pointing sandstone

[davidbrac]

I am gong to repoint the sandstone around our windows and pillars on the house corners. I have pointed before but have never pointed sandstone or anything with a small gap as these have. The other issue l for see is that white lime putty will stand out like a sore thumb.

Does anyone have any hints and tips on method and mortar mix?

 

[Zebra]

The colour of the mortar will depend on the colour of the sand you choose. This can get to be a complicated topic, but needn't be. You can get your existing mortar analysed, and be sold a particular type of sand to match exactly your original mortar, but if I were you I wouldn't go that far. I would simply purchase some soft sand (also called building sand), some sharp sand, and mix it in a ratio of 2 soft: 1 sharp; 1 lime. As for the type of lime, there are a number of options. You can buy hydrated lime from your local builders merchants - check first that they've got it, they don't all carry it. You can buy putty if you can find a specialised supplier. Or if you have time, you can make putty out of hydrated lime - it turns into really nice goo after being stored in a sealed tub for three months. Or you could buy some NHL, but it's harder to use.

 

[plasticpigeon]

Hello David

I'm sorry zebra I wouldn't use sharp sand. Every bag of sharp sand from a builders merchant I have ever bought has grains up to a few mm wide, maybe as large as 5 or 6mm. In my opinion this makes it very very difficult to work with and makes the finish very rough. The very thin joints in the masonry would make using sharp sand almost impossible and I think the finish would be too rough for such refined masonry. Plastering sand has sharp grains but is much finer than "sharp sand" and makes the mortar easier to use and I much prefer it as a base for mortar with building sand added for colour. Are the joints wide enough to use a finger trowel?

 

[88v8]

I'd use NHL, the grade would depend how hard your stone is, the pointing should be softer than the stone.
Sand... silver sand is 'sharp', but very fine. Might be more easily found at the garden centre.
Bit of old hessian sack to rub the surplus off the face.
Unsunny day so it doesn't dry out before calcifying.

Is that a bell pull to the left of the door? Lovely detail.

Ivor

 

[davidbrac]

what does NHL stand for?

 

[mattice]

" "

We don't have sandstone but the gaps are very small. I made myself a little pointing trowel of some flattened steel wire that fitted the gap, just enough to pick up a bit and slide it in

 

[Zebra]

Natural Hydraulic Lime, isn't it?
I can see what you're saying plasticpigeon about sharp sand being to coarse for such a small join, plastering sand seems a good solution.
NHL comes in three different grades - 2, 3.5, and 5.
NHL2 is quite weak, though not as weak as a putty mortar.
NHL3.5 is stronger, and NHL5 stronger still. For every increase in strength, there is a corresponding decrease in breathability, until you get to cement mortar, which is very strong, but not very breathable. Whereas putty mortar is weaker but more breathable. The weaker the mix, the more it will allow for movement without cracking.
So as Ivor says, it depends on how soft your sandstone is, because you shouldn't use a mortar which is harder than your sandstone. A supplier would have more information, you could try the practical guides and technical information on the Old House Store's website.

 

[Andrew Bodenham]

David,

As you can see from the variety of replies there are many approaches you could take....

Some basic principles to consider: the mortar should always be softer than the surrounding stone, so as to act as a sacrificial element in the wall. Soft mortar will yield to thermal movement and allow water to evaporate quickly from the joint. Hard mortars tend to trap water and can cause spalling of the adjacent stone. However, if the mortar is very soft then durability may be compromised....

Colour can only really be gauged by making trial mixes and seeing how they dry. It's the sand which give the colour - for the small amounts you need I'd be looking at the local builders' merchants to see what their sharp sand is like - you need a gritty sand with a good range of particle sizes from about 3mm down, and the grains should be angular, not rounded. Others have mentioned the problem of large particles in fine joints, and it's certainly the case that your largest particles should be easily able to fit in the joint. For doing very fine joints ( a couple of mm or so) you can always sieve out the larger sand grains - this will help to keep a consistent colour - rather than use a completely different sand for some joints.

I would buy ready-made lime putty in a tub to make your mortar. I would not try to make putty from bagged hydrated lime powder from the merchants, as the quality will be inferior. Ready made putty will keep indefinitely in its tub if you cover it with water - just pour the water off before using.

Cleaning out those fine joints can be done effectively using a carbide grit cutter on a Multi-cutter power tool (Fein, Bosch GOP etc.) which can bought quite cheaply these days. Alternatively a fine bladed scraper or chisel may suffice. Never clean out joints with an angle grinder. In all cases take great care to avoid damaging the stone.

Wet the joints thoroughly before pointing. Mist the work if it's a warm / drying day, and cover with wet hessian for several days after completeion to allow slow development of strength. Mix should be quite dry and well packed into the joint. Once just set you can compact it with a stiff brush (stippling action, not dragging) which will knock off any stray material.

If it all goes wrong you can always rake it all out and start again!

 

[Roger H]

Hi David – is it the finely dressed stone jams and cills that you are looking to repoint or the surrounding walling with the thicker joints? The windows surrounds don’t look that bad to me. Do they definitely need repointing?

I recall that there is some guidance on repointing fine ashlar joints in Edinburgh New Town somewhere on the web but it is quite a tricky business to do neatly – masking taping surrounding joints was recommended.

If I were you I would practice on the cement pointed areas to the left of the window, or somewhere discreet until you have mastered the basics.