Offset Chimney

[Frank P. Macak]

I came across an offset concrete block chimney. Concrete block in the offset section is not set horizontal and stepped of little by little, as it is done with brick usually, but is actually leaning to its side for the length of 7 blocks. The actual offset turns are not really visible, with the exception of one mortar joint that has an uneven amount of the mortar on its perimeter, as to allow it for the turn (bend) and for the transition back into vertical/plumb section. The turn is completed in between the rafter and then is transitioned into brick chimney above the roof. There are no visible cracks in the mortar or the block, it's just I have never seen an offset built that way. How safe is that? The chimney is probably original to the house = 75 years old, and shows no signs of movement. Should it have some bracing?

[Jeffrey R. Jonas]

Frank, post a pic if you have it, for better understanding of what exactly you observed. I don't recall seeing concrete block installed as you described. Brick is the norm in my area.

[Rick Marton]

Frank, we have that in our area. Its called corbelling. We were told that the chimney can come up into the attic, but is not close to the ridge, so they slowly corbell it or offset over it closer to the ridge. There are restrictions on how far you can corbell it over. I googled (Chimey Offset Corbell) it but it won't let me copy and paste it. sorry Rick

[Frank P. Macak]

Originally Posted by jonas

Frank, post a pic if you have it, for better understanding of what exactly you observed. I don't recall seeing concrete block installed as you described. Brick is the norm in my area.

Unfortunatelly I don't have a good picture of the whole chimney. But notice this top block gap on the left is about 2", while same joint on the right would be standard 1/2" mortar. They did it to turn the block back into vertical position. There has to be another block installed in the same manner on top of this one, to get back to the plumb line from the offset. I am familiar with the brick corbel, but I have never seen CMU layered angled without any bracing like that. But it really looks old, without any movement all that time.

[Matthew K. Klein]

I thought I had seen this discussion before in the INACHI forums. Go here: http://www.nachi.org/forum/f2/why-ch...lt-lean-27522/. Marcel included a pretty detailed discussion about corbelling there.