Sagging ceilings (35mm at worst), sister joists advice

[goconnor573]

Hello.
2 storey 1920 mid terrace and the bedroom ceiling upstairs was lime and lath was mostly loose so pulled it down and joists are sagging in spots and out of level in general.

The joists are 3x1.5 approx and the room is 3m x 2.3m with joists running the 3m length. They have sagged in the middle by 35mm at one wall and varying amounts on the other ranging from 5mm to 20mm.

The offset makes me think the joists weren't level at install initially but anyway now to level it out for plasterboard and skim going to put up sister joists 4x1.5 for the worst side and then the rest in 3x1.5 (using 2.4m lengths) so the largest gap still has a 3/4 hold on the sister to original joist. Plan on leaving a 10mm approx gap between walls/sister joists and a gap where I am moving from the 4 inch to 3 inch to allow some movement.

Plan on using 5mm x 80mm screws staggered either side to sister them.

The joists are solid, impact struggles more driving into them than fresh timber lol

Anyone seen any issues with this plan?

 

[Jurassicspark]

Pictures speak a lot of words.

You can also do with some noggins attached to the joists.

 

[goconnor573]

Yeah agreed, apologies don't have any on my phone at the moment.

Room is 3m by 2.3m joists running the 3m length with no noggins.

Ceiling is sagging 35mm at worst point, not all are that low.

Looking more at it and think coach screws would be better than what i was originally using. M8 by 70 or M10 by 75.
Only concern with M10 would be they are too big?
Think M8 should be ample since loft is just used for storage

 

[blup]

Add a perpendicular binder above the centre point and sitting each side on the wall plate, which will stiffen the ceiling joists

 

[Ivor Windybottom]

Have they sprung back up or are they still sagging after removing the plaster?

 

[goconnor573]

Aye a couple were still sagging after lime and lath removed.

Ended up screwing with M8 80mm coach screws and glueing with gripfill (construction adhesive) 75mm x 38mm x 2.4 and then 100mm x 38mm for the ends as 30/35mm was too close to centre line for me. Was a random bit were the chimney was previously removed so I just added bit of timber which wasn't there before so I have a fixing point for board.

Sitting on wall plate both ends.

I know I probably should have staggered and screwed through all 3 but although sag the joists are solid and main goal was to level ceiling for plasterboard and the added strength is a bonus.

Still need to add noggins down the middle, 1 row down the middle should be ample for a 3m span.

Massive difference in movement without the noggins yet and from the weight of the rubble bags my guess is board+skim will be lighter than lime+lath so seems to have been a decent job.

 

[catlad]

Add a perpendicular binder above the centre point and sitting each side on the wall plate, which will stiffen the ceiling joists

Then when you have done that and wound the ceiling back level, add a few hangers from the rafters to the binder.