New plaster, old stains

28 Oct 2016
Reaction score
United Kingdom
We had a stain around the top of our chimney breast that we assumed was caused by damp. We've had the roof fixed, but the stain kept growing. We thought then that it could be salt staining from the chimney. As we were having the room redecorated, we chipped off all of the plaster in the vicinity and replaced it with cement mortar. This was then skimmed over, and everything was fine for about two weeks. However, we now have a stain showing in the same place.

Does anyone know why this could be? And what we could do to fix the problem? Thanks
Sponsored Links
Have you got caps on your chimneys, have you got loose lead flashings on roof around chimney, have you got cracks in the render (if you have render on the chimney) Have you any loose tiles or the chimney vented if you have caps on it ? Just a few things it maybe. Is the chimney used?
The roof and chimney were looked at and repaired approx. 6 months ago, so I hope there are no issues with loose tiles or flashings - although I haven't been up to check myself). The render to the top of the chimney was replaced at the same time, and there are caps on the chimney. There is a flue from the gas fire that uses the chimney, but it is no longer used otherwise. There's a vent in the chimney breast in the upstairs bedroom, where the old fireplace has been blocked in
Might be condensation forming in the chimney . Take the vent off (If you can ) and see if there is a draught, as there might be something causing condensation in the flues, just light a taper and see if the chimney draws the smoke up. Pity you cant get a chimney sweep to rod it for you and you would know that there was no blockages then..
Sponsored Links
Do you mean by "caps on the chimney" an approved gas ventilation terminal for the gas fire?
Same with "render to the top of the chimney" - perhaps you mean the flaunching on top of the stack? The render referred to above means, presumably, rendering around the face of the chimney stack?
Unless you go up yourself or have a video from a roofer you cannot be sure of any work on a roof.

I'd suggest that you open up any blocked off fire place and have the flue(s) swept.
If you block the openings off again then use a vent.
All flues must be vented at the bottom (in the room) & at the top (stack terminal) - dont use metal vents on terminals, they rust out immediately; use clay terminals..

The "upstairs bedroom" fireplace has a separate flue - as do all fireplaces.
Have you been in the loft & examined the chimney breast(s) up there for damp or staining?

You will have to remove all stained render & re-render with a 3:1 sand & lime mix and a remedial finish. Dont use any gypsum.

Photo's of the exterior chimney breast and/or the stack might help?
What is this 3 to 1 sand and lime mix and a remedial finish?..

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local