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Discussion Starter · #1 · (Edited)
I have a 100+ yr old foundation that is fieldstone below grade and brick above. The stones are predominantly New England granite. Much of the foundation has stones that are 6"-12" across, but one area is a gnarly rubble wall of small stones and brick. The rubble wall has been refaced on the exterior with granite block, but on the interior, it's ugly and leans in a bit. It's time to repoint this baby. If anyone could tackle one or more of these questions, I'd appreciate it.

1. For larger stones that appear reasonably well laid, can I remove as much mortar as I want within a small work area? If so, how small should I keep my work area?

2. For my slap-dash rubble wall, how much mortar should be removed prior to repointing? (I'm hoping you won't say "rebuild.") How small an area should I work on at a time?

The existing mortar is a mish-mash. There is plenty of lime based mortar, but also significant areas that were repointed with portland cement based mortar.

3. I'm thinking of using an NHL mortar to avoid the time & workflow issues of straight lime. I assume my biggest criteria in a mortar should be that it helps preserve the existing lime mortar, rather than picking a mortar that is high strength to match the hard rock, correct?

4. I know adding PC mortar to an existing lime mortar will cause the lime to deteriorate, but is there any downside to using a lime mortar when there is likely to be some existing portland based mortar behind the new lime? Or should I remove as much of the old PC mortar as I possibly can?

5. I've seen recommendations that maximum aggregate size should be 1/3 of the joint width, up to 8mm. Most mason's sand I see contains pebbles no larger than 1/8" or 3mm. Should I track down 3/8" pea stone to add to mortar for the larger joints? And if so, what ratio of pebbles to mason's sand? (I assume there is a reason that no one uses concrete sand - which is what they call a coarse sharp sand popular for laying patio pavers in this area.)

6. I have also read that large cavities are better filled with dry-pack (i.e. same mortar mix but using just enough moisture so it will form a clump in a closed fist.) Agree/disagree?

7. (Keeping in mind I'd be using a hydraulic lime mortar) Is it ok to do this work in the basement interior over the winter months? My thought was just to do the below grade work where the exterior temperature will be warmer. BTW, I'm in Massachusetts, not northern Maine, so it's gets cold here, but takes some time to start freezing the ground.

 

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Discussion Starter · #2 ·
Bump…

If anyone could tackle even a single question above, I'd really appreciate it.

Thanks in advance!
 

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That is actually called a Rubble foundation. There is nothing that you can do to fix it. Other then raising the structure, to put in a proper foundation.
 

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Don't confuse me for a mason.

I don't know the sources or the validity of some of the claims in your post.

I would mortar in after removing any obvious loose stuff. I would work in small sections. I would make my own Portland mortar mix and add a bit of lime to increase the sticky factor. Done quite a few repairs on old stuff this way and nothing has fallen down yet no have the repairs failed.

For the final step I would stucco the whole mess with the same mix to make it look as good as possible. Over the next 100 years there may be negative interaction between the various mortars. Do you really care what happens in 50-100 years?
 
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Discussion Starter · #5 ·
Colbyt: Agreed, I'm unlikely to outlive the house, and if I do live to be 150, I'll have bigger problems than my foundation. :wheelchair: Having said that, I'd like to do things in a way that preserve the existing mortar, and most everything I've read says that repointing with a strong portland based mortar is the kiss of death for an existing lime mortar. So, I'm feeling pretty set on an NHL.
Taking small areas at a time makes a lot of sense. But how small? A couple sqft?

Gregzoll: Thanks, I know there are plenty of rough looking rubble foundations that have been standing a long time. I'd hope that taking the mortar joints back 3/4" and repointing would extend its life. It's no Hadrian's Wall, but it's been around a while already.
 

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I would skip around, trying not to undermine any particular rock or brick to the point where it might loosen or fall. I probably would not go more than 1/3 the depth of any stone or brick. The footage involved is a judgement call based on what you find when you start raking out the old.

I find pointing a frustrating, messy job and don't do very much in one session. A couple of summers ago I watched several guys take most of the summer to do a short 2 story exterior so I must not be alone in my thinking.
 

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Discussion Starter · #7 ·
OK, consolidating to my unanswered questions. Thanks for the help so far...

4. I know adding PC mortar to an existing lime mortar will cause the lime to deteriorate, but is there any downside to using a lime mortar when there is likely to be some existing portland based mortar behind the new lime? Or should I remove as much of the old PC mortar as I possibly can?

5. I've seen recommendations that maximum aggregate size should be 1/3 of the joint width, up to 8mm. Most mason's sand I see contains pebbles no larger than 1/8" or 3mm. Should I track down 3/8" pea stone to add to mortar for the larger joints? And if so, what ratio of pebbles to mason's sand? (I assume there is a reason that no one uses concrete sand - which is what they call a coarse sharp sand popular for laying patio pavers in this area.)

6. I have also read that large cavities are better filled with dry-pack (i.e. same mortar mix but using just enough moisture so it will form a clump in a closed fist.) Agree/disagree?

7. (Keeping in mind I'd be using a hydraulic lime mortar) Is it ok to do this work in the basement interior over the winter months? My thought was just to do the below grade work where the exterior temperature will be warmer. BTW, I'm in Massachusetts, not northern Maine, so it's gets cold here, but takes some time to start freezing the ground.
 

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Look up using Shotcrete for Rubble Foundations. Not only would you knock out the patching, which the crew would do. But they would also cover over the stone with a dyed color if you want for the final coating. After they are done. It would basically look like a regular wall, unless you want to leave some of that detail in there for effect.

They go through and put in anchors to stabilize the walls, before they do the work. Then once you get a nice surface, you can put up Pressure treated walls and insulate.

Picture examples and explination of the process. http://www.foundationsupportworks.c...wall-stabilization/wall-anchor/shotcrete.html There are other examples out there if you do a image search.
 
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