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.
