How to close an unused chimney

The chimney is damp

we have a problem with our chimney, or with the part of the chimney that is in the attic that we are currently expanding.
It is strongly discolored brown, the lime plaster crumbles and is crumbly. If we have rainy weather for a while, the chimney becomes damp at this point, sometimes even brown droplets run down it.
The head above the roof was renewed in the 80s and a stainless steel pipe was pulled in. The old gas boiler was connected to this pipe.
This chimney is currently not in use. The new condensing boiler in the attic uses its own outlet pipe. But after we have remodeled the living room, we want to connect a fireplace.

The roofer says: the seal on the roof tiles is OK, the chimney draws water from the joints in the chimney head. He would have to be delivered.
But the monument protection does not want that and recommends a new formation of the head.
The roofer says: nonsense! The problem then reappears after a year.
The chimney sweep recommends: Remove the pipe and a clay sacrificial plaster and point out that everything will be fine again if a stove is connected and the chimney is heated dry. But that may take another year and a half before we're ready.

We are now slowed down with our loft conversion and are puzzling over who is right.
In this forum we have already read about a barrier that can then be cleaned - with clay or air lime?
Are there experiences, recommendations, proposed solutions that have proven effective?

Thanks in advance!

Totally rotten

this happens when a chimney becomes damp. Then the soot is drawn into the brick in a capillary manner and then at some point arrives on the outside as a brown broth.
This does not happen with a locally heated chimney. Because the chimney is dried from the inside by the hot exhaust gases and the moisture "migrates" inwards. Therefore it cannot take any soot outside with it. That's why they say that you should only burn dry wood. Wet things bring very moisture and relatively cool exhaust gases. These can then condense on the inside of the cool chimney.
In short, your chimney sweep is right. You have to get the chimney dry. So heat up as quickly as possible.
Put a small (oil) stove in the attic and connect it to the vented chimney. Then it will be warmer when converting the attic. If you heat differently, the heated air can condense on the cold chimney above and you get brown water pearls from above.
A small note about the roof extension of the chimney sweep does not speak of sacrificial plaster for nothing. It'll come down pretty soon.



Red Riding Hood


One thing is certain: the chimney must be dried. I.e. Marko Lindner and the chimney sweep are right: With hot exhaust gases (by heating a solid fuel stove) this works well, and if you can keep the heating up, you can also create a permanently stable condition. That means you have to constantly heat away the penetrating moisture (also in the future). If this is a problem and the chimney is only to be used sporadically, you should perhaps think about where the moisture is coming from and work on it. (If the roofer is right, and the water comes through the joints, then you should work on it. But slating is certainly not the only option. After all, there are many airtight chimneys that are not slated ...) But it is It is also conceivable that the problem is at least partly due to the fact that it is raining into your chimney. Then caps made of stainless steel or, more beautifully, as shown here, a masonry cap can help.

Best wishes,
Andreas Schmeller


Hello Annette,

I can only agree with the previous speakers. Heating helps to dry the chimney, and it may also be burned out - that's what the chimney sweep does. Impregnation is very helpful for the stains on the inside and the protection of the head on the outside. The water absorption is greatly reduced, the masonry is strengthened and there is a high probability that no further penetration in the interior will occur.

I am gladly available for questions

With best regards from Berlin

Werner Gensky

Heating and chimney impregnation

Hello, thank you very much for the answers, which, however, present us with a brain teaser: Where can we put a stove during the renovation phase without the chimney sweep beating us or tripping over the stove? Well, that's not the only brain teaser: Which impregnation do you recommend Mr. Gensky? Best wishes! Annette


Hello Annette and Jörg,

the impregnation is called Altrosil Dual for the chimney head and Altrosil Beto for the inside.
The masonry remains open to diffusion, becomes solid and water-repellent through crystallization in the mineral structure.

With best regards from Berlin

Werner Gensky

Hello, When the outside of the chimney is tight and dry again, the soaked plaster or the masonry can also be prepainted with clay and fresh cow dung 1: 1 and then plastered with clay plaster with cow dung about 2 cm thick - has become over Proven centuries in such cases and is made by us
still used. We have already made entire smokehouse habitable with it.

Shit isn't always shit, it just has to be fresh

In this sense
and best wishes for the new year
Rolf Eggersglüß let clay and clay

Sacrificial plaster that's good

Hello Annette,
I tried an old recipe. The mixture sounds a little getting used to, but prevents the renewed penetration of the sott.
Lime mortar plaster (sump lime) with cattle manure additive. If the problem is still current, I will check my documents for the correct proportions.
You could now remove the steel pipes and create a generous ventilation opening on the ground floor. The chimney head should be covered with a concrete slab (put on spacer stones). This allows the chimney to ventilate well and, best of all, to dry it out.
If something is still unclear, you are welcome to call through.
Greetings and good luck
A. R. Thams