DD_BobK wrote:
On Jan 11, 7:57 pm, Limp Arbor wrote:
My house was built in the 1970s when apparently there was a huge
shortage of either lumber or smarts.

2x8 joists, 12" O.C.,14' span
1x3 X bracing in the center of the span
1/2" plywood subfloor nailed with spiral nails - no glue
Oak hardwoord floors nailed perpindicular to the joists - the flooring
is short with most pieces less than 3'
Easy access from below in full basement

Floor bounces noticeably from normal walking, knick knacks rattle,
etc,

I tried on the other side of the beam with 16" O.C. 2x8 joists solid
blocking with little success.

I've seen 3 suggestions and want to know if anyone has tried them and
what the result was

1 Metal X bracing installed on either side of the current bracing
(splitting the difference between the center and the ends of the
joists)

2 Sistering the joists with glued & screwed plywood on both sides
(All plumbing & electric are either below the joists or parallel
to them)

3 Installing a plywood 'ceiling' in the basement glued & screwed to
the bottom of each joist.
(I guess the idea here is to make the whole joist sytem a giant
torsion box)

LA-

Your floor system is a bit a "skimpy" side.....1/2" plywood & 2x8
joists.


If oyu do decide to sister the joists.....I would highly recommend
propping the current joists with a series of jack stands to relieve a
bit of the load and to make sure the new wood take some of the dead
load.

I would glue & brad nail (or staple, M2 type) the hell out of the
sister. Adding 3/4" plywood sisters to each side of the joists would
pretty much double the floor stifness. Adding a single 2x10 per joist
would nearly triple the floor stiffness.

I'd go with the above and definitely *jacking the existing joists while
sistering them*! I'd run a 4x4 perpendicular to the joists and jack
under that so they are all even. If you go with the 2x8's you will need
2x4 spacers between the 4x4 and the existing joists to allow room for
the 2x8. Tack the spacers in place to make things easy. I did
something similar to my front porch and let it go weeks raising it a
little more at a time. For your job I might jack them so they actually
start to bow upwards (not much, just a little) then sister them, again
taking your time to jack it up letting things bend back slowly.