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Stabilising the top of old stone walls
I live in France in an old house. Outside, we have the remains of surrounding fortified walls. Some are at least ten foot high. Most are a couple of feet wide and made of large stones with an in fill of rubble and lots of lime. They are dangerous, and every winter bits fall off which is a danger to anyone beneath them.
I don't want to rebuild them to the original height, just to replace missing coping stones, so they are safe.
I appreciate that this is an awkward question, but my technical French isn't too great, so asking locals would be difficult.
What proportions of lime, cement and sand should we use?
Hello well thats an exellent professional answer from Peter Ruddick above, my opinion being much the same, try remember when possible to lay coping stones on their natural bed as they usually have three sides exposed. Bring the pointing flush with the edges or arises as they are known it offers more weather protection for the wall lime pointing should be treated, ie roughed up or stippled this brings the aggregate to the surface acting like a natural concrete and advances the carbonation process.
Answered 10th Feb 2014
We have just finished repairs to two of this type of old stone wall on a grade 2 listed 300 year old house, probably a bit higher than yours.
Walls were failing due to water ingress at the top stones, causing the base to bulge and crack, so you are right to be concerned about the water tightness of the top stones.
Often there the core of the wall at the base is lime/soil/small stones, and if it is wet it pushes out one of the outer leafs of the wall.
There are a few methods of introducing a water proof capping, a line of horizontal thin stone, or tile/slate under the capping stone.
Re-bedding the capping stones on a thick bed of 'Hydraulic' lime mortar [ note hydraulic , not hydrated ]
Mix is generaly 1-3 hyd. lime/ grit sand ------- yes, no building sand!
Mixed in advance and let stand. [ Wear surgical disposable gloves ] at all times . Probably a 3.5 lime [St Austel , French ] for summer use, winter is not a good time for lime work.
Do not use cement at all, modern, too hard / brittle.
Have fun ----
PS, Try work from above the wall , or use props /acrows / scaffold, if a section falls out it will crush you!! There is often just dust holding it together.
Peter Ruddick--- Beamish Const. Services.
Answered 18th Oct 2013