How to stiffen an existing timber floor

[James Newport]

I have an existing room that is 4.5m wide spanned by 7x2 ceiling joists. Because of the size of this span, every other joist has been doubled up. In the two years we've owned the property there was never any significant deflection of the ceiling.

We have recently done a huge amount of renovation work, which included a new roof, replacement of dormers, and relaying the floor above. Cracks in the ceiling developed following the lines of the joints between the plasterboards.

 

A 1.8m spirit level laid on the chipboard floor in one of the rooms above shows a 4mm gap between the spirit level and the chipboard in the middle of the level. I guess this must be similar in our now carpeted bedroom, because I get to sit in bed and look at two chests of drawers that are both ever so slightly out of level. What's even more annoying is that when I walk between these into the sidle-in wardrobe there's an annoying rattle from some of the stuff my wife keeps on her chest of drawers. 

 

My original plan has always been to underboard the old ceiling and move on, but now I'm contemplating whether I should take the chance to do some work to try to stiffen up the ceiling. 

 

Any work can only happen from below. 
Simply adding extra joists isn't an option because of the amount of cables and pipes in the way. 

 

I have read about several things I could try
 - brace the joists together
 - glue and screw sheets of plywood to the sides of the joists to stiffen them up
 - plywood the entire ceiling from below
 - glue and screw 4x2's on their side to the underside of each joist to form something similar to an I-beam engineered joist
 

Has anyone tried any of these? It's going to be a lot of work to take down the existing plasterboard, pull out all the acoustic insulation and then try one of the techniques above, and I don't want to go to all that effort if the improvement is going to be minimal.

[Russell griffiths]

I think your only option is either floor up or ceiling down. 

Then look at your options, before you stiffen it up I would get a fat mate to stamp around upstairs and look at the deflection. 

I would also try jacking the timbers up in the centre before you brace them, so sort of pre loading but in the opposite direction. 

A lot of joist movement occurs because the joists move at the point they are fixed to the wall, they flex down as they move away from their fixed ends, you might need to block between the joists at the wall fixing points. 

 

Will be a big dirty job, probably only worth doing if you intend staying for a few years. 

[markc]

When you say re-laying the floor above, do you mean you removed the floor boards etc. and replaced with new? If so, that is where your problem lies. The old floor will have deflected until the boarding (in compression) took a lot of the weight. Removing this and replacing without jacking/pre cambering means your joists and floor now need to settle until the flooring is again in compression.

If you dont want to remove the floor then the options are reduced to removing the ceiling, pre camber and then ply/OSB on the joist sides. Just adding more bottom cord will do very little.

[James Newport]

Yes, old floorboards came up and were replaced with P5 chipboard. The floor has been down for 9 months so I guess it has settled again! There is definite joist movement though - I have taken a small section of plasterboard down and I can see the joist move a little when someone jumps up and down above. 

44 minutes ago, markc said:

Just adding more bottom cord will do very little.

Are you referring to the 4x2 on its side idea?

[markc]

2 minutes ago, James Newport said:

Yes, old floorboards came up and were replaced with P5 chipboard. The floor has been down for 9 months so I guess it has settled again! There is definite joist movement though - I have taken a small section of plasterboard down and I can see the joist move a little when someone jumps up and down above. 

Are you referring to the 4x2 on its side idea?

Yes, a joist or beam works in equilibrium with the load shared by top cord in compression and bottom cord in tension. Simply strengthening the bottom cord with a 4x2 etc. will not stop the top cord compressing and will likely cause the beam to rotate/want to flip on its side if not adequately braced.