Re-pointing cellar interior wall - advice please

Husband Roy has decided to tidy up courtyard cellar and make use of it as a summer room.

He has started to dig out the 'live' pointing cement and now needs to know the best mix to replace it. It's the interior side wall (exterior side is underground to neighbour's garden; as is the whole of the back wall, to our garden and about half of the lower depth of next side wall with final front wall and door access to courtyard) Hope this makes sense! Any guidance would be great as Im the one thats got to go and buy it! cheers :)

I am not sure how to advise you Michael. I have repointed all the inside of our stone walls and then on the ones I have insulated fitted a layer of multi-foil over battens fixed to the stone wall and then plasterboard over another set of battens over the multi-foil. This there is a 25mm air gap between the the wall and each layer. The PB is then plastered. Other parts of the house have been plastered a long time ago with what looks like lime plaster and I have generally left these alone. In the bathrooms I have also insulated the ceiling with multi-foil done the same way. Our roof has been redone with a multi-foil membrane under the tiles.

There is a lot of discussion about multi-foils and their effectiveness and I am not entirely sure that it will be as effective as I expected when we did it. I note that current data sheets advise using it in conjunction with some form of wool insulation which suggests the the early confidence in its effectiveness might have been misplaced. However its done now and it certainly makes the upper rooms a lot cooler in the hot weather. we aren't there much in the winter so I can't say how effective it is in cold weather.

I would always be very careful about using cement against stone. If in pointing it will be likely top cause cracking in the stone (unless its granite) and as render it will almost certainly crack as the stone wall moves - they always do.

Thinking it on then maybe I only apply one cheaper coat of render without any damp proof additive. This would bind and tidy up my inner wall stone surface to receive two coats of the Synthaprufe which could then be perfectly married to my Synthaprufe concrete floor damp proof membrane! Alternatively I thinkthat some of the Gypsum plasterboard with insulated backing do have an integral vapour control layer?
Do you feel any resistance with the principle generally of an internal wall being tanked but with an externally lime based pointed breathing stone wall?

The joint width would be no problem. If you look at the Pnupoint website you will see a video of pointing brickwork. The key to the system is that it injects the mortar to the back of the joint which is not easy to do by hand as there are often voids in the mud which is used to settle the stone and will have settled, washed out or been disturbed by mice etc in the cavity between the two leaves of the wall which is filled with a mix of mud and stones.

If you are gong to insulate the inside of the walls you must make sure there is an effective moisture barrier between the cold external wall and the "warm" inner leaf of plasterboard. The render won't do this as Brian has pointed out as it will inevitably crack. There are various systems or you could use foil backed plasterboard but this may risk the insulating material behind it getting damp in the winter when you will want it to be at its most efficient.

Not an expert, just some diy experience with old houses and read a bit about damp and lime and…
Is it a 16th century barn or just a barn built with old stones? Is the mortar currently lime or cement? Is it going to remain a barn or be used for habitation? Not sure why you would want to dot and dab a load of placco after rendering the inside walls of a barn. Placco does not take well to abuse and is hard to clean/easy to get dirty.
I’m thinking you want to use a hopefully waterproof render to try and stop the placco getting damp/moisture entering the gap between wall and placco. I say hopefully because most renders crack at some point and allow moisture ingress. Concrete/cement gets pretty hard so any movement can cause cracks.
Damp can come from all sorts of directions depending on the environment. One of the ways it can enter a wall is from the outside surface. Are you hoping that any water coming from outside will happily turn around and go back out the same way?
Some very competent experts on heritage buildings denounce the, ‘myth’ of rising damp. Issues (not general upkeep needed on an old building) are generally caused by people screwing with the natural status quo of the original (traditional) build method. Is there currently a damp issue?
Without knowing any real details I would fix any possible damp ingress issues then either re-point with the correct lime mix for the situation or re-point with the correct lime mix then lime render the walls. I’m a Luddite and would find it sacrilegious and detrimental to use anything but traditional methods on possibly a 16th century building.

