Expert: Bruce E. Johnson - 2/7/2009

Question
QUESTION: I would like to build an addition on my home with a full poured basement and two stories. I am wondering what the best method is to join the new foundation walls to the existing foundation walls. We live in a cold climate where frost is a concern although the basement will be heated.

ANSWER: Hi Stella, this question requires more information.  Is your existing home on a crawlspace foundation?  How deep is the frost line and how deep are the extisting footings of your house foundation?

Say your house has a crawlspace foundation that sticks two feet above the ground and the footings are three feet below the ground and the footings are twelve inches thick, this puts the bottom of your existing footings at six feet below the bearing elevation of your floor joists.  Say you are going 8 feet deep with your basement walls with a footing below that of 16".  This would put the top of your basement footing two feet below the bottom of your existing house footing.

At this point it is good to point out that your house footing sticks out 4-6" from the actual foundation wall since the footings are generally wider than the foundation wall.  So once you have excavated for the new basement you have a couple of options.  One is to pour a short (stub) concrete wall under the old footing and utilize the remaining portion of the existing foundation as your basement wall on the adjoining side of the two foundations.  This will give you a 4-6" step in the wall where the footing jogs over to the foundation wall.  This is not a big deal because you can furr the wall out with studs and make that step go away.

If this is the way you opt to go, the best way to attach the two foundations is to undermine the old footing 6-8" and drill up underneath 4-6" and epoxy some rebar dowels coming down from the footing into the new concrete wall.  Or you can make this stub wall thick enough to "key" the old footing into it by pouring around it.  A bentonite or clay expansion mat is often used in these cases between the old and the new concrete.  The mat compresses under the weight of the concrete and then expands when moist to seal the joint from water intrusion.

You will need to get an engineer involved to determine the best method for you and to design the particulars for the new foundation itself.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Thank you so much for you informative and rapid response. The existing foundation has 8' walls on a 12" footing and to the best of my recollection the footing is about 24" wide. We intend to pour the new foundation in the same manner. Because the addition isn't going to be as wide as the existing house we will need to join two foundation walls at a right angle. The frost line here is about 3 1/2 - 4 feet.

Answer
Hi again Stella, okay, cool.  If your existing foundation is a basement also then you will need a vertical column of dowels inserted into the existing wall.  Each dowel will correspond with the horizontal steel of the new foundation wall.  This will include the footing steel also. The joint where the two foundations meet should have a bentonite expansion joint of 1/2" if available or a 1/2" joint of standard expansion material.  After the pour the expansion material is cut out to a depth of one inch one each side of the wall.  This  joint is then packed with a foam backing rod and caulked with a urethane caulking.  Then a strip of peel and stick ice and water shield rubber flashing is applied over the corner joint (one half on the existing foundation, one half on the new foundation.  This will give you a waterproof joint that should never need replacing.  You also do an interior caulk joint with the backing rod and urethane.   I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com