Structures are built from the foundation up, not the reverse.
Investigate the foundation, prior to any excavation.
To maintain the structural stability of the foundation, within the soil, no excavation can cause an angle to that foundation exceeding 45 degrees of slope, IE. 1' vertical: 1' horizontal.
Here is a link to structural span tables;
http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/
Typically, cathedral ceilings have a structural beam as the ridge which carries part of the roof loading to the ends where a vertical post transfers the load to the foundation.
One of the symptoms of an inadequate ridge beam, is the walls deflecting, IE., out of plumb.
There are several methods of correcting this. Planning the walls would be a last resort, as it does not address the root cause.