Make a motar mix for stone roof tiles?

[Mallimak]

Hi everyone

Can anyone give me a good recipe for mixing some mortar to repair the pointing used between stone tiles on a cottage roof?

Is it just a mix of sand and cement or are there other things I should be adding?

I can see a few places on the roof that need repairing and I'd like to get the repairs done before winter sets in, weather permitting

[Odsox]

It used be that you added lime to the mix or even as a substitute for cement, as it makes the mortar more flexible when it sets, stops the joints cracking when the tiles flex through temperature and wind action, but that might be old hat nowadays.
Can't remember the ratio though, probably 3 to 1.

[diggernotdreamer]

We are using lime mortar at the moment for repointing our stone house we use one part hydraulic lime (St Astier Lime Co) to three parts sharp sand, it is important that you use the sharp sand when making the lime cement and not builders sand. You mix half the sand and half the lime in the mixer until it is mixed, then the rest of the sand and lime, add a little water, let it mix, mix more water and keep mixing, the mixture will 'fatten' up just be careful adding water, not the same as making concrete, you need to aim for a mix that will clunk nicely in the mixer and not slosh. Lime mortar is essential for old buildings as it allows the building to breathe properly and is a more flexible medium than modern concrete

[grahamhobbs]

I agree in principle with the previous replies but be careful there are different types of lime, hydraulic as diggernotdreamer is using and hydrated which is the lime normally stocked by builders merchants and then there is lime putty. Traditionally lime putty was used in mixes 2 or 3 sharp sand to lime putty (2 to 1 being stronger). You can make a passable lime putty using hydrated lime by mixing with water and leaving for 3 or more days. You can't do this with hydraulic lime, because it 'sets', it is as if the lime has a small portion of cement in it. By the way, in France most lime is hydraulic.

Having said all that I see that you are in the Orkneys where I guess the weather is pretty bad and one would normally use a stronger mix in such situations. However the mortar should always be weaker than the stone you are working with.

Lime putty takes about 6 months to achieve it's full strength, even then it is relatively weak, which is probably too long anyway in your weather. Hydraulic lime will set quickly and achieves its full strength straight away. You can achieve a similar result by adding a small amount of cement to the normal hydrated lime, so perhaps a 1 lime : 1/4 cement : 3 sand. This is still a fairly weak mix. A 'normal' mix these days is 1:1:6 and 1:1:3 the strongest you would ever have for brickwork at roof level.

Sorry if that has complicated things, but it is a balance of strength against the weather, but not being too strong for the stone and having lime for flexibility.

[Skippy]

Just to complicate things a little further.
I've spent most of my life in restoration albeit working with wood but it's the kind of profession where one picks up a lot from other trades.
I've worked at times with Stafford Holmes who is allegedly one of the foremost "experts" concerning lime. When we first heard of him we all thought that it was an organisation like wimpey homes , doh.
Anyway, very small amounts of cement in lime mortar can actually make it weaker. Some builders have added a bit to firm up the mix and make it "set" but it's not the best idea. We have normally used lime putty and have at times added ground brick dust to mixes where colour allows. The brick dust has what is known as a "pozalanic" affect on the mix and acts like cement. Pozalanic comes from Pozula in Italy which I believe is where the romans come up with their volcanic ash based concrete.
Follow the advice about having the mix weaker than the stone and don't let it dry too quickly , it may need a spray over with water periodically.
Should you wish a little light reading then a quick look here may help-http://www.buildinglimesforum.org.uk/building-with-lime


Pete

[Mallimak]

Thanks for the feedback everyone.

I'll have to find out what type of lime is available up here, may be a case of picking some up when I'm on my next trip South.

I'll probably be doing small areas at a time so it is likely to be hand mixing rather than mechanical. That's should keep me fit

[diggernotdreamer]

www.masonsmortars.co.uk, they are in Scotland and stock the St Astier lime