Crumbly mortar on chimney brest

[blasphemer]

The chimney in my loft needs some attention as the mortar is little more than sand in most parts with numerous bricks needing replaced as they are also pretty soft and some have large chunks cracked off.

This has happened i assume because of long term water damage caused by bad pointing on the stack on the roof and old flaunching around the pots.

I recently got these fixed and was wondering what is best to do with the inside.

I was thinking of replacing the very bad bricks and repointing it all. or should i just get someone to render it. The mortar is so soft/ sandy that most of it could be raked out almost by hand.

Thanks a lot!

 

[^woody^]

Sandy mortar on a chimney is normally the result of sulphate attack due to the flue gasses eating the cement. Normally this is prevalent on the external face due to rain soaking in and forming the acid solution, but in your case a leak would do the same thing internally.

Repair with a sulphate resisting cement, but if the flues are in use the mortar may continue to degrade

 

[blasphemer]

no the flue is no longer in use and we were advised that we would need it lined if we wanted to use it again. so really it's just to stabilize the chimney brest and seeing as it doesn't matter if i make a mess of it i might do it myself.

would a 4:1 mix be ok for re-pointing it and how deep should i rake it out because realistically if i went hard at it i would imagine i could rake most of it out but don't want the house to fall down.?

cheers

 

[r896neo]

any advice on the mix and the depth to rake out woul be much appreciated? thanks a lot in advance!

 

[KenGMac]

Several potential problems.

Sulphate attack occurs because all fossil fuel contains sulphur, for example there ar enow low sulphur diesels on the market, likewise low sulphur coal is being used in Power Stations.

What has happened is that over a long number of years, the sulphate in the fuel has found its way on to and into the brickwork, especially the mortar, that is probably a lime Based mortar.

introduce moisture, whether it be rain water entering the chimney pot [if un-capped ] or rain penetrating the exposed area of chimney head, or as will happen even when the flue is in use, water vapour is given off as the result of combustion, once in the Flue the water can and does "condense" out to attach itself to the soot in the flue.

If the flue has been un-used for some time, and if the chimney pot is not capped, rain water will have entered. conversely, if the chimney pot is capped, with a breather vent attached, but the old fire place is not vented, then again condensation WILL occur, and the entire cycle starts all over again, even if there is no fire and no flue gasses to assist the process. Blocked off flues must be vented at room level

When the moisture encounters the Sulphate, a weak form of Sulphuric acid is formed, it is this that has and still will be attacking the mortar.

I have seen on one occasion a thick steel industrial chimney burning oil that was completely destroyed by such actions, the flue Gas temperature was not kept high enough to allow he gas to exit the chimney before it hit its Dew Point, the vapour condensed on the inner walls of the flue and completely rotted out the bottom bend of a 20.mm thick steel flue some 400.mm in internal diameter.

Problems for you are.
1/. How stable is the section of chimney head protruding above the roof line?
2/. Ditto for the area of brick below the roof line
3/. As for re-pointing, I would be inclined to apply a render coat to the entire inner area of brick within the loft area, this to ensure that you are stabilizing the brick, the mortar Et-all.

 

[stuart45]

any advice on the mix and the depth to rake out woul be much appreciated? thanks a lot in advance!

Rule of thumb is don't go deeper than 1/3 into the joints. You will need to go deeper than a normal re-point for unstable brickwork to stabalise it though.
Rake out and point in sections, rather than doing the lot at once.
Brush out and damp down the joints.
4/1 is OK, but use the cement woody suggested.

 

[blasphemer]

hey sorry to ressurect this but i am getting round to doing it today and tommorrow.

What size areas could i do at once? i dont want it to fall down. Is about 1.5m wide by 2.4 tall.

Could i rake out and do half today and half tommorrow? By half i was meaning looking at the face of it and split it into 4 quarters and do the diagonally opposed corners together each time to spread it out? if that makes sense.

OR would i be better just splitting it down the middle vertically and do one today the other tomo? Or do i need to prolong it over more days?

Lastly there are a couple of bricks that are pretty badly bust up and i am going to replace them but can i use any old brick to put in there or should it be something specific?

Thank you

 

[blasphemer]

I'm really sorry to be so impatient but a was hoping to get to this this evening or tommorrow if anyone could clear it up for me.

Thanks, and sorry again.

 

[^woody^]

You can do it all at once if you are not raking out more than 30mm.

Presumably this sandy mortar wont require much chopping out, so can be raked out by scraping - a wire brush or nail in some timber

You can use any old brick

 

[r896neo]

Cheers.

 

[explodingcarrots]

How did you get on with this little job? I think I've got the same thing needs sorting out - sandy mortar, slivers off the face of bricks on my chimney breast in the loft.
I can see evidence of a fix with tar or somesuch so it could've been an old injury that nobody's noticed again til me.
We use the fire sometimes and the chimney is open - can you cap it but still burn well with enough draw?