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 couldn't 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.

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