Difference between buff and terracotta chimney pots?

[RigidRaider]

Went up to repair a rattly cowl yesterday evening (dry roof and no gale for the first time in a month) and noticed that the buff chimney pot is beginning to crumble around the rim, presumably thanks to sulphuric acid being formed from the flue gases. I'm quite surprised; we have only owned the house and been burning solid fuel for 7 years. Is there a difference in hardness and acid resistance between buff clay and red terracotta clay pots?

I also put a new layer of flaunching on the old about five years ago using a 3:1 mix and this has almost all been washed away, with a couple of "valleys" formed in it by running acidic water.

So I will renew the lot next summer. What kind of mortar should I use for the flaunching? Are all pots equal or are there some better than others?

 

[cotswoldbuilders]

I dont know about the pots, but the sand/cement mix sounds ok, did you use a sharp sand or granite chippings in the mix for the flaunching.

 

[RigidRaider]

Thanks, I don't know what I used. What do you recommend?

 

[cotswoldbuilders]

Id use a granite mix, sets like concrete, you will probably only need 1 bag from the builders merchants.

 

[^woody^]

Fired clay should be resistant to flue gasses, whatever colour it is. You may have had a badly fired pot which has been attacked by frost rather than acid, or it may be just at the end of it's life

As for the flaunching, it would be unusual for these valleys to be acid related. More likely a poor surface finish, and surface not evenly angled to allow run-off

To protect form flue gasses, you need to use a sulphate-resistant cement, and not ordinary portland cement, but you should not need this for the flaunching

To give the flaunching a good weathering finish, you need to trowel it nice and smooth with a steel trowel when it is starting to set, to bring the cement up and close the pores and harden the surface

 

[RigidRaider]

Ah! A bit like polishing plaster!

Great advice all round, thanks!