I live in the coastal Northwest, and am rebuilding the deck on a 1979 house.  The deck consists of two sections; an original deck that I assume was built the same year as the house, and a later addition framed with treated wood.  The later addition I have finished re-planking with 1X6 ironwood, fastened with stainless screws.  It’s the older deck I have a question about.  Part of the framing is a 4X12 beam that has rotted due to moisture trapped by sandwiching to a treated 2X10 that forms the ledger for the new deck, and also by a 2X4 fascia applied to the upper portion of the beam.  The beam will obviously have to be replaced, but the rest of the framing appears to be in good shape.  The structure is high on a hill and receives good western exposure, so it is not continually shady and damp like so many locations in this area.  Working alone, it would save me considerable time and energy if I could use the existing (hem/fir) framing.  Is it more susceptible to rot after the many years of exposure, or could it be expected to last another deckade or so?