"Chris Lewis" wrote in message
...

If you can't (both ends boxed), you have to resort cutting the joist
shorter (one wall width plus span length, plus a diagonal fudge factor),
which means that the ends of the joists aren't fully bedded into the wall.
Which isn't optimal. Sistering to an existing beam is fine, however - a
little less end support makes no difference if you're just trying to
stiffen
it up.


This is a good point. What is it that we are trying to beef up here, the
shear strength at the bearing ends of the joists, or the bending strength
(read deflection) at the mid span of the joists under new larger loads than
in the original design? If we feel comfortable with the shear and bearing
strength of the original joists under the new conditions, and are only
concerned about bending strength, then there is really no need to bear the
sisters on the supports - assuming of course that the sister joists project
far enough into the low bending stress zones at the ends of the joists and
they can be fastened properly to the original joists to share the load in
bending. As always, it is best to have a professional look at the existing
conditions and evaluate the situation for safety... but this could be a
cost saving measure if found to be a safe alternative. One word of caution,
however... If you are not planning to have an engineer evaluate your
situation, disregard my comments and do what must be done to bear the
sisters on the supports.

Good luck!
Giz