Lime or cement mortar for extension to stone cottage?

[civilgingerbeer]

We are currently having a single storey extension built for our 1800s stone cottage. The external wall will be a cavity wall with a local sandstone facing to match the rest of the house.
I understand that the extension wall won't breathe in the same way as the rest of the house.
As the stone will match well with the rest of the house, we would like the mortar to as well if possible. The rest of the house looks like it has been pointed with lime.

Is it worth building the outer skin of the extension with lime mortar or cement mortar (if the builder can get a close enough match). Or would it make sense to build up the outer skin with a cement based mortar but point with a lime mortar (to match the rest of the house in appearance but also act as a sacrificial layer to prevent the stone spalling)?
Thanks in advance!

 

[Stevers]

A crucial point is that the mortar must be weaker than the stone or brick. A mortar made with Natural Hydraulic Lime (NHL) would have most (if not all) of the benefits of an old fashioned lime mortar, which might have been partially hydraulic anyway through contamination of the lime or aggregate. NHL mortars are no different in use to a cement or cement/hydrated lime mortar, and readily available from your local builders' merchants.

 

[stuart45]

If you want a good colour match, the sand is really important. Stick some of the existing mortar in acid to leave you with a sample.
Are you using a backing wall for the stone?

 

[civilgingerbeer]

A crucial point is that the mortar must be weaker than the stone or brick. A mortar made with Natural Hydraulic Lime (NHL) would have most (if not all) of the benefits of an old fashioned lime mortar, which might have been partially hydraulic anyway through contamination of the lime or aggregate. NHL mortars are no different in use to a cement or cement/hydrated lime mortar, and readily available from your local builders' merchants.

Great. Thanks for the advice.

 

[civilgingerbeer]

If you want a good colour match, the sand is really important. Stick some of the existing mortar in acid to leave you with a sample.
Are you using a backing wall for the stone?

Thanks.
The stone facing will be tied back to an inner skin of clockwork.

 

[Stevers]

If you want a good colour match, the sand is really important. Stick some of the existing mortar in acid to leave you with a sample.

Somerset has a range of sands and stone dusts that are local to the area. Most of our builders' merchants offer a range, but the choice will vary from store to store depending on demand largely driven by the local building stone. For instance 'ginger' sand definitely a Yeovil thing!

 

[civilgingerbeer]

Somerset has a range of sands and stone dusts that are local to the area. Most of our builders' merchants offer a range, but the choice will vary from store to store depending on demand largely driven by the local building stone. For instance 'ginger' sand definitely a Yeovil thing!

Thanks!
Do you know if it would be better to use NHL 2 or 3.5? The extension faces west so will get the brunt of the weather.

 

[Stevers]

My local merchant does a 'cement free' NHL 3.5, and that's all I ever use. NHL3.5 is classed as 'moderately' hydraulic rather than 'feebly' hydraulic for the NHL 2.0. The average brickie won't find anything odd about the initial set of NHL 3.5, and an exposed position would suggest it's the better choice.

Of course in terms of the final appearance, there's huge artistry in what might be termed the 'bonding' of stonework, So coursed rubble vs. random rubble, vs. random rubble brought to course etc. My builder asked one of his blokes to fill in an opening in our end wall, but once he saw how badly it was going he took over and was able to blend the new work in with the old with what would be an invisible repair if the 'new' stone wasn't so clean on a cold north east facing wall riddled with chimney flues. Interestingly this was 'just' a cement:lime mortar with sand and stone dust as aggregate. This will be the effect I'll be aiming for when I stone in the back of my basement fireplace - gulp!

 

[aockenden]

If i were doing anything in sandstone I wouldn’t let any cement touch it (neither bedding nor pointing mortar). I would also personally go for the NHL 2 over 3.5 for sandstone.

 

[civilgingerbeer]

My local merchant does a 'cement free' NHL 3.5, and that's all I ever use. NHL3.5 is classed as 'moderately' hydraulic rather than 'feebly' hydraulic for the NHL 2.0. The average brickie won't find anything odd about the initial set of NHL 3.5, and an exposed position would suggest it's the better choice.
View attachment 13202
Of course in terms of the final appearance, there's huge artistry in what might be termed the 'bonding' of stonework, So coursed rubble vs. random rubble, vs. random rubble brought to course etc. My builder asked one of his blokes to fill in an opening in our end wall, but once he saw how badly it was going he took over and was able to blend the new work in with the old with what would be an invisible repair if the 'new' stone wasn't so clean on a cold north east facing wall riddled with chimney flues. Interestingly this was 'just' a cement:lime mortar with sand and stone dust as aggregate. This will be the effect I'll be aiming for when I stone in the back of my basement fireplace - gulp!

Thanks again for your help

 

[stuart45]

Thanks.
The stone facing will be tied back to an inner skin of clockwork.

So it's going to be inner blockwork, insulated cavity,Surecav and then the outer skin of sandstone?

 

[civilgingerbeer]

So it's going to be inner blockwork, insulated cavity,Surecav and then the outer skin of sandstone?

Pretty much yea. Don't think Surecav has been specified though.

 

[stuart45]

Pretty much yea. Don't think Surecav has been specified though.

It depends on the stone. If the stones were random thicknesses a block backing wall was needed, until Surecav arrived.

 

[civilgingerbeer]

It depends on the stone. If the stones were random thicknesses a block backing wall was needed, until Surecav arrived.

Just been googling this and was completely unaware of this requirement. We are using 150mm thick split face stone. Building control signed off the drawings, without backing block or Surecav specified.

 

[stuart45]

If the stone is a regular 150mm thick it's not an issue. It's when you you have some stones 100mm, others 150mm etc. In this case you need a back to fill in to with mortar on the narrower stones.

 

[civilgingerbeer]

If the stone is a regular 150mm thick it's not an issue. It's when you you have some stones 100mm, others 150mm etc. In this case you need a back to fill in to with mortar on the narrower stones.

Thanks again for your help. I think the quarry say "up to 150mm" so there may be a bit of variation. Perhaps the architects intent was to fill mortar directly against the insulation as they specified Xtratherm PIR Cavity Therm Full Fill Insulation... but not too sure.

 

[stuart45]

Xtratherm has got rebated edges to stop any moisture that penetrates the stone from teaching the inner skin.