Repointing with lime

Grenage

My 1905 house has lime mortar and the front is sorely in need of some TLC.  My neighbour repointed his about 20 years ago using a cement mix; it looks good and none of the bricks have suffered, but I'd be more comfortable replacing like for like.

I've never repointed so I'll probably practice a little down the side before I get the scaffolding in.  Some reading suggests 1:2.5 lime to sand, but has anyone got any tips?  I assume the working time is longer with lime than cement mortar.

stuart45

The working time is longer than with cement, but it also depends on the type of lime such as hydraulic lime goes off quicker than an air lime. NHL 3.5 goes off quite quickly when used for pointing, although is quite a lot slower when used for building.
Make sure the brickwork is well damped down, but allow the surface to dry.

Grenage

Cheers, Stuart.  I don't know what lime was originally used so I was thinking of using lime putty - the non-hydraulic stuff.

Apodemus

You might be best to use a lime putty, rather than dry lime for your mix and you can probably go up to 1:3 or even 1:3.5. You can easily make your own putty, just dump a whole bag of hydrated lime (not hydraulic!) into a cheap plastic bin, top up with fresh water, stir it well and leave it for a week (or longer if you can).  Wear a mask and eye protection when handling the dry lime.  As Stuart says, wet the wall well and give it a fine spray after pointing as well.  If it is going to be very dry and hot, cover with sacking and damp that down, too.   Working time is almost indefinite as long as you keep it damp - the left over mix in your bucket at the end of the day can be covered in water and used the next day (or the next week) by just pouring off the excess water.  In fact, I've always found that the leftover stuff is a lot more plastic than fresh (I am possibly too impatient and don't mix the fresh stuff well enough).

It might also be worth paying attention to your sand, too, if you want to match the original pointing.

Grenage

Hi Apodemus.  Thank you, I assume you mix the sand in before leaving it for a week - or is that just to get it ready for mixing?

stuart45

What sort of pointing style are you going for? eg Flush, Weather struck etc.
A lot of people are starting to use hot lime now, as this is probably closer to the original. Analysis of some older mortars has shown the mix to be closer to 2/1 or stronger at 1.5/1.  This is due to hot lime being mixed at 3/1, but it expands quite a bit and makes a stronger mix. 
The sand you use depends on the size of your joints as well. Small joints need a finer sand. 
If you ever want a sample of the original sand, stick some of the original mortar in acid and you will be left with a sample of the sand.

Apodemus

Grenage said:

Hi Apodemus.  Thank you, I assume you mix the sand in before leaving it for a week - or is that just to get it ready for mixing?

No, you mix the dry lime with an excess of water in the bin. The lime settles to the bottom and over days and weeks, forms an ever thicker putty under a layer of water - it eventually looks and handles like mozzarella. You dig the putty out from under the water and mix that with your sand/grit. Some folks would advise you to buy lime putty, but it is so easy to make!

 If you want to get fancy, the water off the top of the bin, which is actually a weak calcite solution, can be brushed into the gaps in the pointing and the calcite helps to bind with the existing dusty residues and provides a good key for the new pointing.

stuart45

Hot-mixed Mortars: Advantages and Limitations (buildingconservation.com)
The Myth in the Mix: The 1:3 ratio of lime to sand (buildingconservation.com)
A couple of interesting articles you might want to read Grenage. 
It's also worth getting some advice from your local lime supplier. Mortars vary with regard to the climate, exposure and type of masonry. A general rule of thumb is that you repoint with a similar mix to the original, only just a touch weaker. 

Grenage

Thank you both, that's great information.  The existing mortar joints are what I would consider pretty standard thickness.  It's a fairly solid white colour, but I'll try the trick with dissolving the lime.

Grenage

Hi guys,
I've knocked up a load of putty using a few bags of lime.  Do you normally add any water to the sand and putty when mixing?

I was going to make it in small batches using a mixing paddle, not adding any water unless it seemed dry.  Some online sources add water, but others don't.

stuart45

See what it's like as you mix it. You want it to be workable but not too wet. Mixing it it well get's more air into the mix without adding too much water which is a common mistake when people first start using lime.