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Can I throw the cat amongst the pigeons here please?

The pnupoint system looks good but we have a very large stone barn in Normandy built with old 16th century stone but the stone pieces are much smaller than the usual stone sizes used for external stone walling. The joints are very slender too and I do not think that the pnupoint system would really be compatible with say quarter inch wide pointing? What do you think?

The other cat is that the walls are about 750mm thick. I fully understand the lime pointing argument for allowing the wall to breath but I cannot help but find myself drawn to a more economical and quicker wall treatment for the inside. That is to rake out the millions of joints and apply two coat damp proofed sand and cement render tanking (say Sikadamproofing additive). The render is required to bind and prepare the walls to receive dot and dab Gypsum plasterboard with insulation backing which will be joint taped with skim coat plaster finished. Most of the internal stone walls are not attractive enough to expose but we may be able to keep some.

The point here is that the wall can still breath and moisture can evaporate to the outside. I appreciate that we could use the fibre board with lime based rendered finish but the costs would be astronomical in comparison as this barn has a two storey ground floor footprint of some 200m2.

Newtonite lath I do not consider an option as it would be difficult to mechanically fix here. The corrugations of this are supposed to allow the wall internally to breath but where to if not to the outside? Hence surely my render option is worth a consideration?

I look forward to your comments with great interest.

At £25 I would jump at it. I haven't regretted spending £250 as it has saved so much time and effort. You may need a few spare parts but the sealing washer for the top could be cut form any small piece of wetsuit most of the other parts can be found in M Bricolage I suspect.

The manual and some advice about types of mortar can be found here if you haven't already found this

Good luck.

After my friend bought his from a returning to UK builder for €25 (!!!!) he had to go about it without a manual or anything. He has now got it off to a tee and my OH and I go to help him (play really) now and again. Being French he should insist on being traditional but not at all. He also showed us how to sand blast properly on local materials and the exact sand we need for here. Even school teachers are useful in France - says an ex-don who traditionally should have no idea how to do practical things ;-)

Anyway, thanks for that. I just downloaded stuff from them when I saw it on this post a couple of days ago since until then I had no idea it is a UK product, so went searching and got the manual which I shall refer on to our friend (his English is pretty good I think) but at around £250 new, €25 was a bargain so I am planning a raid on his workroom soon ;-)

Brian. I have attached my notes on the mortar mix and a few hints about using the Pnupoint which you may find useful. It needs careful attention to a few critical points and if you do this it is really easy to use and much faster and less work than doing it by hand.

Oh yes!! We too are built with the stonework bonded but not mortared with red mud. Pressure washing would be a disaster... I'll be acquiring our friend's PnuPoint for the outside work but first we have a lot of hard mortar to chip out. The other danger there is that it loosens the stones and also pulls the dry mud out. Using a mechanical method is far too risky, so it has to be by hand.

We are somewhat South of you Brian and our stone is quite soft sandstone. I use the soap to ensure the mix flows well with the Pnupoint. It may not be necessary of the work is done by hand I agree as "flow" is irrelevant.

Another point which occasionally comes up is the preparation. Some have advised clearing out the old mortar with a pressure washer. As, in our house, the stone is mainly bedded on mud and the pointing is fairly shallow and is only there to stop the mud being washed out. Pressure washing would be disastrous as it would wash out the mud and make the wall very unstable. I just rake out any really loose mortar carefully and then fill after that. In case where there are voids, such as in corners, the Pnupoint is brilliant as you can run in a lot of mortar to fill the void very quickly. Very hard to do this by hand.

Since we have repointed the inside of our walls we have become virtually "mouse free". I used to catch 4-6 every day but haven't caught any over the past two years.

