Hearth / fireplace question

guy84 · 24 May 2010

Hi all,

First time poster long time reader on the forum!

I have formed a concrete hearth in my lounge for a fire place and have bought some natural oyster slate floor tiles to tile the hearth with. I hav also bought a small 5kw multi stove and had a flue liner installed.

My question is can i use a mortar mix (say 4:1) to bed the tiles to the concrete hearth or do i need a specialsis product?

I have a load of sand and cement left over from other projects i could use.

Thanks for any help. I have had a good search on the forum but could find the answer.

Richard C · 24 May 2010

I always use a decent powder cement adhesive which will be good up to 150 degrees C. A 4:1 straight sand/cement mortar mix is rather strong for the heat involved & I use sand/cement/lime render mix 5:1:1 as a base coat in a fire hole & on the wall surrounding the fire as it's more tolerant of heat & less prone to cracking.

I do hope you haven’t used any sort of Gypsum plaster on the walls surrounding the fire; it won’t stand the heat & will crack & blow in a very short time. Heres a link (with sub links) that may be of interest to you;

//www.diynot.com/forums/viewtopic.php?t=211524

guy84 · 24 May 2010

thanks for your reply richard.

Yes litterally had it skimed on saturday morning the chimney breast has been d&d'ed with 12.5mm wall board and skimmed.

Its too late now so will just play it by ear and see what happens when we light the stove.

Our plasterer is pretty experineced and he didnt mention that when doing the skimming.

The other prob is its a small fire place so the stove will be half in half out of the fire place opening if you know what I mean? It there anyhting we could do to stop the plaster blowing off do you think?

Richard C · 24 May 2010

Your spread may be very experienced at skimming (a lot of them are) but if he understood Gypsum plaster products, he would not have D&D plasterboard around a solid fuel fire opening (the adhesive & PB will eventually fail) or skimmed it with Gypsum plaster. In his defence, there are many that won’t be aware if there scope is limited but you’ve presumable paid good money for the work so it's not your problem when it goes wrong; he'll find out the hard way as I did!

Gypsum will not tolerate temperatures much in excess of 50 degrees C for too long. I don’t know if you’ve read my link but I got caught out with this some time ago when diversifying into the odd stove fitting. I’ve measured temperatures in the wall surrounding an 8Kw multi-fuel cassette stove (flush with the wall) at full tilt well into the 90’s at a distance of 500mm above & 300mm to each side of the stove; gypsum plaster of any sort will not stand that! Remedial work later, lesson learned & I now know what works & what doesn’t. 5Kw isn’t that much in the scheme of things but from what you say (½ in ½ out) I don’t hold out much hope. Nothing you can do really except make sure you keep his phone number, burn a couple of logs at a time to keep the temperature down & keep your fingers crossed but what’s the point of that; I guess you will know for sure around mid November time!

guy84 · 24 May 2010

i have attahced a photo of said fire place with the log burner in the aprox position it will be installed within the opening. As stated the finish is plasterboard dot and dabbed with sstandard skim.

does this confirm your suspecions?

Richard C · 24 May 2010

It doesn’t really make me feel any more confident about the area immediately above the fire & particularly around where the flue will be (I’m using my imagination here). You may have got away with the area to the sides of the fire if it was render over brick but the fact it’s D&D PB doesn’t thrill my heart with hope & joy. I don't know who you relied on for the instal but unless you really want a full scale "drains up", all you can really do now is wait & see what happens, initially in the first week or so but then in the longer term. I know it’s a way off yet but I would appreciate feedback as to how you get on with it but do post back if there is anything else I can help with.

Just so your aware about the boring bit; are you doing this under HETAS registered instal or Building Regs. submission? I assume you are aware it’s notifyable work for which you will, at some point, need a certificate of compliance!

guy84 · 25 May 2010

I sort of did it myself with regard to the plaster board, i assumed it would be fine due to the fact half inch board gives 1/2 hour fire protection.

With regard to the hetas/building regs...........urm, i'm not going to bother, we intend staying in the house for a considerable time hence why we are spending so much doing it up. When we do eventually sell, i'll just say it was already installed when we moved here and that we were not given any certs etc.

guy84 · 25 May 2010

Oh yeah and i'll post back on here how i got on, probably wont be using the stove til autumn anyway as there is still so mich to do before the room is ready.

Richard C · 25 May 2010

guy84 said:
I sort of did it myself with regard to the plaster board, i assumed it would be fine due to the fact half inch board gives 1/2 hour fire protection.

