About 3 1/2ft from ground to bottom of deck.

That's more room than I have in my crawlspace. I was picturing you having
to slither in on your belly. Sounds like plenty of room to work.

The metal overlaps the house siding, but I have a special moulding
which is made for trailer houses, which locks everything in place and
seals it. I also applied some silicone to make sure it stays dry. I
used this method because there was no way to go under the siding
without doing a lot of of siding removal which would have likely
damaged it. If I ever re-side the house, I can then go over the crawl
space siding.

I still can't picture what your conditions are.

You obviously don't have a continuous perimeter foundation. So, I assume
the floor joists extend to the edge of the house, then the house siding
comes down to the bottom of the joists. Is that correct?

Is there a reason you couldn't preassemble a wood frame and screw it to the
underside of the floor joists (essentially "hanging" it from the joists).
Set it back just far enough that the metal siding would be positioned
behind the house siding.

If the house siding does not extend below the floor joists, maybe you could
slip a thin piece of metal flashing between the back of the house siding
and the front of the metal crawlspace siding. This should prevent water
from wicking around the bottom edge of the house siding.

Take care,

Thanks

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com