Chimney repointing

[mikegc]

hi,

I am looking to get my chimney stack re-pointed. It looks like it is lime mortar. The rest of the house has previously been repointed using cement, as I can see a few places where it is now failing and is a white crumbly mortar underneath.

Does it matter on the chimney if it is pointed in cement or lime, as it is all external anyway and shouldn't cause damp? Or should I specify to the builder to use lime?

Thanks!

 

[pilsbury]

Not sure on specific use of either lime or cement, but I’d suggest the repointing is failing due to not raking out the old mortar deep enough and just smudging new mortar on the top so it looks pretty until the workman has gone.

 

[Doggit]

You would be better of using a bag of ready mixed mortar form Wickes, as they already have lime in them, so would be more in keeping with the original construction. As Pilsbury suggests, make sure they rake out to an inch deep, and do the job properly.

 

[John D v2.0]

The mortar is a kind sacrificial layer supposed to degrade before the bricks so should be softer. With modern bricks that can be cement mortar as both are extremely hard. With older chimneys they probably used a strong mix ie nhl5.
So with an older chimney if you repoint in nhl5 it'll need doing sooner, but you won't need to rebuild the whole stack as soon.
Either way as above it'll need raking out well.
Edit: line mortar doesn't have any cement in at all, as soon as you add a bit of cement it becomes hard.

 

[Blagard]

Two important aspects of mortar are it should be weaker than the bricks and when repointing as noted above, deep enough.

If you think the old bricks are soft then use a mix like 1.1.6 which is cement. lime. sand In this case the lime acts more like a plasticiser. The worst you can do it repoint in a strong cement mortar.

For what it is worth I use that mix all the time as it is strong enough and workable. The only time I use a stronger mix is with blues at DPC level and when capping walls with blue engineering bricks

 

[Doggit]

If you think the old bricks are soft then use a mix like 1.1.6 which is cement. lime. sand

And that's what Wikes premixed mortar bags have.

 

[mikegc]

Thanks for the advice, I have asked the roofer to use 1:1:6 mix.

He has mentioned spraying it with water repellant once finished. We don't have any problems at the moment, so wonder if it is a good idea or just leave old 1880's bricks alone!

 

[Nige F]

Blue circle quality assured mortar mix 20 kg from Wickes - has lime+ plasticiser