Tie Existing Stone Foundation to New Concrete

shazapple · 04-11-2012, 11:21 AM

Hello all, My house is 100+ years old and has a stone and mortar basement foundation. It is about 7 feet from floor to joist because at some point in the past concrete footings and floor were added (the floor is lower than the footings by about 8", so I assume the footing does not extend all the way underneath the original stone foundation).

We are putting a 15x25 addition on the back. A contractor will be hired for the foundation work and I am looking to educate myself on where the old and new meet.

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Name: foundationlayout.jpg
Views: 477
Size: 25.0 KB

1) The current basement is about 7 feet, and the new will be 8 feet. What should the contractor do to ensure stability of the old wall when digging around it?

2) How will they tie into the old? From what I've read they'll scribe the form work to let the concrete 'key into' the old. Would there be any advantage pouring a concrete wall parallel with the old, in other words 'sistering' the old wall with new concrete?

3) How to continue the waterproofing/insulation from the new concrete onto the old stone foundation? While it is dug up I might as well waterproof it, as the bottom right of the old foundation faces up hill, so it sees the most water.

Any information is appreciated!

shazapple · 04-13-2012, 02:58 PM

Bump!

jomama45 · 04-13-2012, 09:57 PM

Quote:

Originally Posted by shazapple

Hello all, My house is 100+ years old and has a stone and mortar basement foundation. It is about 7 feet from floor to joist because at some point in the past concrete footings and floor were added (the floor is lower than the footings by about 8", so I assume the footing does not extend all the way underneath the original stone foundation).

We are putting a 15x25 addition on the back. A contractor will be hired for the foundation work and I am looking to educate myself on where the old and new meet.

Attachment 48819

1) The current basement is about 7 feet, and the new will be 8 feet. What should the contractor do to ensure stability of the old wall when digging around it?

Ideally, the excavation & concrete work will be done relatively close to each other time-wise, which will cut down on the chance of alot of rain undermining the existing wall. Also, they can stay a foot or two away from the existing wall with the dig, but this will create a wider "shelf" in the basement.

2) How will they tie into the old? From what I've read they'll scribe the form work to let the concrete 'key into' the old. Would there be any advantage pouring a concrete wall parallel with the old, in other words 'sistering' the old wall with new concrete?

Pouring a wall directly against the existing would be overkill, especially when you're eleviating much of the later soil load against it. Also, it would require one-sided forming, which is slow & tedious compared to standard forming with panels on each side with ties holding them together. I would expect that you'd be paying a hefty premium if you were to request it to be done, and you'd see little gain.

As for tieing the two together, yes, the two will key together fine, but the joint between the two is a potential leak point in the future. A liberal coating of liquid damproofing at the joint helps as well as additional stone over the draintile in that area. Mechanically, the connection is fine, as the soil backfill will always be exerting a lateral load on the new addition foundation, keeping it from separating with the existing.

3) How to continue the waterproofing/insulation from the new concrete onto the old stone foundation? While it is dug up I might as well waterproof it, as the bottom right of the old foundation faces up hill, so it sees the most water.

Any information is appreciated!

I can't speak for how the 100 year old walls were constructed in your area, but I can tell you what they all look like here. The inside is rough & wavy, but it looks like a laser-straight line compared to what the exterior looks like. Typically, they dug the hole to the exact dimension of the house & used the dirt bank as the back form for the wall. They had no intentions of anyone ever exposing the backside, because they certainly aren't pretty. You'd need copius amounts of mortar to get the wall flush enough to damproof, and even more to get any kind of foam insulation to sit against it.

To be perfectly honest, 100 years ago when the basement was new it wasn't meant for anything other than shelter from serious storms, a house for a cistern sometimes, and a cool dark place to store your veggies from the garden. Try as you might, it's near impossible to make a wet old stone foundation much more than that, unless you're willing to spend some serious money, at which point you'll likely be better off with an underpin.

shazapple · 04-14-2012, 09:14 AM

Thank you very much! The original stone foundation is more of a mechanical room than livable space, and I agree that it's not something that can ever be converted into a living area. Even the addition foundation area will be storage/work space, so a shelf/step where new meets old would be fine. Should we consider some sort of interior drain or drain tile under the slab where it meets the original foundation?

There is mortar on the exterior exposed portion of the stone foundation, but I wasn't sure if it went down all the way. I guess the best plan of action is to assume it doesn't and have mortar on hand for the areas we are exposing and where the waterproof membrane will tie in with the old. Good grading and down spouts will have to do the rest.