bouncing joists cracking plaster

[nishnish]

2 sheets of 12.5mm will be to heavy.

This is the first time I have seen this. I thought it was a common acoustic treatment to put up 2 layers of 12.5mm Soundbloc.

I can't see how that can be true for properly sized joists.
Gyproc SoundBloc 12.5mm is 10.6Kg/m2. So two layers is 21.2Kg/m2.
Take a 4mx4m room as an example, that would be a total ceiling weight of 16*21.2=339.2Kg.
A person can easily weigh 100Kg. So this room, supposedly cannot support a weight of 339.2Kg spread evenly across it's entire area. Therefore it would be unlikely to take the weight of even one or two 100Kg persons whose weight would be concentrated in a much smaller area?
That doesn't seem correct?

 

[noseall]

I think Hertsdrainage post was referring to the fact that 12.5mm boards are heavy to lift.

 

[nishnish]

I think Hertsdrainage post was referring to the fact that 12.5mm boards are heavy to lift.

Oh. He confused both me and ajrobb then

 

[2929twin]

Surely its not often 30 yr old joists shrink/warp in an upward manner.
Could it not be from the removal of the ceiling weight? Maybe to maintain a level plane under joists the top of joist sockets should be packed downwards?

 

[jeds]

I'm still trying to work out how the joists are floating. They must be sat on something?

 

[ajrobb]

Before I start, let me say that I lived in this house from when it was about 2 years old in 1980 through to 1991. The front bedroom floor was always bouncy - the whole room shook whenever I walked across it (I was 11 stone then). Since 1991, the house has been let and the last tenants were there for over 10 years. This is the first opportunity I have had to have a good look at the place.

I'm still trying to work out how the joists are floating. They must be sat on something?

I suspect the joists are floating from a combination of warped floorboards, herringbone bracing and taking down a partition.

The joists run across the house from sockets in the party wall (probably not allowed now) to sockets in the end wall. The partition I took down ran along a pair of rotten joists under one set of roof truss joints - about the stiffest part of the upstairs ceiling. I only took down the downstairs ceiling after I took down the upstairs partition, which revealed the rot.

Untitled

• ajrobb
• 24 Apr 2010
• 5

It was only after the partition and ceiling were down that I could push up on the joists to find which ones weren't sitting firmly on the end wall. The mortar I've injected into the sides of the joist sockets in the end wall seems to be holding the joists in position for now and the upstairs floor no longer shakes when I walk across it (I am nearly 17 stone now - ah, contentment). Anyway, it now feels 'proper'.

I'm going to fit an end block mid span between the end joist and the front wall. This should help protect the joint between the downstairs ceiling and front wall. It should also make the upstairs floor firmer by the front window. I hope it firms up at the back of the room when I tie the joists to the roof trusses with a new partition. (The old paramount partition was only attached to the floor at 3 points.)