Expert: Bruce E. Johnson - 3/29/2010

Question
Good day, I have a 16x13 additon on stilts (pillers). I would like to know how to go about building a block foundation under neath the existing addition and break throu the existing foundation for a door. Do i need to hold up the exisitng structure?

Thank you
John Johnstone

Answer
Hi John, I am assuming that the existing foundation of your house is a basement otherwise what would you need to break through the existing foundation for a door..Right? First of all you need to determine what is actually holding up the main floor joists of the addition.  They are probably 13' joists and span from one wall of the main house and out to a beam on the outside of the addition which is in turn supported by posts or piers.  You call them pillars.  Are they masonry pillars, or just posts?  House moving companies have large beams that they can strategically place to support your floor system while you dig and pour your footings and then build your walls.  If you want to do the shoring yourself you can shore up the joist bearing wall inside a couple of feet so that you can remove your posts and dig the new foundation.  Digging under the addition is the hard part.  That's why I like to use house moving companies to do my shoring.  Their beams are long enough so that they can be supported outside your addition footprint.  Once the floor system is supported you can remove your posts and start digging.  Without looking at how your addition is built that's about all the information I can supply about that.  As far as cutting a doorway goes?  Chances are your existing foundation has a bond beam of two courses  of concrete filled masonry at the top of the wall.  If this is the case you can just cut out the door opening to fit..Nothing special should be required other than a good concrete cutting company or a good concrete cut saw..I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com