We're building a new deck, and yesterday I removed the old ledger board from
the old deck and to my horor, discovered that for years, water had been
seeping between this board and the rim joist on the house, and the rim joist
is now badly rotted. In fact, there's about a 12-18" section that is all
but not there now, once I poked many holes through it with my finger...

So now I need to know how to go about replacing it, so I can continue on
with the deck. I've googled for info without much luck. The little info I
found via google assumes that the rim joist is perpendicular to the floor
joists, and that it's right over the foundation. In my case, it's parallel
to the floor joists (and on the gable end of the house), and since it's a
split-entry house, there is a short (approx 5') wall between this joist and
the block foundation below.

My questions a

a) do I have to replace this joist as a complete span, or can I cut it out
and replace the damaged chunk? The posts dealing with the joists right over
the foundation imply you can just cut out the damaged section, but since I'm
over a wall, I wasn't sure if this was more structural. For what it's
worth, the wall's top plate is only a single 2x4, not a doubled header plate
like you'd expect if it were load bearing.

b) What, if anything, do I need to do to brace the house temporarilly during
this repair? As I have vinyl siding over "build rite", I can get to the
framing reasonably easy from the outside, but both levels of the inside are
finished and would be a major problem to remove anything down to the framing
members. The damaged chunk isn't doing anything structurally now, but I
want to make sure I don't have "the walls tumble down" if I remove a
slightly bigger one...

Any advice is greatly appreciated. I"d like to (at least start to) tackle
this project today, so any info I can get ASAP would be wonderful.

-Tim

P.S. By the way, I'm pretty handy with repairs and with a hammer -- just
never had to tackle anything of this nature before (and hopefully won't
anytime soon after this!