I am near the Dordogne/Vezere confluence where, for instance, the lime is hard but porous, unlike the sandier stuff down the other side of Bergerac. Because of it being porous the mix sticks nicely anyway. Everybody who knows hereabouts, and there are many of them who have done their own building for well over 50 years as is normal in rural France, says there is no need for soap or any other material for adherence, a small amount of cement is always used though. But sure, with a cellar it is only a case of there usually being less movement unless it is an alluvial base, wet or with earth tremors. Here we, again for instance, are above huge great slabs of bedrock, old glacial deposit, that is stable. If it ever goes then the whole hillside would go, forget a mere detail like a house. Anyway, in a cellar I doubt the Batiment de France rules are especially pertinent given it is not visible on the surface and the stone's qualities are by far the most pertinent. For all of that, I would not like to have a builder of the type my father was and exist in large numbers here who buy cheap grey cement and the standard builder's yard sand and that's what you get.

Not sure I agree with Brian over cement. As the stone walls will be built without foundations I can't see why the inside is any less likely to move than the outside. I agree it depends on where you are. If its built on rock it won't move. If in Brittany the stone is likely to be granite and thus harder than the cement so OK in Gironde the stone is quite soft sandstone of limestone and will be much softer ans weaker than the mortar is there is much cement added. Using soap makes it pretty sticky.

I absolutely agree with him about seeking local advice not only about the stone etc. but also about local rules over colour if you are likely to be covered by Batiment de France rules (i.e. less than 0.5Km from a church or ancient building or monument.

If you decide to use a Pnupoint let me know and I will send you details of the mix etc.

Generally with you, except the mixes with a bit of cement in will work here being a cellar. Add more chalk - chaux arrienne - to reduce the cement, being indoors in effect the cracking is minimal. Ideally, use 100% chalk (lime, if people must call it that) but if mortar has been used in the past (as in the walls we are doing right now) the cement helps it stick. Also, if you are rendering after mortaring and have cement in the walls, when you get to the render you will no longer need any cement.

The one thing we have all omitted to say or account for is how local materials differ. Make sure you get advice from a couple of people who have done their own work, preferably 'old timers' who have done it several times who can advise on what works best on local stone.

Hi Michael, Thanks for all your info regarding mixing ratios and the pnupoint, we are just starting out on our French adventure and have got plenty of pointing to do,

We have been using the Pnupoint pointing gun and the mix is critical.

I tried several plasticisers and found that the best by far was soap powder. We use PAIC (Colgate-Palmolive).

Its important to mix using a mechanical mixer as the lime mortar needs a good 15-20 mins mixing and then a rest of a few minutes before a second mix for about 5 minutes. Be sure to use only lime mortar (5% DHL) without any cement in the mix as cement mixes are too hard and will crack the stone if there is any movement in the wall (there always is).

Some people use the premixed mortar as they think it’s easier. It may be, but it’s a lot more expensive (€12+ for 25Kg of premix compared with about €2 for the same amount of mix using our method)(2013). We use 5 parts of the coarse greyish sand with one part of yellow sand to get the right colour for our area. This varies from place to place but if you are covered by Bâtiments de France they may specify the colour you have to use outside.

I would use a mix of 1 third cement to 2 thirds sand, but most important is to use a good waterproof plasticiser in the water when mixing, when the whole wall is dry give it a coat of clear damproofing solution, will stay good for years.

This might be useful as the mix is the same for pointing or render.

My father was a house builder, when he and his ace bricklayer partner sold the firm on my mother told them to empty the shed. I grabbed what I could. I have old type trowels that do not rust or handles break, nice old plumbing tools that were old when they got them and chisels/bolsters that have split thousands of bricks and stone. I wish I had hired a van and taken more.

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When I think of the tools gone missing over the years it makes me weep. My father was by trade a watch and clock maker and his manual skills were exceptional. Whilst he could seem to renovate most heritage houses we owned with just his old RAF axe and a dodgy screwdriver I remember quite a few tools that would do me nicely now. Unfortunately I was never around when these things were passed on.

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