The method used to fix the problem depends on the reason the joist is sagging and the number of joists.
How to fix sagging floor trusses.
Thre ways to fix a sagging or sloping floor.
Temporary adjustable floor jack tall enough to reach the floor joists from the basement floor.
Set a hydraulic jack and post under the beam and jack up the joists about 1 8 in.
Deteriorating floor joists posts or beams can be caused by termite infestation water damage or dry rot.
Tack a beam under the sagging joists.
A severely arched bulged cracked or sagging floor joist can only get worse over time eventually deforming the floor above it.
Floor joists and posts support the flooring and damage to them is the most common cause of a sagging floor.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
I suppose somewhere in the world there are floor joists that need to be repaired that have wide open clear joist bays with no pipes wires or obstructions.
Then set new joists same height as existing ones alongside the old.
A day until they re level.
Nailing two 2x4s together will work to span about three joists unless the sag is under a weight bearing wall.
Two four by six wood beams of 8 to 10 feet each as straight as you can find them.
Replacing joists costs around 100 to 300 for each new joist.
Place a screw jack found at any hardware store between the 2x8 on the floor of your attic and the sagging truss where the 2x4 is attached.
I recently had to repair a floor joist that had cracked.
Clever fix for a sagging floor learn how to level floor framing in an old home by lifting joists with temporary walls installing a midspan beam and adding new studs to distribute the load down the the foundation.
By andrew grace issue 286 oct nov 2019.
Four or five beams of the same size as your floor joists typically two by eight and as long as the room.
It s best to identify problem joists and fix them before installing your underlayment and new floor covering.
A dip or slope in the floor could indicate one or more saggy joists beneath the subfloor.
Correcting a problem joist is an easy repair and makes a big difference in your finished floor.
Temporarily screw a 3 foot 2x4 to the bottom of the sagging truss so that the 4 inch side of the 2x4 is flat against the bottom of the truss and in the center of the sag.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
A dozen metal rafter ties.
Deteriorating floor joists or posts.