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Discussion Starter · #1 ·
It got me thinking about leaking chimneys and ive been wondering how feasible it would be to thoroseal a whole block chimney that will have thin stone applied to it?

Would the cement bond correctly onto it?
Would the coating make the drying of the mortar take way too long and risk having stones falling off from the weight?

Would milk in the cement essentially accomplish the same thing?
 

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thouro seal on block is not good under any cement application. I've seen many walls with it flake and peel. Perhaps it was put on in hot weather or cold but I would not stick stone to it!

All chimneys will leak a bit no matter what one does. If thru a tall attic it doesnt matter much as they dry from the mass of masonry and surrounding air not causing any harm [unless leaking down the flues]
But on any exterior chimney water and or moisture will migrate to the warmest side which is in the air space behind the chimney.
We've replaced the sheathing on many f p's and sometimes rafters and studding.
Use 60lb felt and always flash at headers ie just above them and pitch the flashing channel to each end...drop some pea stone or broken stryofoam peices or packing foam pellets and let the chimney do it's normal wet / dry cycle.
For years we've put flashing above headers and if the chimney is tall I'll slap one at the second floor too. Nice fire stop also....
Try and use conc block 4'' solids if your doing 4's the lights suck water and expand with heat, alot!

I had a chimney fire once.. [my attic is about 14' high] and the chimney got very hot.....the 4'' block expanded about an inch off the course below ...cracked at various places, even with air spaces at the flues. It did contract back to original position when it cooled.
If the H O is going to burn a long fire or have many of them...if and when you pour a concrete hearth slab, keep a expansion gap between the three exterior walls behind and at each side of the firebox....and if it's a flush hearth try and keep the slab as low below the firebox floor as possible[even a few brick courses help] This is possible with 10 and 12'' joists..or if the basement is never to be finished,or you can keep the slab lower than the ceiling below and slope the hearth upward. ie like a 1/4 barrel arch as done years ago but with cement merely slanted.

Thinstone with dry look joints is a water sucker......and even with tooled joints water resistance is compromised by the lack of a decent brown coat [done with any continuity] behind it. Even with a decent stucco coating, block and heat don't work well together.i.e. cracks are inevitable.

I did a 36'' rumford f p for my son a few years back. It had a tall exterior brick chimney with 8'' brick surounding the back of the firebox as well as around the 12x16 flue...the chimney was 48'' wide by 33'' or so...used 53dd reg 8'' brk.
He burns it alot and it gets hot as it's a slight slant rumford with a cutoff vestal which he closes down to around 3''.....

Even with all I've spoke of I noticed a slight horizontal hairline about 5' high at the exterior brick work behind the fire box. It doesnt leak as I sprayed it with sure clean xilozane [whatever the spelling is] and put a stone cap on with a silicone bead drip around /under the cap's perimeter to prevent rain water and flashed the header and half way up the nearly 20' chimney.

I have a stone fp at my home which is similar but has 8'' real stone. It is header flashed and has a stone cap and a 12 by 16 flue also. No leaks at the siding or header but I after 18 years or so I'm seeing effloressence at the firebox back side just under the damper. I looked up during a storm and noticed water driping down the flue from the bluestone cap's bottom nearly at center.
I recalled my wife's uncle, an old German Carpenter, running a 3/8's bead under his stone cap just about 3/4'' in from the perimeter. He said it was a drip!....I laughed....I did it on mine subsequently and it worked.

Straight back rumfords have less of a smoke shelf to absorb water leakage and any that comes in from the flues will not be blotted at all.

JB a fireplace on the exterior is burdened with water infiltration from the top /sides and back not to mention under a stone cap.

I quess it's a no brainer to do 8'' around the flues and box then thin stone. The flues will maintane warmth [regardless of the h o's intended usage]and the backup brick will blott nicely. You can do used brick backup and 4'' solids otherwise there is no jiffy solution. Ps I'd use a jointed thinstone also....but all in all I can't see any longevity in any thin masonry applied to chimneys which are thermally effected.
 

