Basement Moisture and Repointing

Most of our home improvement work over the past couple of months has been cosmetic stuff – getting the nursery ready and so forth.  But now it’s time to get back to work on some more structural issues.   For one, we want to tackle the wood stove issue before facing a winter with a baby in the house.  That’s a subject for a whole other post but is not entirely unrelated to the subject at hand here – drying out the basement.

What we have is unfinished (to say the least) fieldstone basement with a floor that was at least mostly cement at one time, but is so badly degraded now that it’s hard to tell in places whether it might have just been a dirt floor all along.  We did a bunch of air sealing work down there last spring that should be helping to keep it a bit warmer, but it’s hard to know how successful it might have been given that it didn’t address the permeability of the actual walls.

On rainy days  like today, the humidistat vent we had installed is nowhere near powerful enough to dry out the space down there and we end up with several areas where the moisture shows on the floor and walls.  If the water table rises enough, we do have a sump pump that (so far) is successful in preventing actual puddles from accumulating on the ground, but there’s a big gap between the point when the pump turns off and the point when the basement seems even vaguely dry.

Beyond eliminating the health risks associated with a damp basement, we want the space to be usable for storage and a workshop for DIY projects.  We also might eventually want to have a root cellar down there, which actually requires high humidity, but that’s a project for another day.  For now, getting the space dried out is definitely an important project.  Actually, it’s more like three projects.  As far as I can tell we need to:

  1. Fix some land grading around a few of our downspouts and on the north side of our yard (which slopes toward the house) in order to keep runoff to a minimum.
  2. Repair the floor to create a more effective barrier to moisture coming up through the ground.
  3. Repoint the fieldstone walls to prevent moisture from coming through the walls.

I’ve started looking into the repointing and, as I might have expected, it’s turned out to be a more complicated prospect than we might have hoped.  In some areas of the basement, the mortar seems to be fairly intact, while in other areas it has disintegrated far into the wall.

One section of the wall with recessed mortar.

One section of the wall with recessed mortar.

This section is under the Bilco door, so technically outside out building envelope, but still, there's sunshine coming through the masonry here.

This section is under the Bilco door and technically outside out building envelope, but still, there’s sunshine coming through the masonry here.

Considering this wasn’t just a surface band-aid job, I wanted to really make sure that I didn’t do something that would damage the foundation of the house.  And if it ends up that I need to bring in a professional, I want to make sure that that person really knows what they’re doing and don’t just automatically patch over everything with a cheap and readily-available product.

My online research started out strong with a series of helpful YouTube videos by a mason in New Hampshire.

His explanation of the process seemed clear and I felt capable of following his instructions.  I just needed some supplies – and that’s where things got tricky.  Based on the video above and several articles I came across online, it seems probable that the mortar used in the original construction was made of lime.  An article in Masonry Magazine lays out the following timeline:

There are always exceptions to the rules, but based on my work and observations, if a structure was built between the mid 1880s and sometime in the 1930s, it most likely needs to be repaired with a mortar that consists of lime putty, some Portland cement, sand and color pigment — considered a traditional lime-based mortar.

The information I’ve come across online suggests that trying to repoint over a historical lime mortar with a cement-based product (i.e., any of the pre-mixed mortars that are used in contemporary construction) is a really bad idea because the concrete with trap moisture behind it, causing the rest of the lime mortar to quickly disintegrate.  Furthermore, cement-based mortars may be too rigid to deal with the seasonal shifting of a stone wall due to temperature and moisture variations.  Indeed, in some areas where prior owners applied cement mortar patches it seems to have detached from the stone.  Of course, the old mortar doesn’t seem that well bonded either, but then it is over 100 years old.

cement repointing

The problem here is that Portland cement-based mortar has become so established as the industry standard for mortar use that lime-based mortars are extremely hard to come by.  I went to two local hardware stores that carry mortar mix, but neither had any idea about the lime content or what to use in the repair of historic fieldstone.  I also went to a masonry yard, but was directed there to use a Type S Mortar Mix which is apparently the exact right product to further mess up the masonry.

There are companies out there that sell the hydraulic lime that one would use to mix (with sand) your own lime mortar, but they seem this product isn’t cheap and many of the companies are at least a few hours away…unless you want to pay to have 55 lb bags of lime shipped.

My best lead so far is from an inquiry with Quikrete customer service, who recommended “Portland Lime Type N or O mortar” rather than a Type S Mason Mix.  Recognizing that the lime mortar is not so widely available, they recommended I contact their local production plant, which is only about 20 miles from us.  So now I’m waiting for a call back from the plant to confirm what the have in stock and what product the recommend.

Of course, the big lingering question is whether the condition of the walls is such that I should be leaving it to professionals to do the repointing in order not to destabilize the foundation.

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6 Responses to Basement Moisture and Repointing

  1. Lucas Jean Peterson says:

    Thank you for sharing your experience with old stone/mortar foundation. You probably saved me a wasted and potentially harmful repair, especially thanks to the info regarding new/old cement mixes. I’ll be visiting a local plant soon instead of opting for standard hydraulic cement.

    ~L

  2. Long says:

    Looking for an update on this topic. Did you every end up repointing the fieldstone foundation? What type of mortar did you use? Did you have to make your own limestone mortar? Was a commercial quikrete version compatible to be used? Quikcrete Mortar Mix Type N or O?

  3. Stu says:

    Just bought an 80lb bag of Type S Vita-crete. Both the masonry supply shop in Quincy, Massachusetts (“This what they use in all the old houses here in Quincy”) and the Vita-crete manager (“it’s already mixed: lime, sand, bonding cement”) tell me just add water and get it into the joints. Use gloves, grout bags, trowels, and keep the rocks moist.

    I am presently scaping out the old white-dusty mortar which looks very lime-y. I hope they are right about this Type S, no idea where or how I would find Type N or O or anything softer. No one seems to sell it here in Boston area. The Vita-crete does come from a very local quarry in Milford, so there is that. Our foundation dates to 1874. Fieldstone topped with a layer of brick.

    • phil says:

      I definitely had a similar experience with local shops not having a wide selection of mortars since there isn’t much demand for anything besides a few core products. I also ran into various professionals assuring me that regular old masonry was fine, that it has lime in it, that folks use it all the time, etc. I think much of that is indicative of the fact that lime mortar is outside the universe of contemporary construction and a lot of folks just don’t know what is or isn’t appropriate in caring for it.

      I don’t know anything about Vita-crete in particular, but I’d be careful about just going for it with a cement product.

    • Long says:

      I can easily find Quickcrete Type N Mortar in the Boston North area at Home Depot. I’d be happy with Type O but can’t find those. There would be no way I would use Type S when Type N is easily available.

  4. Doug says:

    Thank you for this post! Just curious, what was the total cost to rehab this basement? Did you get any estimates from professionals during the process? Just trying to estimate what it might take me to do this same remediation for a similar basement.

    Thanks!

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