My house was built in 1860 and I have a field stone foundation. I have some areas on the exterior that I need to reapply some mortar on. I know some people are using type N and adding sand and some people are using type O for their foundation. If use type N how much sand should I add? Or should I use type O for the exterior?
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Although I would use Type N for the repairs, according to an article I found online, Type O is a very weak mix and is typically used indoors. It did state that it could be used for outside work for repointing, and other non structural work, such as you are suggesting. I personally would not use it at all.
Type N is good for repointing above grade, "softer" than Type M or S which are much stronger and suited for below grade and structural purposes. If the cement used for repointing is stronger than the material it is applied to it will pop the surface of the stone in your case. Type N will be more compatible with these types of materials.
The amount of sand I was taught to use when I was a mason tender was 18 shovel of sand. Some other folks may use 20 shovels of sand, no harm in that either. First thoroughly "chop" the 2 dry mixes first, then add water on one side and pull the dry into the water, mixing a little at a time, but place as much water as you dare with out over wetting. You can alway add more, but cannot take it back out, only ad more dry mix...
Jack
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