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Question DetailsAsked on 4/28/2016

What can I clean this with?What should I patch the plaster with or do I leave it alone. Do I seal it with shellac

The house was built around 1920. In the winter I smelled Ammonia in my finished attic room and found out it was coming from where the chimney goes up. It looked discolored and the plaster was cracked, like it was leaking from water coming down the chimney. I scrapped all the places where I could see where water got behind the plaster. It's like crystallized ammonia forming on the cracks. The smell went away after I vacuumed and swept. So I just had my chimney guy come and look at it. He put a chimney cap on and I should not have any more leaks. Now I need to clean it. As I was scraping pieces of the plaster came off. Under neath it is more crystallized ammonia. (I keep saying ammonia but don't know exactly what it is). What is this stuff? The plaster that comes off is a brown color. Some areas it is a light beige color with no crystals. The chimney itself is mortar or cement under the plaster. The chimney itself is brick. The rest of the room is fine no leaks.

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The brown would be either wood oils, rust, or creosote staining from the fireplace - depending on whether the leak was from outside the chimney (like flashing or through the bricks) or from inside to out like due to a missing/leaking rain cap. Sounds like the latter in your case.


I would suspect what you smelled was creosote smell, or just wet lime - not ammonia. The "bloom" or crystallized material you see is lime and gypsum compounds like anhydrite from the drywall and possibly the mortar in the chimney as well, dissolving and leaching through with the leak and forming crystals on the surface as the water evaporated. I would not breathe it in, and can cause mild skin irritation on some people so clean with latex gloves on, but will remove easily from surfaces by scraping or with vinegar.


Depending on how wet it got, the drywall may have just been contaminated and stained, pretty much disintegrated, or a combination. In a finished area, you would normally tear off all the drywall that is cracked up, soft, crumbling or significantly stained and replace it because the staining is likely to come through paint, and you may be creosote odors in damp seasons. Generally, you would tear back to where the drywall was unstained and also remove (with a solvent for oils) the creosote staining to the extent possible, because when it gets damp creosote puts out a pretty acrid petroleum odor - sniff a warm telephone pole or blakc treated railroad tie up close and you will recognize the smell.


In an unoccupied or unfinished attic, you might just tear out the worst of the really breaking up and crumbling drywall and replace it, then paint over the stained-only area with Kilz stain primer (oil based is FARRRR better than the water based stuff), then normal paint over that.

Answered 2 years ago by LCD




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