So I’ve found a few questions ask that are similar to my question but not quite the same (and many of those are unanswered). I’ll ask mine and include some pictures in hopes that I’ll get some input.

I’m in a 100+ year old house with field stone foundation. The mortar, for the most part, is doing well except for the base of the very bottom layer where the mortar is disintegrating.

A cement floor has been poured by a previous owner that goes just to the edge of the bottom stones. Under the bottom stones is just sand.

It appears the mortar under this bottom stone is completely gone and the stone is resting on the sand.

I’d like to put mortar in these gaps after cleaning them up.

I know there are considerations for using the same type of mortar as was originally used in order to accommodate moisture. However, I was thinking of using a mortar that is more solid (being that it is the foundation layer) and not necessarily one that would allow moisture (given that I’m not doing all the wall)

So, what kind of mortar should I use? enter image description here enter image description here enter image description here Thanks!

1 Answer 1


What you want is type-N mortar. You can buy it premixed, or mix your own -- I always prefer to mix my own, as it's cheaper, and having the raw ingredients around allows me to make other types of mortar as I need it. The ratio is 1:1:6 cement:lime:sand. If possible, get an electric mortar mixer and set it up in the basement. I always do, even if I have to disassemble and re-assemble the machine in the basement. Then do small batches and pace yourself.

Realize that repointing the foundation is just part of owning an old house. It's something you will have to do every 10-20 years. The purpose of the mortar is not so much to hold the house, but to prevent erosion from destabilizing the field stones, ie keep things in place. The field stones are holding the weight of the house. Typically, you don't want a mortar mix that is harder than the building materials it's holding together. That's how you get bricks with broken faces, as the mortar compresses the bricks. But no mortar is going to break field stones, so you can go with type-N.

  • Thanks for that information. I’m glad you provided it! I was going to go with the type S because I was thinking this was below grade (basement) application. But I guess, although it is below grade, it is actually not “in the ground”. Thanks. Nov 8, 2023 at 19:58

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