Wednesday, February 4, 2004
How to connect a new foundation to an old one
By TIM CARTER
Q. I am building an addition on my home and realize I must connect the new foundation with the existing one. What is the best way to do this? Is there a way to waterproof the connection? Will this connection become a hinge point in the future, causing cracks in the walls? I am very concerned about how to make a connection between the structures that will stand the test of time. P.M., Orono, Minn.
A. I can remember many years ago having these same thoughts when I was building my first room addition. I couldnt figure out how the shallow crawlspace foundation would stay connected to the full basement foundation since they were at different elevations. My geology professors would have frowned at me had they been at the jobsite. I was not thinking the situation through.
Many people fail to realize that a typical foundation wall is, among other things, a beam. You begin to see this if you look at the vertical web in a steel I-beam or even a simple wooden floor joist. A steel I-beam derives much of its strength from the thickness and height of its vertical web section of the I-beam, not the flat part that iron workers walk on as they erect the steel. A foundation wall is no different other than the fact that its vertical height is 10 or 15 times greater than a typical I-beam.
If you accept this principle, then it stands to reason that the foundation will not flex up or down if the soil beneath this beam is solid. This is perhaps the most important aspect of connecting the two foundations: making sure you have great soil conditions under the foundation of the room addition.
If the soil under and around the existing house is solid and the house foundation has no structural defects, you will have no connection problems in the future. When new homes are built, it is often a common practice to pour different parts of the foundation at different times and at different heights within the soil profile. The fact that you are installing the foundations years apart instead of days will make no difference.
The masonry foundation or the slab should also contain structural steel. Dual horizontal steel bars 16 inches from the top and bottom of poured concrete foundations help to create a stiff concrete beam. Concrete block foundations can incorporate steel truss fabric that is installed in every other row of the concrete block units. Filling the hollow cores of concrete block foundations with a sand/cement slurry also adds strength.
Concrete slabs can contain 1/2-inch-diameter steel bars on 2-foot centers in both directions to stiffen the slab substantially. A residential structural engineer can easily specify the best solution for your particular situation.
You have numerous options with respect to making a physical connection between the two foundations. I have successfully installed steel pins that are epoxied into the old foundation. On other jobs I have bolted a steel angle iron to the existing foundation. These building components then become encapsulated by the masonry materials that are used to create the new room addition foundation.
It is always best to hire an engineer to create a design for this connection. This is especially true if you live in an area with expansive clay soils or an area that is subject to seismic movement.
There are special waterproof connection materials that work well when connecting a poured concrete foundation to an existing one. These flexible waterproofing materials look like weatherstripping on steroids as they have small fins that project from the center of the main strip.
The base is epoxied and sealed to the existing foundation and the strip projects into the center of the foundation form. When the wet concrete surrounds the strip, you have a waterproof joint so long as the foundations do not pull apart from one another.