This is an idea I thought about to seal and waterproof my fireplace chimney.

Problem with using silicone and other sealants is that the chimney constantly moves. It EXPANDS when hot and SHRINKS when it cools down, so any attempt to seal the chimney and ceiling is temporary. Soon the heat and up-down movement of the chimney, will form a crack in the seal, through which water can enter. A 4000 mm chimney, will expand by aprox 5 mm in length when hot. This is quite a movement, and on a daily basis.

So, what I did is use a metal tin sheet and built a “skirt” that seats on the chimney, close to the floor/roof (in my case its a floor you can walk on, but it doesn’t matter, it can be a roof) , and use another sheet to build a round “wall” that seats on the floor/roof.

Rain then drops on the skirt and drips outside of the wall, while the hole in the ceiling, remains water free.

Both skirt and wall should be water sealed (use heat-resistant sealant), but that’s not a problem, cause there’s no expansion of metal there. Skirt just moves with the chimney and wall just seats on floor/roof.

Tools you will need: