cfrizzle:

Do not cut the joists; shim if you must have flat.

If I may be so bold, perhaps you should rethink old house living. Slightly bowed ceilings and floors are part of old house character. If you want dead flat, build a new house with TGI's and drywall.

I'm not looking for perfect,flat ceilings, but I am looking to keep this house trucking for another 100 years. That plaster on that ceiling was in bad condition. Now we have an opportunity to even out the ceiling. The floor above will remain dipped.

The joists under the closets are carrying all the extra wall weight; as you have found out, old plaster is really heavy. I would not try to level the ceiling by cutting; the best approach (if you intend to drywall) is to average out the deflection with a minimum amount of planing ( an electric plane is dandy), making it even with no bumps across every three neighboring joists. You want to turn the ceiling into a perfect upside-down bowl, with no ripples, and straight along the walls; then the deflection will be undetectable.
The perfect job would entail gutting those closet walls, jacking, sistering all the joists with engineer-specified lumber (perhaps adding a steel beam) which would get you to a perfectly level floor and a flat ceiling, but at a somewhat greater outlay.
Casey

Given a choice between the structural integrity of the house and not having to remove and reinstall some old crown molding ... I'd make sure the bowing was not from some serious problem (like rot degrading the beams) and make sure it was fixed before going any further.

Do what Casey said ...

If the ceiling is open and you can get to it, you might want to install joist hangers. The inspector had us do that.

We are in the middle of a kitchen and dining room renovation and have a similar problem, so we finally removed all the plaster. What they have done is run a level line at a right angle across under the joists to find the low point. Then they screwed new 2x4s to the side of each joist and ensured each 2x4 is level with the low point. They will attach the new ceiling to the 2x4s instead of the original joists. Our ceilings were at 8'4" and they had about 1" of plaster and rock lathe. We did not have any major sagging, so after the new sheetrock ceiling is installed we will lose less than 1" of height.

I'm not a big fan of sheetrock replacing plaster, but with an uneven ceiling and all the new holes for lights and other wiring changes I finally gave in.

Bruce

The 2x10's are on span for floor joist and the ceiling plaster caused the deflection.

Remove the crown, sister 2X4's, drywall, crown and paint.

snoonyb:

Bad advice. Do not remove the crown. Sister the 2x4s to level and call it good.

The depth of the joists cannot be compromised. Cutting them will only increase deflection.

Trebruchet,

I'm pretty sure snoonyb meant "remove the crown moulding", not "cut off the crown in the joists."

" greg"

Exacaly, Thanks.

It is essential to know the span, the actual amount of deflection and whether or not there is any strapping below the joists.

In New England lath was never directly attached to the bottoms of joists. A small amount of wood was removed from the low points of the joists and shims added to the high points in order to level a 1x3 strapping grid. If that is the case and the deflection is not too great, the use of thinner strapping and shims could level a new drywall ceiling and save the crown molding.

So, I went back and measured the deflection. It's about 1.5" lower in the center than the outside walls.

After talking to our contractor, we've decided to sister the 3 pairs of joist in the middle with new 2x8s, then we'll shave off the bottoms of the 2x10s to match the new level ceiling height. The floor above will still dip a little around the closet. But the floor should now be strong enough to handle any weight we put into those closets.

The other option would have been to unload everything from top down and basically rebuild the floor there to make the ceiling and the floor level.

We went with the more reasonable option price & time wise.

Thanks for all of the feedback.

I would use LVL's, because why not? They are stronger, harder and are not prone to having defects.
Casey

Definitely use LVLs; they are about 50% stronger for the same size and they won't shrink. They are 1/3/4" thick X 7 1/4", 9 1/4" or 9 1/2" deep. Use a row of TimberLOK screws top & bottom to attach the LVLs to the existing joists.

You might consider jacking the joists up to reduce the drop in the floor above but it might crack some plaster. I suspect the wall above has been inadvertently loaded from the attic/roof at some time because a closet is a light load and old joists rarely sag that much if they haven't cracked.

greg 2010:

Thanks for the clarification. He had me scared there for a minute.

"Would shaving off a section of the 2x10 just lead to more bowing in the future? Or less load capacity?"

cfrizzle:

Yes.

Build a temporary header perpendicular to the joists supported by plumb 4x4s sitting on jacks. Crank the the 1 1/2' out of the joists (or as much as will go) and install 10" LVLs with a very heavy nailing schedule and lots of structural adhesive. When you remove the temporary header your ceiling will stay flat.

Please give up on the shaving idea.

Use TimberLOK or other types of high-strength self-taping lag screws in order to increase the transfer of the load to the LVLs. Sistering on both sides is even better.

Span deflection calculator shows zero difference between 10" vs. 9.5" depth of joists. That is to the .001 inch place value. No difference if you trim off up to 1/2" of material from the bottom.
Casey

Drywallers shaved off the bottom of 2x10s to level ceilings in a 100-year old house I had. Using hatchets. That was 30 years ago. Still no sag. (Can't say the same thing about moi.)

With the cause of the sag unknown, I'd invest in greater strength.

The issue with cutting off part of the bottom of the joists in question is not the increase in deflection at the design load because the old joists are already so stiff and close together that they would far exceed modern code design standards with an inch removed.

But because of the excessive sagging, the relevant issue is the condition of the joists and the current loading which we cannot know without inspecting the structure in person.

I suspect it would be OK to cut off whatever is needed to level the ceiling and reinforcement would be a hedge against unknown conditions but to be safe (and legal in most jurisdictions) it would be necessary to get the written approval of an engineer who had actually inspected the structure.