Expert: Dan Griffin - 4/20/2008

Question
QUESTION: ok i plan to add on my small house and i am going to do it my self because of the cost,  and i am trying to figure out ,
on the foundation my house just sits on Big rocks,  so i was thinking about useing blocks,  but do i have to pour a footer?


ANSWER: You have me a bit lost.  Basement, crawl space, or slab?

Many homes built before 1900 did not have concrete footings.  The normal approach was quarried stone slabs of sufficient width laid in a running bond type pattern to a sufficient depth to carry the loads imposed.  I have no earthly idea how to calculate one.  I did an extensive remodel on a WPA stone building with 24 foot tall sidewalls about 27" thick.  When we went to dig under the footing for some new utilities, the footing stones were much wider and we were down over 15 feet and had not found the bottom.  We ended up doing the utility install a different way.  They had dug down to bearing shale/rock.  I know the WPA was used to put men to work, but this whole building was phenomenal, and done by hand, horse, and mule according to the old timers.

If you are planning on working with CMU, I would think for your own sanity and use that a concrete footing would allow a good level point of beginning that would allow chalk lines and marks for layout purposes.  I would think, also, that the AHJ would demand an engineer's stamp on anything that was not absolutely normal.

---------- FOLLOW-UP ----------

QUESTION: ok there are no footings that i see, when i said rocks,  the house has several big rocks that the house frame is actually sitting on.  and i was wondering if i should just do it the same, or if i should dig and pour a footer and put the house on blocks?

Answer
Quotation:
If you are planning on working with CMU, I would think for your own sanity and use that a concrete footing would allow a good level point of beginning that would allow chalk lines and marks for layout purposes.  I would think, also, that the AHJ would demand an engineer's stamp on anything that was not absolutely normal.

If you are anywhere that will require an inspection, the Authority Having Jurisdiction (the city engineer) will have the final say.  It is hard enough to build anything when you can chalk and mark corners, windows, etc.  If you don't need code compliance and feel comfortable resting the place on rocks, have at.  Are you asking about setting the existing house on a footer and block stem wall?  It would sound like the right approach for a long-term solution.  Just curious, how is the gap under the house dealt with at this time?