I found parge coating falling away from my fieldstone foundation(1906). I used a hammer to tap on the remaining parge of the first section I am going to repair. Some of the coating sounds very solid while other parts sound hollow I removed the portions that sounded hollow. I have two questions. Should I remove the portions that sound solid? How deep should I remove the loose sand mortar? I think I could go all the way through the foundation and not find solid mortar. The foundation is about 18" thick I have gone 6" or more into the loose mortar in some places and am afraid to go much deeper without some advice.
Removing old mortar
foundationrepair
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Best Answer
I think that probably depends upon where you live, and what type of stone that fieldstone is. If it's dense nonporous fieldstone, you could remove most of the loose stuff near the surface and have a contractor shoot the foundation with gunite. If it's porous fieldstone, though, that'd be the worst thing in the world for you - gunite is essentially nonporous (which is why it's used for swimming pools), so instead of the (inexpensive) mortar being sacrificial the stones would become sacrificial - your foundation would crumble. Porous fieldstone calls for "soft mortar", which represents a lot more work for you now.
I don't think I'd dig any mortar out of the interior of the wall, either way - the stones are at this point stable, the mortar is primarily there to slow down breezes & rainwater. A deep surface reparging should be fine.