Mortar mix for pointing granite.

[sb1202]

Need to do remove and repoint a granite wall where the previous owner seems to have slapped some postcrete type mortar. Spent the last 2 hours reading/googling on which mortar to use and I'm more confused than when I started. Some say, as its granite and very hard, to use a hydrated lime mortar of 1+1+5 or 1+1+6, other say use NHL5 and my head is bursting. Walls are granite 600m with a rubble in fill.  Pic shows the original good pointing but I have no idea what mortar mix this is.

Any advice appreciated.  

[nod]

4-1 

[sb1202]

4 hours ago, nod said:

4-1 

4-1 what?

[MikeGrahamT21]

Doesn’t look lime based, not light enough.

 

as @nod said, 4-1 sand and cement will be ideal for granite 

[Mike]

If the build is as old as a 600mm thick rubble-filled wall suggests, then the chances are that it's lime mortar, so I'd repoint in that to maintain / restore the vapour permeability & flexibility of the original mortar. You'd need to knock through the existing pointing to see what's behind if you think the original could have been cement-based, but that's only been commonly used since start of the 20th century.

 

1:3 NHL3.5 or NHL5 to sand would be the easy mix; since it's granite the latter would typically be chosen to match the hardness of the stone (NHL5 is usually, though not necessarily, stronger than NHL3.5). Lime putty would be the traditional material with superior properties (more permeable & flexible), but would be harder to use on granite - it would take longer to dry - and isn't suitable if the wall is regularly wet. Having said all that, I'm not a lime expert.

 

Edited January 21, 2025 by Mike

[sb1202]

5 hours ago, MikeGrahamT21 said:

Doesn’t look lime based, not light enough

It's definitely lime based. It was renovated in 1982 and the plans stated the pointing and binder were lime mortar - it doesn't specify what though.

[sb1202]

1 hour ago, Mike said:

If the build is as old as a 600mm thick rubble-filled wall suggests, then the chances are that it's lime mortar, so I'd repoint in that to maintain / restore the vapour permeability & flexibility of the original mortar. You'd need to knock through the existing pointing to see what's behind if you think the original could have been cement-based, but that's only been commonly used since start of the 20th century.

 

1:3 NHL3.5 or NHL5 to sand would be the easy mix; since it's granite the latter would typically be chosen to match the hardness of the stone (NHL5 is usually, though not necessarily, stronger than NHL3.5). Lime putty would be the traditional material with superior properties (more permeable & flexible), but would be harder to use on granite - it would take longer to dry - and isn't suitable if the wall is regularly wet. Having said all that, I'm not a lime expert.

 

It's definitely VERY old so it's not cement. It's at least 300 years old according to historical maps and possibly goes back to the 1400s.  Our deeds say it was part of an estate and the castle was built in 1604 on top of an existing watch tower from the 1400s. The walls aren't regularly wet - we're in Aberdeenshire which is broadly similar to the SE of england for rainfall. I've been researching what I can but drawn a blank. I do know that the rubble isn't bound with a mortar - it's just mud/soil. The folks at historic scotland said this was common and this type of property last hundreds of years with the original walls. 

[lookseehear]

I've gone down the rabbit hole multiple times with lime, and the only thing anyone seems to be able to agree on with pointing is that it should be softer than the bricks or stone.

 

The heritage route would be a non hydraulic lime mortar with some pozzolans (natural additives like brick dust, wood ash which act as a source of carbon to help the mortar go off). This would be best given that your walls are constructed with mud, as its soft and will help to keep the joints dry. It's also very slow to go off and can only be done in the milder months.

 

This YouTube channel has a lot of information on lime https://youtube.com/@michaeljamesdesign?feature=shared if you want to do some research.

 

A mild 6:1:1 sand to cement to hydrated (bag) lime is probably the pragmatic answer if you want to get it done in a hurry.

[jfb]

I’d just go with 3.5 nhl (not hydrated) at 3:1 ration sharp sand/nhl

doesn't want to be done if there is  any danger of freezing and needs to be done by somebody who can work at the right pace (not like cement where you can have a section finished in a day - definitely be brushing off a section a day after the mortar has gone in).

 

[Mike]

21 hours ago, sb1202 said:

It's definitely VERY old so it's not cement. It's at least 300 years old according to historical maps and possibly goes back to the 1400s.  Our deeds say it was part of an estate and the castle was built in 1604 on top of an existing watch tower from the 1400s. The walls aren't regularly wet - we're in Aberdeenshire which is broadly similar to the SE of england for rainfall. I've been researching what I can but drawn a blank. I do know that the rubble isn't bound with a mortar - it's just mud/soil. The folks at historic scotland said this was common and this type of property last hundreds of years with the original walls. 

A very interesting history! I've not seen earth mortar before, though in several regions of France they built (as recently as 100 years ago and occasionally still today) 5 or 6 storey buildings from a mix of earth and stones, compressed in-situ (pizé) - so it can be very strong, if protected from the weather - they're usually covered in a lime or clay render. You'd certainly never use cement on them.

