I have a small barn (20' X 24' - approximately 11 years old) built by a
previous property owner. Nice job with one major exception, he didn't
pour a footer (don't even ask me why). He simply built right on top of
the ground. The sill plate is pressure treated lumber, but I'm
concerned about rot. None visable yet, but I'm looking to put some
money into the structure to use it as a workshop, and wanted to deal
with this before I did. My dad thinks the treated lumber will last
another 15 years, but frankly I'm planing on being here much longer
than that (I also don't believe I'll get 15 more years before having a
problem).

A couple of details that you may need to know:
1.) While I was having an addition put on my house, I had a 5" thick
cement floor poured inside the barn, but it is isolated from the sill
plate by the radient insulation (think bubble wrap with some foil on
both sides), so the walls should be able to move independently of the
floor.

2.) The bottom foot of pine siding will have to be removed because of
rot. I'm hoping to use Trex or something similar to give it the look
of a foundation, and move the siding away from the ground. I would
like to eventually cedar shingle over the pine if the cement foundation
happens.

The big question is: Can I potentially dig up under the sill plate and
put down some sort of cement foundation/footer? Can I just do small
sections at a time and pour cement or buld a cement block wall just a
couple of blocks high. Maybe just a few cement piers? What about
jacking the whole structure up and digging pouring a complete footer?

In general, I'm not afraid of the work and have all the time in the
world, I just don't know what would be the best way to tackle the
problem. I would appreciate any comments from somebody with some
experience in this area.

Thanks,
Lagnaf