My husband and I are designing an addition with poured basement for our northern NE home. Does anyone have suggestions on details relating to tying a new foundation into an existing concrete foundation to optimize structural and watertight integrity? We are considering the typical rebar pin in epoxy filled hole and are looking for tried and true ideas to block water infiltration. Lots of crushed stone and drainage pipe are called for but how about sealing between old and new concrete.
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The best would be to run the waterproofing system for the new basement right over onto the old. One of the best systems consists of sheets of really sticky, stretchy rubber that comes on a roll. It will stretch over any crack that would conceivably develop between the old and new work.
By the way, the rebar in epoxy hole is an excellent method.
i had this exact deal on my house and i did the rebar pin,drain tile,3.5" of gravel. after about a year the concrete has pulled ever so slightly and in a hard rain i get a little moisture. if i had it all to do over again it would have been so easy to have run a circular saw with a diamond blade down the center of where the new fondation meets the old fondation and put a spline in so that ther was a barrier all the way down.i would use a 3'' wide by 1/8 thick pc of steel to spline it. it might eventually rust away, but so can the rebar pins. no matter what this is a detail i think you should address as once it's poured you never have another shot at it.larry
hand me the chainsaw, i need to trim the casing just a hair.
We pinned it with rebar and epoxy, and heavily tarred the joint. After the concrete was cured and before we were done with grading we got a heavy enough rain that we saw a little moisture. So we got epoxy injected into the cold joint and that (plus finishing the grading) seems to have done the trick. It was another 2 years before we drywalled and we never saw any moisture.
Dundee,
Starting from the ground up;
Precompacting under the new footings is because the existing structure has compacted the soil under it and the new would do the same, thus settling some.
1" rebar is because there is a lot of shear going on between the existing and the new, even with precompaction.
The drain plane is to prevent the capillary pressure of water in the soil from pushing water into any seams.
I just finished retrofitting a basement so I have a picture to illustrate.
SamT
How did you train that dog to do footings?
David,
He doesn't 'do' anything. He's the Super.
SamT
LOL, I thought you would say, "he isn't that good he never gets the concrete estimate right."
What Sam recommended would work, but is probably way more expensive than necessary. He's right about bearing on good soil, but if you are on undisturbed soil so long as the footing is proper width you will be fine. Only in unusual conditions, like building on peat moss, will you have difficulty using standard footings for a structure as small as a house.
Constructing a spline as mentioned before is sometimes done, but I would recommend buying the rubber material made for splining rather than using steel. The steel will expand when it rusts and in a strip the full height of the wall could crack the wall. Large contract supply companies can get the rubber spline for you.
But, I don't think you'll really need the spline. An expansive membrane will bridge any slight movement and stay totally waterproof. Tar will not, it will crack with the concrete.