Spalling on exterior chimney

[paddytt]

Hello All
Advice for spalling on an exterior (retired) chimney stack, as per photos.

The lower part of the stack is not visible from the ground because it’s in a gully between two apex roofs, so instead of replacing or turning some bricks, could we have the lower part of the chimney lime rendered? It’s obviously worse where some cement re-pointing was done, but some bricks seem poor even where only lime mortar is present.

I’m also worried about the part under the thatch… any advice on that would also be appreciated. Its water reed thatch replaced 12 years ago, so likely a long time before its removed.

 

[Cubist]

Irrespective of anything else you would be wise to replace the heavily damaged bricks and re-point the entire chimney stack with a lime mortar, after of course raking out the cement mortars used in the past. This would be the best immediate action as you do not want the stack to become unstable in high winds. If you can, check the lead flashing to make sure its in good order and the mortar flaunching beneath it and also that around the chimney pot bases is in good order, and repair as necessary. The ridge of the roof, unlike the main body, will be straw and as such has a much shorter life expectancy. Typically that's in the order of 15 years so it may be due for replacement sometime in the next 2-3 years.

As to whether to render the lower sections of the chimney. That may be a good idea insofar that much of the spalling will result from heat/cooling variation and the greater accumulation of moisture in the faces of the bricks in sheltered spots. Almost certainly a thatched property would be listed today so if you decide to do this it's worth checking with your CO before applying a render. If its not listed then happy days - for now; they will come back whenever you apply for permission to do something significant.

As a matter of interest, what heating systems do you use and is the chimney equipped with a suitable flue?

 

[paddytt]

Thanks for that good advice - the chimney is “retired”, heating is gas central heating through a separate flue.

 

[Cubist]

Then it's probably also wise to check the seals, if any, at the entry to top of the old flue. These can be a source of water ingress to create damp issues elsewhere.