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Discussion Starter · #7 ·
I tend to agree and will be using a flushish joint. The chimney will be 8" block with vertical steel and bond beams. It wont crack.

I am interested in the dry block Stonecutter mentioned.
 

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Thoroseal is NOT similar to the various "waterproofing/water resistant" coatings (DryLok, etc.). In my experience, "Dry Block" was a material added to concrete block and mortar that is not very permanent or uniform as far as waterproofing.

Thoroseal is a cement-based coating and is a bear to mix and apply correctly. If it is saturated well in advance (days) and a latex was added to the mud coating, it could work.

The application of Thoroseal gives some clues to the properties. - Thoroseal is properly applied to a surface that is misted lightly just before and it is a mixture that bonds to and becomes a part of the the substrate. Because of this integral property as second coat must be applied within 24 hours that is misted and a latex admixture is recommended to get a good bond the coating applied a day earlier. That means that is quite tough to get a bond to for something applied much earlier.

I have used Thoroseal numerous times to cover masonry and concrete repairs (dams and poured concrete) and it is bullet-proof, but it must be mixed properly AND is must be applied according to the instructions.

One of the problems with Thoroseal is that it is so good that it is tough to adhere to (not like bare block or brick), so bonding agents must be used if the Thoroseal is a few days or weeks old.
 

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I tend to agree and will be using a flushish joint. The chimney will be 8" block with vertical steel and bond beams. It wont crack.

I am interested in the dry block Stonecutter mentioned.

jB all the beams and rods are not it when heat is at play...I mentioned a horz hairline which ment lift from the splay area at the throat smoke chamber area. This an inevitable expansion . Do you realize there is vetical as well as sideways expanson.
You all think I'm hy pathetical and I feel it.....My insurance sent an engineer with cameras to look into why my chimney fire raged so high!

He deducted it was because oxygen was induced into the fire by a faied flue miter where the woodstove flue had to angle a bit and the straighten then go plumb out some 20'.

He insisted it also might have come from the cap wash because I mentioned it was cemented tight to the flue.
I told him I had a second thimble in the basement and when the chimney fire started i shut all air off....then the fire died untile I hear the basement smoke alarm go off! I went to the basement to find a raging fire from a stack of waynescoating I had stacked ther under the thimble which had been stuffed with a sweat shirt.
The kids had a party down there and removed the insulation to use the flue as a ventalator ....and replaced it with a sweat shirt or similar. When the fire strted hot creosote fell down and ignited the shirt and let in a plethera of air.....and the fire re strarted . It raged and had absolutely nothing to do with my miter or cap being cemented to the flue........Yes it pushed the chimney flue at the miter [went back]but what can you expect under those extreme conditions.......

They claimed the flue has to be plumb from the basement to the top!!!! and the engineer said "why didn't I use brick"...

I told him he had no experience and it should be reqired because 75% of engineering is building with experienced professionals and actually using what you build for years!

Till this day I never did a thing to the chimney and burn it kowing it's air tight...I cemeted the basement thimble...back then it was legal to share two wood stove on a single flue.

Finally,.. if you are using 8'' block then thru wall fash it with copper at intervals with a 4'' and 3'' block! What is your problem dude!

The dry mortar is a joke on a chimney... water will get in plan for it!!!
 

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Discussion Starter · #10 ·
Good info Concretemasonry!

There wont be any heat from the fireplace effecting the exterior masonry at all. It is large enough that I have the luxury of creating another chimney of sorts inside the chimney protecting the exterior until the flue is set, then I will most likely run a 6" partition wall inside for the 2 flues.
 

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you missed it....vert expansoin is lift which is precisely at wher you plant the first flue!. Aside from any thermal cracks [dura logs u mentioned] which probably this H O won't cause, your main concern was leaks due to thinstone on block....now if you say you are providing a chimney within a chimney as you have plenty of room then do the double 4'' wythe and scrap the 8'' block......As I first said a 4'' solid and a 4'' used brick infill is very fast and easily integrally bonded.....smoke chamber and all,.. we've done a ****load of them! We used to stock pile 20,000 used brick just for that!
 
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