Joists are 2x6 on 16" centers, lighter than average 2x8 construction and
could use some help. House is 2 1/2 yrs old.

Don't waste time and money on this jerry-built dud, Start planning now to dump
it and move as soon as you can. You may have to sell at a loss since disclosing
faults is required in many locales. Just dry your tears and remember your tax
accountant can make it less painful next year.
Before you shop for a new house, start making a a check list now of things you
want, things that are signs of shoddy wiorkmanship and present the list to your
realtor when you're ready to go. This is no time for sentimentality. Good luck

Joe