What kind of mortar should I use to point a fieldstone foundation?

Question

What kind of mortar should I use to point a fieldstone foundation?

The old mortar is crumbling, and sandy mortar is falling out onto the basement floor. I plan to remove the crumbling mortar (see related question on how to remove), then point the foundation with new mortar. But what kind of mortar should I use?

I have had a hard time finding definite answers about fieldstone foundations on the Internet, but I'll include some of what I've found here. First, David Valley has this to say about fieldstone foundations:

Most fieldstone foundations have, or had at some time, a thin mortar coating on the face of the stone. The purpose of this coating was to assist in holding the stones in place. This thin mortar coating will inevitably flake off from any moisture migration, revealing the surface of the stones. As this coating continues to erode, the mortar between the stones will begin to crumble and the soft, sandy mortar begins to slowly fall out onto the basement floor. It looks like a small pile of sand at the base of the foundation. When this occurs, tuck pointing is needed to refill the voids where the old mortar has fallen out. It's very important that you scrape away or chip away the crumbling mortar (in between the stones) in order to establish a small cavity or key way which can hold the new mortar that is being applied. Never apply new mortar over any old crumbling mortar, as this is a temporary fix and it will only last a couple years. All crumbling mortar must be removed in order for the newly applied surface to bond properly. While upgrading your field-stone foundation, only work on a section at a time. Do not remove the old mortar throughout the entire basement all at once. Complete all removal, tuck pointing and parging one section at a time.

To avoid annual tuck pointing, you should finish the foundation with a complete top coating of mortar. Be sure it is lime based. This top coat does not have to look like a stone artisan's creation: It merely has to serve the purpose of keeping the newly installed mortar in place. It's sort of like applying a frosting to a cake.

This page seems to suggest that I should be using not straight type-S hydrated lime, but type-S mortar, which consists of one part Portland cement, ¼ to ½ part lime, and an amount of sand equal to 2¼ to 3 times the total volume of cement and lime.

Do I need to analyze the existing mortar to make sure that the new mortar has the same lime content, as suggested by some?

Should I expect the new mortar to start crumbling and releasing dust soon, or should it stay solid and dust-free for many years?

Answer 1

It is not so much about mortar as it is about the working process. You should clean out the old crumbling mortar to make place for the new one. Remove the old mortar so that all parts are firm and nothing is crumbling - you can even scratch out a bit of the old mortar. The very important part is to wash the wall properly with water so that all parts hold firm and there is no dust - otherwise the new mortar would just fall out. Than press the new mortar in the joints and fill them, make them smooth on surface.

As for the mortar the lime content should not be important for the lifetime, but you can add a bit for better workability. Generally there should be no or very little lime in a stone wall. The new mortar should consist of 1 part of portland cement and 2-3 parts of sand.

If you do it properly (well prepared surface, good mortar), the new mortar should not crumble or dust for tens of years.

A personal experience: it is a very time consuming monotonous work, but the result can be very good if done with care.

Answer 2

I have been working on my field stone basement as well (170 years old), and here is what I have learned:

Having hired an actual stone mason (that's right; they still exist) to repair an area of wall that was very decrepit I learned some things that are helping me now, and I have also learned that some of what he was doing was not necessary.

I have begun my tuck pointing by chiseling away the loose mortar; sometimes there is a lot, and sometimes just a little (sometimes I can clear a cavity inside the wall bigger than a softball, and sometimes I have to chisel away sound mortar to have enough of a keyway to hold the new). This process starts by actual, but gentle, chiseling, and ends by scraping with the chisel until I am satisfied that any mortar left is sound. (Old lime mortar is more like scraping very hard playdough that the kids have left out accidentally). I chisel this away strategically, never completely freeing a stone from the wall, if I can avoid it, and never removing to large an area at a time; as I progress, I work from and join into the areas I have recently completed, so that the wall is well supported. Then I vacuum out the cavities completely before I begin mortaring.

I am using a type N mortar mix (already to go; just add water). Since this is about 25 percent cementitious material (lime and portland cement), I am adding sand to bring the percentage down to about twenty percent, to more closely match the hardness of the old lime mortar, although with all the conflicting advice regarding this, I doubt this matters as much as some say. The sand, however, makes the mortar a bit more workable, and enables me to pack it into the crevices a bit better.

I am using a trowel and tuck pointing tool to pack the mortar thoroughly between the stones, and completely fill the cavities (I mean COMPLETELY). Then, after curing for a little while (till the moisture shine is off of the mortar), I use a damp paint brush (really cheap one for this) to brush the mortar joints smooth, and to bond all the edges thoroughly to the stone. The result is pleasing to the eye, and completely sealed. After this, the stone mason advised me that a parge coat (completely coating the wall) is unnecessary; in fact, it seems to me that if repairs are needed in the future, that coating will make them harder to see early on, and complicate the repair.

I hope this helps. My repairs are progressing nicely, and look just like what the stone mason was doing.

Answer 3

I'm on the same quest. I have the exact same type of fieldstone foundation as the posted picture above, where the original mortar is falling out and creating a dusty mess around the interior of the basement. After some research, my wife started the tucking process using a type N mortar mixed with sand but she never completed the project. This was about 5 years ago and it still looks like it was done yesterday. Now that I have some spare time, I'm going to jump in and start where she left off. Logic and research is leading me to this conclusion: 1. Since my house was build in 1870 and Portland cement was not widely used, The original mortar has got to be a lime based mortar and I should probably use a lime and portland mixture. (which will be much harder than the original) 2. The fieldstones in my foundation consist of granite and quartz type rocks, which are extremely hard, so my mortar is never going to be harder than the rock. 3. Since the foundation is below grade and not subject to freezing and supported by dirt on four sides, the only concern I have is moisture problems. In conclusion, since type N can be a substitute in the chart for type S, and I've started with type N and it looks like its holding up well, I should continue with the type N and sand. Having said all of that, I would be interested to know what Mr, Ljosa decided to go ahead with.