I fitted a wood burner to my new build taking the flue out the gable wall on a 45 and up the outside. I put on stainless plates around the pipe coming out the wall and sealed it all with silicone.

I have had water running down the outside of the flue which has came in somehow and caused damp on the plasterboard behind the fire. I am not sure how I can seal it.

I was thinking of changing the 45 for a 90 out the back of the stove and making a new hole in the gable.

  • Welcome to Home Improvement. Please post some clear, focused pictures showing the wall penetration from both inside and outside - 1 good overall shot, and as many detailed close ups as you think are necessary. Often times, in a situation like this, seeing is understanding. You can click edit and drag the pics from your computer right into the text edit box and the site will upload and host them for you. If you have issues with that, load them to imgur.com, post the links here, and someone will embed them for you.
    – FreeMan
    Apr 27, 2021 at 17:04
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    "taking the flue out the gable wall on a 45" Are you saying that the flue passes through the wall at a 45 degree angle?
    – Alaska Man
    Apr 27, 2021 at 17:34
  • Yeah, I've never seen a flue pass through a wall at a 45.
    – isherwood
    Apr 27, 2021 at 18:39

1 Answer 1


If the pipe you ran on the outside of your house is single wall stainless steel and you are using the wood stove with a minimum combustion air or the outside temp is fairly cold the water could be from the flue gasses condensing and running down the inside of the pipe. You could also be condensing the flue gasses and the water gets into the inside through a pipe seam. Post a picture of flue pipe so we can get a good look at it. I hope that you used a good wall thimble or left a lot of distance from the flue pipe to any combustible material when you exited the outside wall of the house. There could also be other reasons for your problem.

  • A good point about condensation, but; as i read the question it is about water on the outside of the flue. "I have had water running down the outside of the flue which has came in somehow and caused damp on the plasterboard" I suspect an improper boot/flashing issue because of the 45 degree penetration.
    – Alaska Man
    Apr 27, 2021 at 19:31
  • Thanks guys, it’s twin wall stainless flue, cut through the plaster board through the cavity and out through cored block with all required distances. It seems to be getting in at the 45 but I have siliconed it to death. Is a 90’ ok to use instead of a 45?
    – James
    Apr 27, 2021 at 20:16
  • @James since most of the comments are about confusion around a flue penetrating a wall at a 45° angle, not a 90° angle, then you can safely assume that yes, you can redo this to go straight through the wall.
    – FreeMan
    Apr 28, 2021 at 12:59

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