Expert: Paul S. Brown, AIA - 4/7/2009

Question
We are adding an addition 26 x 30.  We had footings done on solid ground that has not been disturbed in many years if ever.  However, our mason did not do anything (rebar etc.) to tie into the existing foundation.  It's basically just butted up to the old foundation.  Is this o.k.?  Should this new foundaton be attatched to the existing in some way?  What are our options now that the new foundation is up?  Thank you in advance for yur time.

Answer
Hi Sheila:
Good question.
If the foundation for the new addition is bearing on good solid undisturbed earth, and below the frost line, then you should not have too much differential movement between the new and the existing portions of your building.  It would be better if the new foundation was doweled into the existing using re-bar.  Depending on where you are, this might also be a requirement for seismic reasons.  You should check with your local building department.  A retrofit for seismic should be possible without too much difficulty, if it is needed.
Back to the question of settlement: it is an inexact science.  Even with the rebar dowels, you might still get some minor differential settlement, resulting in minor cracking of finished surfaces inside.
This differential settlement is somewhat more likely without the doweling.  I would suggest that you install a flexible 'v' joint where the new finishes meet the existing in the upstairs addition.  And don't wall-paper over these joints for the first few years.  After two or three years, any small surface cracking can be repaired, and then you should not expect to have any further problems.
I hope that helps.  Enyoy your new space!