Joist spans for suspended ceilings

[ChrisKingston]

Hi,

I am trying to put in some new beams to hang a ceiling off, I will put them in between existing ceiling joists so I can weave acoustic matting between the old and new joists.

So the new joists are only going to hold up the ceiling, they will not touch the floor above and there will be no other load on them at all. I may put two layers of plasterboard (and a skim) over them, but that is all the weight they will need to hold. But I don't want sagging or cracking, so my question is: what size timber do I need?

The longest span is 3800mm, and the joists are spaces every 350mm. Will I get away with 4" x 2", or do I need something meatier?

Thanks,
Chris

 

[freddiemercurystwin]

50x175 would be more suitable for that span.

 

[ChrisKingston]

Thanks for the advice.

Does it not make much difference whether there's a floor pressing down from above then?

 

[^woody^]

As these wont be supporting a floor, then they would be considered ceiling joists, and so as FMT said 50 x 175 or 47 x 170 will be required at that span

From a design POV, it would be poor to have individual joists spanning 3.8m without any noggins, binders or other interim support. Normally joists work in a load sharing arrangement so that movement, flexing, bowing etc is controlled as each joist gets support from neighbouring ones.

Potentially, isolated joists can sag or twist with changes in moisture content

You can use 50x100 with supports at mid span or better still at 1/3 span.

 

[ajrobb]

Have you considered a suspended ceiling on acoustic hangers?
http://www.british-gypsum.com/systems/system_selector/casoline_mf.aspx

or even just resilient bars:
http://www.british-gypsum.com/produ..._profiles/gypframe_sound_insulating_bars.aspx

These might be cheaper and as effective.

Building regulations might require noggins or herringbone trusses mid span on the floor joists. How will you manage that?

 

[ChrisKingston]

Many thanks; yes, the steel frame looks like it would be easier and costs less. The people I rang suggested a Richter frame instead.

I don't really understand how hanging something off the floor joists above can give the same sound-reducing qualities as a new set of joists, but the numbers seem to prove it...

Thanks for your help

 

[ajrobb]

Isolation works above the fundamental frequency sqrt(k/m). Much above this frequency and the mass is much more important than the stiffness of the mount. Much below this frequency and the stiffness is more important.

By hanging heavy Soundbloc board off resilient bars or rubber grommet frame mounts, the low stiffness of the mounts disappears out of the equation at speech frequencies. Low frequencies can be much more of a problem. For this reason, most performance figures are weighted to speech frequencies.