Expert: Richard Taylor, AIA - 4/14/2004

Question
Hi Richard:
I'm looking at putting an addition over my garage.  I had a contractor come by my house and he was concerned with the support and foundation.  After researching I found out that the garage was build on an 8" slab (about 10" in the middle), 2X4 construction, 16" oc.  Concrete blocks were used and come up about 2.5' from the slab.  The garage is 14'X 20'.  I want to put a master bedroom above.  I'd like to cantalever it out one foot towards the west and one foot to the south.  In your opinion, will the foundation and present structure support the added weight?  Should I not catalever out to keep the walls in line?
The contractor I spoke to seems very cautious... is he overly so?
Thanks very much for your time, Richard.
God Bless.
Sean

Answer
Sean:

Thanks for your question!

It's impossible for me to give you a proper structural analysis in this forum but I can give you some general guidelines.  It is important, however, that you have your situation reviewed by someone qualified to make an on-site structural analysis.

It doesn't sound as if the foundation will be a problem for you - that's probably the least concern (be sure to investigate the footings).  Likewise the cantilevers shouldn't cause any unusual problems, although you'll have to carefully design how you're going to cantilever on adjacent walls.

The real concern is the load on the existing first-floor frame walls.  It's likely that they're not stiff enough to support the additional loads above, especially if they're older.  With the additional loads from living space above, the walls also likely don't have enough lateral bracing and this is very important.

This isn't an area to skimp.  Find a qualified local engineer or architect to help you out.  Your contractor may know someone, or try asking around at the lumberyard.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtaylorarchitects.com