On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
How to level old floor joists.
The solution to sagging floors or the damaged sills and joist ends that contribute to them often involves jacking.
Floors in older properties are very often out of level.
If the uneven floor is due to floor joists that have warped or twisted over time or were incorrectly installed the only way to level the floor is to remove the subflooring and shim the joists.
Use screws to secure the leveling strips to the floor every 16 inches.
A common scenario is to install temporary jack posts and support beams then permanent posts and beams over new footings.
Screw the strips into the floor joists or subfloor.
Leveling the floor with shims is when you attach thin wedge shaped pieces of wood on top of the low areas of the joists to make the top joists even.
Tack a beam under the sagging joists.
A taut string stretched across the floor will show the amount of deflection and improvement.
Nailing two 2x4s together will work to span about three joists unless the sag is under a weight bearing wall.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
If the floor slants irregularly cut strips that bridge the cap at each floor joist.
To do this simply cut several leveling strips the same size as the original strip.
Set a hydraulic jack and post under the beam and jack up the joists about 1 8 in.
This job can run you 1 000 and 5 000 depending on the extent of the job.
A day until they re level.
Using a chalk line.
If you have an uneven floor you may need to think about leveling floor joists with shims.