Lime or cement mortar?

[twm.bwen]

Hi there, I'm going to be building the dwarf wall on my porch out of the local soft limestone.

I know that when it comes to repointing this should be done with Lime mortar, and I will indeed be pointing my porch with it. But I was intending to lay the stone with a cement mortar as I assumed this would be stronger, leaving enough depth for pointing later with lime mortar. (inside will be renderned with waterproofed cement, slurry then render again.

Am I wrong in thinking this (bearing in mind the wall is only single skin 150mm so I I leave 25mm for pointing I lose 1 sixth of the contact for cement)
Should I just build it with Lime paste from the start? (Certainly more hassle)

Thanks for any tips,

Tom

 

[CG]

see

//www.diynot.com/forums/viewtopic.php?t=92462

 

[twm.bwen]

That's a great resource for when it comes to repointing, but I still don't know If my origional idea of building the wall with cement (considering it'll be rendered on the inside) then pointing with lime is ok.

The reason for this choice is that its easier and quicker.

Any problems with it?
Thanks

 

[CG]

I would do the whole job with a lime mortar, ignore the cement as lime will be strong enough once it has gone off - look at all tthe old stone cottages and buildings down in the Cotswolds etc the were built using lime.

The idea that you use a lime mortar for repointing is that the original stone were laid using lime and you are just stopping water getting in the joints by topping it up.

http://www.spab.org.uk/html/advice/technical-qas/technical-qa-11-repointing/

http://www.buildingconservation.com/articles/cement/cement.htm

I hope you are using a lime plaster on the inside as I have just come back from a place that had a modern plaster on the internal stonework and has caused some nasty damp due to lack of breathability etc and has to be hacked off (it was only put on 5 years ago). Also I would not agree it is easier as lime takes longer to set up and you have more time to get it nice and tidy ( also cleans up from spills on stone work face as cement will stain)

This typical of what local councils recommend ( http://www.peakdistrict.gov.uk/repointing.pdf )

 

[twm.bwen]

Thanks for the help.

Due to Aethetics and trying to get max internal space the wall are only single skin. So if i have any chance of stopping penetrating damp it is by tanking the inside with render:hydradry:render. Unless there is another method (except drylining - hate the stuff) this is not an option to lime render the inside.

This is the accepted way of treating damp in rubble filled walls. I assume it is hoped the wall can breathe sufficiently through the outer skin.

My understanding of why cement pointing is bad is that it is harder than the stone ich wears first leaving a gap behind the pointing into which gets water and blows the front of the stone off.

I am hoping that with a good inch of lime pointing that this problem should be sufficient to protect the stone for my life time. Using cement up to this last inch seems easiest.

I am still not aware I why I should NOT do this.
Any idea?
Thanks again

 

[CG]

I think I understand correctly that you are building a new dwarf wall for your porch, and for that I would use the lime - sand mix.

For the single skin rubble wall I think you are looking at it like you would a modern cavity wall with barriers and what not, I would take a look at it from the point of view that you need a breathing wall and for this you need lime plaster on the inside, lime mortar in the joints and lime pointing on outside. This is so that any damp in the inside of the building (cooking, showers and washing machines etc) can be absorbed to the outside via the softer lime and not just condensate on the inside which it will on modern plaster, internal heating and ventilation sorts this. To prevent penetrating damp you must make sure that your gutters etc are tip top and you have good drainage with no ways of water being trapped against the bottom of the wall, concrete paths are normally the culprits but french drains fix this.

I have a solid 9" soft brick house with lime internal plaster (hacked off cement stufff from 1970) and a once nasty damp wall is now bone dry and the room feels much warmer. This is due to a breathing wall

You are correct about the faces of the stone blowing due to cement as it traps the moisture and it follows the path of least resistance which is the stone!

 

[twm.bwen]

Fair enough, I have heard people singing the praises of lime, but I was unaware it could work effectively as an internal render for controlling damp.

I understand about it allowing moisture to move through it and breathe but this surely works on a humidity/saturation gradient as moisture moves from higher to lower levels or saturation. So if it is damp outside and comparatively drier inside the movement will be inwards.

Considering this is generally the case, how can it be that penetrating damp does not reach the inner render?

When the damp specialist came round he was saying that there was never a problem with damp in old stone cottages because the lime render/pointing breathed. BUT that changed with the use of central heating that dried the air and the render forcing movement of moisture into the house. To me this makes sense.

Any idea, why the two ideas conflict?

 

[CG]

It is true that modern life can induce damp but with good ventilation and air flow you can prevent this, just think of your bathroom after a shower it is steamy and the condensation will run down the paint causing mold on the windows etc now if you have a good extractor fan you have no steam what so ever. This is due to ventilation, so if you dont run a tumble dryer etc with no windows open or vent to drain etc you will not have this problem.

If you look at older buildings you will notice they generally have bigger overhanging eaves so water drips away from the wall and if the ground is properly sloped away from the building it will remain dry, remenber your walls are not submerged in water they just get wet from rain and due to evaporation it will dry the walls out (as long as it allows it with no cement pointin). If you have ever been in a straw bale house (some at the CAT in Wales) it will become apparent that this is the most basic of construction but one of the nicest to be in due to the good non stale air and one of the warmest.

When you get some damp firms in, they will knock off the cement render on the inside and out of the lower 1m of the external walls and then fix the timber sole plate, then replaster in lime. This will let the timber breath and not rot due to being encased in its own damp (wood has a moisture content), also there are many houses still standing from 1600s that are nice and warm and dry and they are the ones usually that have not been "modernised" using cement and plastic paints.

Remember that ventilation is key along with working drainage to avoid internal damp