Block and Clay flue Gap

by Michael Boutin
(Windham NH)

I am building a chimney and I am using 8x8 chimney blocks and 8x8 clay liners. There is a 1 inch gap between the two and I would like to know what is the best way to stabilize the liner inside the blocks. I have heard to fill the gap in with ruble (whatever that is) or some sort of RAW insulation. I want to make sure this is correct so what is your suggestion.


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Apr 02, 2012
Block and Clay Flue Gap NEW
by: charlie Cummins

Hello Mike, Where you live has something to do with the gap, I have seen it done both ways. I believe it should be gently filled as you go up with mortar, gently means some but not solid. Make sense? They used to say up north fill it solid because of the parma frost, and earth quakes. They finally figured out that down south with the tornadoes and everything else fill it also.
Assuming your building a solid masonry fireplace, TIME FOR A STORY, here is the way I was taught to do it back in the 60's. Fill the whole chimney solid from the clay flue to the inside of outside face brick from the top of the bottom of the firebox to the top of the chimney. What we called shysters back in those days was someone who filled it in solid for about a foot above the damper, then just wrapped the clay flue with a 4 inch block, nothing else, then about 2 feet from the top of the chimney they put re-bar and filled it solid the last 2 feet, thus making a good cap on top of the chimney. This was in Texas. Later I was in Steamboat Springs Colo building one on a million dollar plus house, this was in 1971. I asked the contractor where the material was as there was nothing but flue liners. He said what are talking about? I said, are we gonna fill this in solid, he said, are crazy, that material costs money. I said well where are the 4 inch block to put around the flue? The contractor said, what are you talking about? Just stack the flues up to the top with nothing around them? Up I went, this chimney was about 6 feet by 3 feet or bigger, can't remember. Now when I got within 2 feet of the top, I asked for rebar to place to hold the weight of the fill for the last to feet before the top. Again the contractor said what??? Just take it to the top, so I did. When I got there he sent his son hoddie/mason helper up with a piece of 1/2 plywood, placed it on top of the brick on the top of the chimney, and put a 2 inch mortar cap on it. THAT CONTRACTOR WAS OUT OF BUSINESS in a year or 2, but then there was plenty more like him to take his place. To learn more go to and click all the links to my other sites, and please note THE GUARANTEE. If you want to get rich building and repairing fireplaces go to youbethebossbrickblockstonecontractor,com Thank you and happy masonry
Charlie Cummins

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