Victorian House - Re-pointing chimney stack

[matt80]

Hi all,

I'm having the roof on my Victorian semi re-slated next week. As part of the works, one of the disused rear chimney stacks is to be removed and slated over and the main one at the front requires some spalled brickwork replacing and some re-pointing.

The wording they've used is: "Overhaul all brickwork and pointing to front side chimney stack, re-flaunch all existing pots using good 3:1 washed sand/cement mortar".

I'm sure I've read on here that Victorian houses generally require the addition of lime in the mortar otherwise it can cause damage to the brickwork over time. Should I be instructing them to use a different mix or will this be OK?

Thanks for any advice.
Matt

 

[skier-hughes]

Depends on what is was built with :
Either sand and cement or sand and lime, not a mixture of sand, lime and cement.

 

[MdB]

General idea is that the mortar should be weaker than whatever it is sticking together. Cement is fine for modern bricks but older bricks are weak which means you should use a weaker mortar to compensate. Lime + sharp sand (absolutely no cement) is suitable alternative and has the advantage that is very good at absorbing and releasing moisture where as cement is waterproof in comparison. This means with lime, it is more likely that the mortar that fails in a frost rather than the brick as it is likely to contain more moisture. This protects the brick but does mean you will need to repoint every few decades or so. Better than damaged bricks though.

For exposed repointing / rebuilding NHL lime is a good choice - NHL 2.0 is probably the best for this application but 3.5 is available from Bradfords and is what I used.

The exception to this is the flaunching of the chimney, for this you want something strong and waterproof as it is going to be hit by a lot of weather. I used cement with a waterproofer in it for my chimney.

Having said all of that, I'm not really an expert so read up what you can elsewhere. Just don't expect your builders to be keen on using lime.

 

[Feltwell]

As MdB says really.

Your roofers are highly unlikely to want to use "pure" (i.e. no cement) lime mortar - if they know it's use then they probably know it only as a plasticiser for cement mortar.

How much you push the point is up to you. If the rest of the house has already been re-pointed in cement based mortar then you may feel it's not worth fussing too much about the use of cement on the chimney brickwork. But - if your chimney brickwork has spalled, that may be because it's been re-pointed in cement mortar in the past.

For the flaunching, as MdB says, cement is best.

 

[Zebra]

Agree with above - your builders are unlikely to suggest the use of lime mortar, unless they're already well versed in its benefits and uses. Many Victorian houses have had their lime mortar replaced with cement, which may or may not cause further damage, as MdB says, depending on the softness of the bricks.
The addition of lime to a cement mix will only affect the workability of the mix, it will still be a cement mortar and behave like a cement mortar, as described by MdB above.
I don't know, it might be fine. I used NHL 3.5 for repointing my chimney, bought ready mixed with the sand from the Old House Store - there is a link on their website showing which mortars are best for which applications, but again, it depends on the softness of the brick.
Agree that cement best for flaunching, I left my cement flaunching in place and it has not caused any damage to soft brickwork of chimney.
If all this is happening next week though, isn't it kind of the wrong time of year to be thinking about repointing in lime anyway? Because lime mortar doesn't set quickly like cement mortar, if lime mortar is subjected to frost, the water still in it will freeze, and blow it apart.

 

[MdB]

I hadn't heard of readymix NHL before so looked it up. http://www.oldhousestore.co.uk/product/Moderately_Hydraulic_Mortar__-__25kg_LPHM0020025
It is more expensive than mixing your own but not by much. Useful to know it is available.

 

[Lime]

Overhaul all brickwork and pointing to front side chimney stack, re-flaunch all existing pots using good 3:1 washed sand/cement mortar

My translation of that is that any new pointing will be superficial but look good.
I doubt that any raking out will be done.