Im getting a weird leak in my chimney. It almost looks like a condensation buildup (due to distance from roof/location), but this seems to occur during rains.

Can anyone give some possible explanations on what could be causing the leak shown in the attached pictures?

  • could be a leak on the outside of the flue .... the roof flashing on the flue may have a hole and the water runs down the outside of the flue – jsotola Jul 17 '18 at 23:36
  • Water running inside the block. Probably a problem with the cement cap. Water when it gets in, runs til it finds its way out. Also not as weird as you think. – Tyson Jul 17 '18 at 23:59
  • Thanks jsotola and Tyson. This makes sense. Ill probably try and get up there and take a look at the cap. – Dennis M Jul 18 '18 at 14:28

I’d guess the concrete masonry unit (CMU) that is wet, is also unreinforced...probably unreinforced all the way to the top.

Not all cells in a fireplace need to be reinforced and filled solid with grout. The corner unit is probably reinforced and grouted solid, but this cell is probably empty. If so, rain could enter the cell anywhere and run all the way to the footing. When enough water fills the cells, then it finds a crack and seeps out.

The CMU is porous so the water fills the cell a couple of inches and then “wets” the shell (wall of CMU)...thus the darkness.

  • Thank you @Lee Sam. Very helpful insight. Do you know if this is an issue that should be addressed or is more of a passive concern? – Dennis M Jul 18 '18 at 14:20
  • Yes, it needs to be fixed. (You don’t want it to develop into mold.) I’d look at the very top of the chimney and you’ll see a “wash” of cement over the top of the bricks/blocks. It may be cracked allowing water to seep in. I’d also check the exposed exterior vertical section of the chimney and see if there is a clack in the mortar joints allowing rain to blow in. Often we’ll apply a clear sealer to the brick to help seal the vertical surfaces. – Lee Sam Jul 18 '18 at 15:12

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