Hello, I want to repair a hole in the flue in a stone wall - rough stone with a rubble filling. Please can anyone advise where I can find the mortar mix I should use? The flue is used for an open fire or a wood and coal burning stove. Thank you, James
I believe Fire Cement is the appropriate material to use.
Perhaps, if it is in the hot area, but it's relatively strong.
Perhaps the (unstated) question was "Will cement or lime mortar withstand flue acids". The Building Regs answer is that the flue should be lined. (But mine isn't).
And expensive...
Lining is a relatively recent requirement. Most chimneys used with open fires where just built from the same bricks/stone and mortar as the rest of the building. The mix might have been different for the chimney but TBH I doubt it.
The other question is what mortar is currently used, cement or lime? I'd try and match any new mortar to what is already there. ie. if it's lime mortar use lime mortar.
The statement "The flue is used for an open fire or a wood and coal burning stove." is a bit odd. I don't know of many people who switch between an open fire or stove in the same fireplace on a regular basis. Is the OP opening up and old fire place and doesn't yet know if there will be an open fire grate or a stove fitted. Personally I'd go for a stove far more heat into the room but the flue gases are hotter, I'd line a chimney used for a stove.
To be honest, for the stove, I would line it at least. ordinary mortar doesnt last long round a fire back. Dull red heat and its gone.
I know several (France) including a few bars.. Stove for the winter, open fire other times. Bostik do a fire cement in 5, 10 or 15 kilo bags in the bricos but I haven't noticed it in the UK.
Many thanks all for your various answers. Perhaps I need to explain the construction further. The wall is built from undressed stone with small rubble in the middle between the inner and outer. I don't think mortar is used in the construction - it's dry stone. The outside is rendered, and the inside is plastered as normal. The fireplace is on the ground floor and the problem is a little above first floor level.Probably in work a few decades ago, a small amount of the rubble adjacent to the flue was removed, and the pargeting has eventually come away, leaving an aperture a few inches square between the flue and the rubble part of the wall. This has eventually become apparent for reasons I won't bore you with. I want to patch the hole, and fill up the cavity in the rubble with stones and an impervious mix that will prevent penetration from the flue even if the lining at the point becomes unsound in the future. I will probably have the flue relined soon in addition to this repair.
So from your various answers, it seems fire cement would probably be the best material to repair the hole. Should I use it as a mortar for some stone? I would use a more "normal" mix to fill in the rest of the aperture, away from contact with the flue - any suggestions for this? For example, would portland cement and lime with sand be suitable? James
It varies.
1949, I was surprised to find the entire outside of the chimney up to the roofline was essentially fire cement. You could drop a bomb and that bit would still be standing. Blue refractory tiles everywhere on the inside too. The builder did thatched cottages and was noted for indestructible chimneys (the bottom bricks bear a striking resemblance to those at the nearby railway around the same time).1951, miserable construction, plain mortar, with obvious voids in mortar perpendiculars and "palpable hot-spots" around the wallpaper. I recall that was part dismantled and firecemented in the early 1980s, refractory tiles up to about 6ft.
Fire cement is not that much, 1kg 5kg 10kg tubs in the UK. Ok, it does not go far and costs much more than mortar.
For a gas fire (which can be particularly bad re acid & CO) fire cement for a repair is ok, but for open fire etc a lining is not a bad idea. If the OPs house is stone with rubble infill, what is the roof made of? What joists "abutt" the chimney? Coal gets hot, wood can get extremely hot - flames can travel a long way up the chimney re burning gases.
And tar from half burnt wood can build up inside the chimney and after many months ignite, it goes like a rocket! [g]
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