That’s just a one off in the event of a fire, there won’t be much left of it afterwards; Gypsum boards (even Fireline) & plaster products are not designed for use where temperatures regularly exceed 49 degrees, this is lifted from their technical data sheets;

Effect of temperature
Gyproc WallBoard is unsuitable for use in areas subject to
continuously damp or humid conditions and must not be used
to isolate dampness. Plasterboards are not suitable for use in
temperatures above 49°C, but can be subjected to freezing
conditions without risk of damage.

A non gypsum based board such as Supalux would be more suitable

guy84 said:
With regard to the hetas/building regs...........urm, i'm not going to bother, we intend staying in the house for a considerable time hence why we are spending so much doing it up. When we do eventually sell, i'll just say it was already installed when we moved here and that we were not given any certs etc.

It's your decision of course but the reason for the regulations is that carbon monoxide kills people & the old chestnut “it wasn’t me wot installed it guv, honest” wont really help if you can’t produce the compliance documentation & your future buyer decides not to proceed or, more likely, the surveyor picks it up & their lender then wont advance the cash on the property due to unauthorised/non-compliant building works. The onus is on you to provide evidence of compliance not on them to accept your word for it. Non compliant building works is causing increasingly more problems for sales now & if you lie on the sellers disclosure questionnaire, you could find yourself open to civil action & a hefty damages claim from your buyers if they find out; it’s happened. Certification is really not a big deal or that expensive as long as the installation complies with Building Regs. but that’s the reason for the certification process.

jctilingservices · 25 May 2010

come winter time when you have the stove burning, you are going to have some real problems, and not just health problems either. The job has not been done correctly OR with the correct materials.
However you have been given some Safe and acccurate info from Richard, how you go on from here is down to you, but i to would look forward to your reply after use of the stove mate

tictic · 25 May 2010

guy84
the advise the guys have given you has been correct...

i would advise you to get your stove up and running asap..why?? cos the walls will fail,and you will be able to get it done properly before this good weather turns lol...and not on a cold winter bleak night..

guy84 · 26 May 2010

Hi guys thanks for you comments.

Do you think installing some kind of fire suround would sort the problem, it would stop the heat from effecting the wall above and to the side right?

Obviously it has been plastered in the way shown on the phot for a minimalist effect, but im sure i could get a fairly contempory surround from somewhere.

Or

Should i just hack off the plater above by say 500m and render and skim in the heat resiatant plaster?

Richard C · 26 May 2010

One of the main reasons original fires/stoves were fitted with a stone, tile or cast iron surround was to cope with the huge heat transfer into the building fabric. Lime plaster copes very well with it but it’s really only suitable if you have a 150 year old clay lump or solid stone cottage; it’s really expensive & will look stupid in even an 80 year old property.

A surround would be a solution but it would have to be stone or cast iron; most contemporary surrounds are only designed for "pretty light mood fires" of insignificant heat output, like gypsum plaster they won’t take any heat at all; you will need to check the surround is suitable for a solid fuel appliance. You will still need to lose the false Gypsum plasterboard surround & re-render for a solid base, if you just stick it over the top of what you’ve already got, it will only make maters worse as the gypsum will get even hotter behind there, fail & then the surround wont be fixed to anything of any substance.

guy84 said:
Should i just hack off the plater above by say 500m and render and skim in the heat resiatant plaster?

If you want a contemporary look, that’s what I’d do but you have to loose ANY gypsum plaster & plaster board as well & replace it with something more suitable; as your fire is half hanging out of the fire hole, I’d increase that 500mm to 1m above the fire.

guy84 · 27 May 2010

thanks for all your advice richard, I'm going to try and get hold of a cast iron surrond (salavage yard ideally), hack off the plaster and board to accomodate it, bolt it to wall and get the plasterer to make good around it.

Hopefully i can enjoy my fire in the comfort of knowing the wall isnt going to fall off now!

There will still be some heat build up in the plaster but the cast iron should act as a big heat sink and shouldnt let it get anywhere near the crucial 49 deg C.

Richard C · 27 May 2010

If you’ve not in a hurry, look up your nearest auction house & check when their next Agricultural & Salvage or Building Salvage sale is on. It’s where the building salvage specialists get their stocks from & there are usually several surrounds to chose from at my local monthly auction house. A blasted, tarted & blacked cast iron surround may cost, say, upwards of £1k from a dealer but you could get it for 10-15% of that at auction; you have to do the tarting up yourself of course.

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Hearth / fireplace question

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