Fermacell to name a few but some are only modified Gypsum based board, posibly similar to Fireline, which only seem to be rated up to around 90 degrees C; maybe OK but maybe not in the long term. As I said, Ive no experienec of using these as I’ve so far only finished solid wall using render/plaster.

A usefull thread I started last year when I first got into fire surrounds; //www.diynot.com/forums/viewtopic.php?p=1264123#1264123
this one’s actually my own fire which had already suffered a failure of Gypsum plaster so I’ve personal experience of that.

Good advice from TM on using pure lime based render/palster mix as well. You could possibly combine the two by using a sand/cement/lime mix for the render base & mix up your own lime finish palster which may work out cheaper than buying the tub mix finish; it may even be the same for all I know!

 

[PULLMYFINGER]

Hi,

I've just read on another site after googling that a render with a 5:1:1 mix would work, sand:cement:lime

Could i use this system on the inglenook back and sides part of the chimney and dot and dab the outside face and size with plasterboard or will this need to be fireboard. Then skim with multi finish or is multifinish a no no for chimneys full stop, just i want it to be smooth to match the other chimney.

The opening is quite high about 1200mm high and over 1000mm wide and quite far away from the stove.

There's not much info around on this problem but i cant be the first to be doing this

Thanks

 

[Richard C]

I've just read on another site after googling that a render with a 5:1:1 mix would work, sand:cement:lime

If you follow my link, that’s actually the mix I used but a little more lime probably wouldn't go amiss.

Could i use this system on the inglenook back and sides part of the chimney and dot and dab the outside face and size with plasterboard or will this need to be fireboard. Then skim with multi finish or is multifinish a no no for chimneys full stop, just i want it to be smooth to match the other chimney.

The opening is quite high about 1200mm high and over 1000mm wide and quite far away from the stove.

Your call, I wouldn’t for the reasons given. You only want to do it once, mine failed after just a few weeks & I had to do all over again because, like many, I was originally oblivious to the temperature limits of standard Gypsum plaster products.

If your tempted to go with boards, don’t just D&D them up, there is a risk the adhesive will fail & the boards may actually fall down.

There's not much info around on this problem but i cant be the first to be doing this

There is, you’ve just got to dig for it.

 

[PULLMYFINGER]

Thanks Richard C

Just read the link you provided i think i'll give it a go at the weekend did you put waterproofer in as one person suggested. I'll use use the 5:1:1 mix for the full chimney inside and out.

And finish coat with Silver sand and lime putty 1sand : 1.5putty as was suggested earlier.

Does this sound about right?

 

[Richard C]

Just read the link you provided i think i'll give it a go at the weekend did you put waterproofer in as one person suggested. I'll use use the 5:1:1 mix for the full chimney inside and out.

I didn’t use waterproofer.

And finish coat with Silver sand and lime putty 1sand : 1.5putty as was suggested earlier.
Does this sound about right?

Not tried lime plaster yet but think that’s what TM has advised; would be interested how it turns out.

 

[PULLMYFINGER]

I only have a B & Q near me and this is the only lime they do. Is the the correct stuff?

http://www.diy.com/diy/jsp/bq/nav.j...refview=search&ts=1264604903457&isSearch=true

Ohh before i for get what are temporary formers ( fot doing the arch bit )

 

[trowelmonkey1]

yup, 1 silver sand to 1.5 lime putty for the finish, 2 coats as with multi but in quick sucession.

have to be honest i'm no expert at lime plastering but this is the information i know to be true and have tried skimming a small area with it but only onto a lime backing coat mind.

hydrated lime is correct for the scratch coat, lime putty is a different thing altogether...well kind of, still lime after all:
http://www.ecomerchant.co.uk/index.php?main_page=product_info&cPath=10&products_id=284

 

[PULLMYFINGER]

When using the silver sand and lime putty for the finish coat do you still PVA the sand cement coat or just wet it, and do you still do the wet trowel stages and dry trowel stage and polish or is it just 2 coates and leave just i'm wanting a smooth finish on the outer chimney faces.


Thanks for all you tips guys!