I'm putting a single layer of concrete blocks on top of our house walls. The walls are made of irregular sized stones with an ancient mortar made of clay and straw. It is quite an uneven and dusty surface so I plan to brush off the loose debris then give it a good wetting with dilute PVA.
Question: should the PVA be allowed to fully dry or should the mortar be spread on top while the PVA is still moist? Or should I do two coats and allow the first to fully dry then apply mortar while the second one is still tacky?
What way would give the best adhesion between clay and cement mortar?
Technically I don't think it makes any difference. It doesn't act as an adhesive, but prevents water from the mortar being sucked into the original surface before it's had a chance to cure. IME it's better to thin it heavily, say 10:1, and apply lots of it.
Mortar will stick to anything (try getting it off a steel shovel!) providing it doesn't lose its water prematurely, hence the pva seal.
I take your point, when you think about it you just wouldn't think it possible for mortar to stick so well to a smooth metal surface. I always take care to wash every bit off my tools - as a kid I was given the task of trying to clean a shovel a few days after it had been used to mix concrete. The shovel was a write off.
By the same principle paint sticks really well to glass. You often see blobs of Victorian paint on windows that have survived intact when the woodwork is shot to pieces.
Dear David Unless it is a modern PVA (?Group/Type 4) Forget it and use an SBR This does not re-emulsify and does act as an adhesive - blankets and brownstuff comes to mind mix the SBR in the bedding mortar as well as putting it on the recipient wall
I have done this many times with mortar and concrete and it always has worked a treat
The single row of blocks isn't for a "structural" purpose. They are just to sit on top of the existing wall. The reason being the new ceiling I'm fitting will actually be about 10cm higher than the height of the existing walls due to having to take the new level from the top of big fat oak beams. So to fill the "gap" and make the structure weather proof I'm adding the layer of blocks.
It's all a bit complicated to try to explain, there is nothing "standard" about the house by today's standards, though it was probably run of the mill standard two or three hundred years ago when it was built using whatever rocks were available in surrounding fields, nearby clay and trees hewn to make beams and joists of all shapes and sizes.
I'm wondering why you want to do this at all. The best bond with the top of the old wall will be achieved if you dampen it down and then extend it using exactly the same material as was used before
That way there wont be any differential cracking, the materials will be cheap / free, the skill set is fairly basic and you will enjoy doing it (think sand and water play at nursery school). The downside is that it will be messier to do and will take longer to dry which might be important if it is structural and you are on a tight timetable
Many would say that I have a bee in my bonnet about traditional materials, but I think they tend to work better as additions to an old house as they keep it as a unified system which always used to work and still does. New materials are great on newbuild (which has deep foundations, is of rigid build, made of impermeable layers etc). Mixing the two systems should be done cautiously * and not at all if it is not necessary imo
Exceptions exist. I am a fan of silicon frame sealant - they would have used it if they had it
There are a number of issues here. It would be "nice" from a romantic point of view to do everything using original materials and methods. And as you say there are also conflicts in the way old and new materials work which can cause problems when they are mixed and matched together in the wrong way.
If the house was of historic significance then that too would be a good reason to use traditional methods and materials. However, the house is just one of tens of thousands of similar properties all over France.
There are constraints of time, money and accessibility of old materials. While superficially clay may sound free, in practice it means finding a source and digging it up, which would need to be agreed to by the land owner, possibly also involving payment. There are similar issues with the old timber shingle ceilings which have rotted. I could get a timber yard to cut oak shingles for me, but again the cost starts to mount up. There is also the issue of what to put over them, traditionally this was clay mixed with straw. Making this sort of floor would be good from an aesthetic point of view, but utterly impractical as an upper floor, not least because of the colossal weight of the clay and the deterioration of the beams over the years. Modern floors are much lighter and put less stress on ancient wormy beams.
In practice I'm doing a balancing act between keeping original features and appearance and using new materials where the alternative is impractical or too costly. There are also issues of insulation to consider. The old materials had very poor heat insulating qualities which meant the typical French family spent most of their time huddled in one room around an open fire and had an outdoor toilet. Modern lifestyles mean more rooms are in use and need to be kept warm. This means it is necessary to use modern insulation materials.
The concrete blocks I am using are in a position where they will be completely invisible, part of the hidden structure of the building, so there is no compelling cosmetic reason to use clay here.
I am very keen on using lime mortar for the pointing. To use cement mortars here would be both cosmetically wrong and also structurally wrong as it can lead to disintegration of the stone as cement mortar cannot breath and allow moisture to evaporate, leading to freeze damage of the surrounding stone.
As I say, it is a balancing act. I think I've got it about right for the nature of the property.
I imagine your approach is rescuing a lot of buildings that would otherwise remain derelict. In some ways the balancing act is more creative than slavishly restoring things and pretending we live in the Middle Ages.
Definitely. There are hundreds (thousands?) of old French properties that are falling into dereliction. I pass several on the drive into the nearby town. Some are beyond renovation and literally have no roof, the walls are crumbling and are heading to being a pile of rubble. This happens fairly quickly when the roof has gone. The elements wash out the ancient clay mortar.
Most French prefer to live in towns in new properties and think the hoards of Brits are mad who move over here and spend our own money restoring their heritage!
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