Cinder block chimney repair

dlabrie · Mar 29, 2004

I live in snow country and I have a large cinder block chimney. There are 3 flues running through it. It is about 2’X5’ and sticks up about 5 ‘ out of the center of my roof just a foot or so down from the peak. It is about 30 years old. The cinder blocks have disintegrated in spots and have been patched up with mortar. Over that is kind of white stucco finishes that keeps flaking off. I am going to put a new roof on this spring and I want to repair this chimney once and for all. I would also, if possible, cover it with some sort of faux stone covering to match the fieldstone fireplaces in the house. Any ideas on how to go about this? Should I tear it down to the roof and start from there? Is there a super sealer I can use?
Thanks,David

OkeeDon · Mar 29, 2004

I'm interested in the replies to this one, also, as I have the same -- but somewhat more extensive -- problem. Our cabin in the NC mountains has a concrete block chimney that's in pretty bad shape. Looking at the attached picture, the chimney runs from the ground and up through the decks on the outside of the cabin. There is only one fireplace, at the level of the top deck (main floor). The damper is rusted and broken just above the open, wood burning, masonry fireplace. I think I'm going to have to tear it down to the level of the damper, replace the damper mechanism, and rebuild it from there. It's still very functional (I can manipulate the damper plate by hand), but the mortar joints in the blocks are in poor condition. The portion below the top deck, down to the ground, is in decent and repairable shape.

I don't think there's any danger of the thing falling down, and what I'd really like to do is re-point the mortar joints without tearing down the chimney, but I don't have a clue as to whether that's possible or feasible. I, too, would then veneer the exterior, but in concrete cast "stone".

OkeeDon · Mar 29, 2004

Here's another shot from the side. The detail is horrible because I wasn't focusing on the chimney, but even in this distant shot, the raggedness is apparent.

Bird · Mar 29, 2004

</font><font color="blue" class="small">( what I'd really like to do is re-point the mortar joints without tearing down the chimney)</font>

I think you can do that if you can get to all the joints you want to re-mortar. It's been about 30 years now since I re-mortared a fireplace, instead of a chimney, that had most of the mortar fall out from between the fire bricks, but there was a special mortar for the high heat and it worked just fine.

GaryM · Mar 29, 2004

On Ask This Old House a while back they had a similar case to yours. Broken damper that is. They removed the broken parts and installed a spring loaded cap at the top. It had a stainless steel cable that ran down through the flue and there was a handle that you used to open and close the cap.

Advantage was no need to tear into the masonary and since it closes from the top there is no chance for birds or other fauna to enter the flue.

This doesn't however address the tuckpointing issue.

BAC · Mar 30, 2004

I am a bricklayer by trade and have encountered many situations as you describe and while it is impossible to make accurate recommendations without actually seeing and inspecting the problem I will give it a shot.

From what you described you should tear down the chimney to just below the roofline and rebuild from there. In most cases the chimney will be sound below that point. In the rebuild you can then use brick, natural stone or you can rebuild with block and cover it with any of the man made products available. Anything less than a rebuild you will not be satisfied with as it will not last for any appreciable time as you have already implied in your post. If the flashing is still good you could only tear down to that point but personally I always go the extra bit and install new flashing as long as I have it down this far.

Disclaimer: In no way should anyone construe this as advise and use this information to base their repair on. Only after a qualified professional has inspected the chimney should anyone proceed with their project. In any event I will in no way be responsible for any action taken on the above. Use at your own risk.

Sorry about this but one can't be too careful.

chucko · Mar 30, 2004

OkeeDon,
If I can see right in your pics you hvae that reasting on a wood floor/deck. If so that is a big no no in building. You will always have problems. The wood will move and the cracks will come back

BAC · Mar 30, 2004

I just posted to the original post in this thread so I thought I would make a few comments here as well.

I can't tell from your photo but it appears that the block chimney has been painted. If the chimney is sound and only the exterior joints are deteriorating then it can be repointed and cast (man-made) stone applied over it. But only after any paint has been removed. The paint will not let the cast stone adhere properly to the block and you will have delamination of the stone in a very short time. I am concerned as to why the joints are failing. It can happen at the top of a chimney due to the acid in the smoke and gases but this does cause problems down lower on the chimney. This should be checked out and the cause determined if it is happening lower than a 2 or 3 feet from the top. Also care should be exorcized in the use of any applied stone product. This stuff has its place but if any water gets behind it will fail in short order. The town I live in recently applied it to a large concrete retaining wall that was built 50 or 60 years ago. It lasted only one year as the hydraulic pressure of the water coming through the 14-inch concrete wall caused the cast stone to delaminate. It then had to be removed and the law suites were flying. On a chimney you should not have this problem.

As far as the damper is concerned someone posted on the easiest way to fix the problem (the damper on the top of the chimney) and this is by far the easiest way to fix the broken damper after the old one has been removed.

If however the chimney was build right to begin with a new damper can be installed from the inside quite easily if the damper is made of cast iron and the throat of the fireplace was formed with masonry. If the damper is steel and it forms the throat then this is harder but not impossible. You may have to open a hole in the chimney on the outside just behind the damper and a new damper and throat can then be installed. It is a lot harder but makes a long lasting, permanent repair. I have done this many, many times.

Disclaimer: In no way should anyone construe this as advise and use this information to base their repair on. Only after a qualified professional has inspected the chimney should anyone proceed with their project. In any event I will in no way be responsible for any action taken on the above. Use at your own risk.

Sorry about this but one can't be too careful.

BAC · Mar 30, 2004

I just noticed in your photo that it appears your chimney is not resting on solid footings. The masonry should run all the way to the ground, below the frost line and rest on solid concrete footings. If your chimney is resting on wood then it is dangerous and should be torn down and rebuilt on solid masonry footings.

OkeeDon · Mar 30, 2004

Nope -- the chimney goes all the way to the ground, through the decks. The portion below the firebox (from the top deck on down to the ground, a distance of about 20' or so to the footer) is solid, because it has not been as exposed as the upper portion. The fireplace has a trap door in the floor so ashes can be cleaned down through the lower chimney, and removed from a door in the basement level.