 

I've just taken a look online and, if you're not already found it, there's a short pdf on clay mortars here: http://openarchive.icomos.org/1801/1/Historic Environment Scotland_Mortars for Masonry Buildings.pdf which, states that "most clay-bonded walls were externally pointed or harled with lime mortar to provide additional protection from the weather". There is a phone number at the back for Historic Scotland's technical advice department, though it looks like you've already talked to them. But maybe they can help with the identify the mix used, if you can find any original pointing mortar behind the existing.

 

Having just looked it up, the process for what we now call 'natural hydraulic lime' wasn't developed until 1756, so lime putty would no doubt have been used - perhaps burnt & slaked on the estate if there is limestone in the region. Lime putty & sand, probably using a 1:3 mix, would therefore be my fist choice. That would mean repointing it slowly, starting no earlier than Easter so it has the best drying weather. There are courses you can go on to learn more if that interests you.

 

Failing that, I'd go with the 3:1 NHL3.5 to sand, as per @jfb.

 

Edited January 22, 2025 by Mike

[Roger440]

Im inclined to agree with Mike and jfb.

 

Id not go NHL5 as its got mud in the wall. NHL5 is nearly as impermeable as what you are taking off.

 

You could of course get some or the original motar out and get it analysed. 

 

 

[nobody]

Since I realised there is a single story stone building at the centre of my house (subsequently extended upwards and sideways with solid brick) that is built with earth mortar I've been reading up online. 

 

Research says that this was used extensively across Europe in stone buildings (eg 75% in Scotland pre 1850, 25% after) but use has been underestimated because it is usually covered to protect it. Or where exposed to the elements has washed out.

 

I'm only at the beginning but the advice in the following article is to try and repair it with something as similar as possible so I'd be cautious about assuming lime putty (which I think I read came later, but please correct me if I'm wrong). 

 

https://www.buildingconservation.com/articles/earth-mortars/earth-mortars.htm

 

There's a reasonable amount of information online if you search "earth mortar" and maybe "renovation". 

[Gus Potter]

As a suggestion for granite walls call:

 

https://masonsmortar.co.uk/

 

The link to their website is above. 

 

6 hours ago, nobody said:

Since I realised there is a single story stone building at the centre of my house

This is a completely different animal. I'm assuming as it's in the centre of your house it is an internal wall? 

 

Try and post some drawings, maybe some photographs, often most helpful and describe what you are trying to do / why and your thoughts. 

 

 

 

[scottishjohn]

If as it looks from picture it has been done with cement +sand --then no point in going lime based

 

 If you are lucky your local sand is greyish -- not red in colour 

 

 my suggestion would be to mix up a small test batch at 4 or3 to1 to get colour to match what you have

 

 granite is hard and non porous  but it can have hairline cracks  in the stones --so make sure you wet where you are going to point to make it stick well --plastiser helps 

 

 but if real problem is water ingress   and pointing is ok -then could be  very small cracks in the actual stones

 

-- then  bite the bullet and coat the walls  and stop any water ingress full stop

 

do not use  cheap ronseal  or erquivalent -- it will not last

 

 do it once and do it right 

 

 i know i sound like salesman for permagaurd -- but having been through this problem with old stone house  and inspected pointing  very carefully before coming to the decision to treat the walls

 

    I know what cured the problem  and easy to do 

 

[nobody]

[Note: this is not the granite building being discussed above - my stone is I think local mudstone] 

Thanks @Gus Potter  I don't have time just now and we're still discovering as we peel off the layers. I'll just say that the stone is mostly internal. This central part is 40cm stone at ground floor (pre 1820), solid brick above to second floor and loft room.

 

One gable end is a party wall against a brick built house (19th C. ?) and the other is external and rendered probably with the wrong stuff.

 

A solid brick extension on one side keeps that side of the stone internal, and a 20 C. single story block work extension keeps the stone wall internal on the other side, with some solid brick exposed, sitting on top of the stone between the eves and the extension roof below.

 

There's damp from two roof valleys on the party wall side, and from the ground elsewhere. Three levels of concrete floors and no central heating so we're removing the internal plaster (different kinds in different areas) and planning to dig up, level and insulate the floors.

 

We'd like an extension that will bring the gable end stone wall inside, but will have to see if we have the budget (and the energy ) after the essential works are complete or at least known.

 

We're aiming to keep the character (old floorboards, two staircases from when the stone cottage and the brick building to one side were separate, the old internal doors etc). 

 

I posted a bit in the introduction a few weeks ago, but have learned more since that. I'll no doubt be back once I understand more and am discussing my builder's plans.

[scottishjohn]

28 minutes ago, nobody said:

Note: this is not the granite building being discussed above - my stone is I think local mudstone]

same applies but that probably is porous 

 sandstone is a type of mudstone and it is porous