I am a novice DIYer who is renovating a second floor bathroom. I
removed the subfloor to move some pipes and now that I am ready to
relay the subfloor I have discovered that one of the joists ( which
are 16 inches apart) and which is in the centre of the room has sagged
an inch. It has sagged where the first floor chandalier was attached.
I removed the chandalier and tried to pull up on the joist but it
won't budge.I am limited as to how much I can push up on the joist
from the first floor because I dont want to disturb the finished
ceiling.

When I lay plywood across the joists there is an inch gap between the
plywood and the one joist. I could proceed and screw the plywood down
and then use a levelling mortar to fill the trough, or I could place
shims on the top of the 2x10 joist so that the plywood would lay
flat.What would be a good material to use as a shim? or how would an
expert proceed?
Brent
Thanks