Expert: Dan Griffin - 10/30/2007

Question
I am adding on to an old house built in 1906.  I just had the house leveled (which was actually very little).  I am building a two story addition.  The addition is going to be on slab and tied to the exisiting home.  We have a high clay soil.  What is the best way to tie the two foundiations together?

Answer
Your title mentions pier and grade beam.  This would be unusual for a house, especially in 1906.  I suggest this is not what you have now.  Commercial buildings have piers driven to bearing, at least through the clay to a bearing shale, and often to bed rock with a continuous grade beam that bears on the pier heads often with carton form under the grade beam to form a void to prevent clay heaving. The slab is usually placed on select fill.  Many places with high plastic index clays use post tension systems to hold the footing together - very prevalent in the greater Dallas area.

A commercial pier and grade beam building might have a footing/stem wall/slab addition, but it would be designed as a separate structure and probably only tied to the original at a hall way or door as the two will probably move differently.  Your addition has the same potential, so it is important to tie the footings and structural members together or to isolate the two structures with construction joints so they can move independently.

If you have a high plastic index clay a f/sw/slab can be problematic.  I had one once that had a plastic index of 42, the soils engineers suggested not building on the site.  They suggested excavating 19 feet and replacing with select fill or lime stabilizing to a depth of 10 feet for a light weight single story.  It is still standing on the lime stabilized subsoil.

I suspect what you have is a footing/stem wall with a wood flooring system over a crawl space or basement.  If I am correct, I would think you should stay with that system for the addition.  It is certainly possible to use a footing/stem wall/slab foundation for the addition, but why?

No matter what system you decide on, you will need to excavate to satisfy the frost depth for your area.  Here it is 18".  It varies all over the country due to the freeze depth in your vicinity, there is no one answer.  Are you planning on removing the esisting exterior (brick?, siding?) and tie all new framing into existing framing?  I'm sure you've considered issues of HVAC, roof line, etc.

If you are working with an architect, engineer, or knowledgable builder, they will have good advice.