plasterboard across ceiling joists or along ceiling joists

[StephenStephen]

Hi all,

I've half a ceiling to board, wanting to cut sound transmission as much as possible

- joists are 5", max span is a little under 2.4m, joist centres are varied - up to a max of 550mm (however, there are regular 1200mm centres)
-These joists support the ceiling only - no live load (floor above on separate set of joists)
-I'm using 15mm plasterboard 1.2m by 2.4m
-the ceiling will be professionally skimmed

Am I better running the boards across or along the joists? (If across then I've little or no space for noggins for board edges between joists in the centre, as I'm weaving insulation between the 2 sets of joists)
If running across joists I assume I stagger board ends?

I'm probably overthinking as usual - thanks for your patience!

cheers, Stephen

 

[PrenticeBoyofDerry]

The bonded edge of the boards run perpendicular to the joists. So the long length of the board would go in the opposite direction of the joists.

 

[StephenStephen]

It's no longer a live practical concern, and I do appreciate your helpful advice, but after all these months I have finally admitted to myself that I cannot sleep at night, concentrate on work, eat cornflakes or otherwise continue my peaceful existence until I have some kind of answer to the following question with regard to the answer to my original question:

Why?

 

[PrenticeBoyofDerry]

Why what? That you cannot sleep??
Or why is the bonded edges perpendicular to joists?

 

[StephenStephen]

Why are the bonded edges perpendicular to joists?

 

[PrenticeBoyofDerry]

Plasterboard's long edges are bonded to add strength to the board.
This edge helps prevent them from sagging (especially on a ceiling application).
Therefore they run perpendicular, as in a normal situation there is very rarely a noggin fitted across the joist to fix the long edge to. The spaces between joist are generally in-access of 300mm, generally 400mm+. This span would cause an un-bonded edge to sag, as it does not have that extra strength (as a bonded edge).
It is advised that boards are screwed/fixed at at least every 200mm along their fixing points, so this cannot be done if a across noggin is not present along the long edge of the board.

Alternatively you could install cross noggins to catch the edges and the centre of the board.

Hope this is clear?

 

[StephenStephen]

Great - so I guess where I've got no unsupported edges at all I can get away with running either way, as long as my joist spacing is small enough to counter sagging of body of board between joists?

 

[PrenticeBoyofDerry]

You could but do not use anything less than 12.5mm board thickness.

 

[StephenStephen]

Cool, thank you - I'm using 15mm so no problem there