adding a concrete barrier to stone foundation

March 10, 2015 11:09 AM

Our house in Boston was built sometime around 1900. It has a stone foundation that is mostly intact and has been kept in fairly good repair. Unfortunately, there are some holes/gaps that need to be repaired along the front wall of the foundation, under our brand new deck. They can't access the holes properly without removing most of this new deck. As an alternative, the masons have suggested pouring a small thin layer of concrete in front of the stone wall. Is this a reasonable approach? Anything we should be considering?

Basically this foundation wall extends about 2 feet above the ground in the front of the house. Our new deck is directly above of this foundation wall. The masons would only need to remove a few boards to gain access for this project. They would build a small frame a few inches in front of the stone foundation, and basically pour concrete into it, encasing the front wall of the foundation in concrete, and thus plugging the holes in the stone foundation.

Something needs to be done as the basement had a mouse issue this fall, and we determined they were getting in through the gaps in the front wall. It's hard to explain why, but it's impossible to repair this particular wall from inside the basement. The ONLY option is to repair these holes from the outside. Several folks familiar with masonry and construction have looked at it and agreed that it can only be fixed from the outside, and I agree.

We've had one masonry team look at it, and this concrete wall is what they recommended. A second mason is coming out once the snow has melted, but this was one of his suggestions too.

I don't really see a good reason to preserve the aesthetics of the exterior stone wall under the deck. It has never been exposed and never will be exposed. The house is a mish-mash of architectural details and the foundation isn't exactly pretty in other spots. It's the only possible front entrance to the house and for a variety of topographical and neighborhood reasons, it will always have this small porch above it.

This small barrier wall does seem like a reasonable approach, and I'd rather not rip up a perfectly good (and expensive) new deck to solve this problem if it's not necessary.

But there's something slightly weird about a stone foundation that has lasted 115 years only to be encased with a layer of concrete. What if the concrete cracks? What if the house settles more? What else am I not considering? I don't want this to be the "easy" solution if there are bigger issues to consider here.

Side story: The front deck entrance was just entirely replaced last summer. We used Apex decking, which is like Trex - it's a manufactured product and very weather resistant but expensive to purchase. The reason we can't just unscrew all of the boards is that the decking compnay apparently used very cheap screws. When we removed a few boards to take a look underneath, the screws were all stripping and they had to destroy a board to get in there. I'm not entirely convinced the deck can't be disassembled without stripping the screws, but I don't have the time to do it myself this spring and our original decking contractor is being a jerk about it.

In summary: is installing a small concrete layer against the stone foundation a decent solution? What should I be considering? If it's not a good idea, why not?

Thanks so much for any feedback or thoughts from folks experienced with this sort of home repair situation!

posted by barnone to Home & Garden (6 answers total)

Concrete tends to crack, so you should probably expect that there will be cracks sooner or later.

This Old House on how to patch a crack in a concrete foundation wall, and how to know if all you need is a cosmetic fix or a structural evaluation.
posted by Bentobox Humperdinck at 11:22 AM on March 10, 2015 [1 favorite]

Given what you say and if the only alternative really is to tear up most of the deck than you have nothing to lose by trying the concrete. If it goes wrong in some way you can just remove it, which yes would require tearing up more of the deck but you'd have to do that anyway if you didn't try the concrete facing layer first.
posted by Wretch729 at 12:48 PM on March 10, 2015

Best answer: I'm struggling to picture what's going on because of the conflicting idea that you can take up just a few boards to build a form completely along the wall and fill it (I'm assuming from the top), yet to patch a few holes in the same wall you have to tear up much more of the deck?

Anyway, I'd have three concerns: A. That the new concrete be thick enough to be structurally sound, you said "a few inches", 2 might be too few, 3 might be enough depending on... B. Are they proposing to put any steel reinforcing (e.g. rebar) in the concrete? that would greatly reduce cracking. And C. Will there be any support (other than dirt) under the new concrete? If they are just pouring the new concrete on dirt it increases the likelihood of cracking and falling away form the face of the stone. Some kind of footer or getting down to other stone or concrete that can support the new concrete would be good.
posted by achrise at 1:09 PM on March 10, 2015 [1 favorite]

I recently poured a concrete layer to make a sound and isolated floor, because I have installed geothermal heating, and also because of money. It is great - now I have a warm house which is in many ways sounder than before. And when I add on solar cells for electricity, I'll be off the grid.
However, the concrete cracked right away, and as I had expected, the old structures are strained and also beginning to crack all over because the concrete layer is so inflexible.

Since I knew this was going to happen and have taken precautions, all will end well when everything has found its new order. But you need to take care.

Once my dad began cementing holes in an ancient house, and the cement was so stiff compared to the rest of the building that the entire building began to split up!

I looked it up and found a method which seems good for you: hand-crumpling a bit of chickenwire into the spots you need closed before pouring in the cement. This seems like very sound advice to me.
posted by mumimor at 2:23 PM on March 10, 2015

Response by poster: achrise, those are good questions for me to ask the masons. The angle is part of the issue - it's hard to see and fix all of the holes in the stone foundation without removing more of the deck. We'd still have to remove some of the deck to build the form and pour in concrete, just less of it. Maybe the difference doesn't matter much and I should do things the "right" way with the regular masonry fixes. I want to do whatever will last the longest at this point, without having to go through this issue again.

mumimor - the cement would be against a wall, not on a floor. But that does give me a question to ask them about whether the inflexible concrete will stress other walls.
posted by barnone at 4:03 PM on March 10, 2015

A couple of things; If the deck screws are Phillips head then you can get a screw extractor that will remove stripped Phillips head screws. Here's
an example.

FYI I use Robertson (square head) screws for decking which may be hard to find in the US but are much more resistant to the head stripping.

Secondly have you looked at parging the wall to seal the gaps and holes? I've done this and it works great. If you mix Xypex into your mortar you can even get some waterproofing added to your wall. FYI I live in an area where frost damage is not an issue.
posted by Zedcaster at 11:26 PM on March 10, 2015