I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

On Oct 2, 6:58 pm, Señor Chris wrote:
I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

Done this a number of times before ( its my day job)..... We always
use 6x2 at 600mm centres unless its a really short span of say less
than 1.5m where you might get away with 4x2. Normally we fix a wall
plate using frame fixings, then fix the cross members directly onto
them. Leave about a 50mm gap between bottom of existing cieling and
top of new joist and you should be fine. You don't want any movement
in your secondary ceiling, you don't want to go to all the effort and
then it still fails to pass regs, so we always tend to over spec.

Señor Chris wrote:

I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

minimum size will be 2x2, but since you want acoustic insulation
the bigger the better, preferably much bigger than 2x2.


NT

ch8rlie2000 wrote:
On Oct 2, 6:58 pm, Señor Chris wrote:
I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

Done this a number of times before ( its my day job)..... We always
use 6x2 at 600mm centres unless its a really short span of say less
than 1.5m where you might get away with 4x2. Normally we fix a wall
plate using frame fixings, then fix the cross members directly onto
them. Leave about a 50mm gap between bottom of existing cieling and
top of new joist and you should be fine. You don't want any movement
in your secondary ceiling, you don't want to go to all the effort and
then it still fails to pass regs, so we always tend to over spec.


Thanks for that. However, 6x2 doesn't leave me with much space for the
acoustic quilt which is supposed to sit on top of the joists. Could I
get away with 4x2 if I halve the centres ?

Señor Chris wrote:

I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

4x2" at 400mm centres sounds like it ought to do it.

You may find it helpful to have a play with the demo version of SuperBeam:

http://www.sda.co.uk/sbw.htm

Model a joist, and apply a uniform load based on how much weight you
need to hang on it, and see how it performs.

--
Cheers,

John.

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On 2 Oct, 18:58, Señor Chris wrote:
I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

The formula is that for a timber beam. It requires some engineering
knowledge to do from first principles but there are bound to be
tables available on the internet or in a library. Basically if it is
lightly loaded as it is - just quilt and plasterboard and its own self
weight - you will not need a huge depth for a span of 4 m. I have used
225 (9") for a similar span that is a floor load that will be
reasonably highly loaded so 150 mm sounds about right to me at 400 cc
odd. Any less and your deflection would be too great and dont forget
duration of load effects if you go too small
Chris

Done this a number of times before ( its my day job)..... We always
use 6x2 at 600mm centres unless its a really short span of say less
than 1.5m where you might get away with 4x2.

CLS equiv may be an alternative to consider 148 x 38

B&Q had a great deal on this ... cheaper than local timber yard .... great
for this type of job as it's all gauged and planed.

Thanks for that. However, 6x2 doesn't leave me with much space for the
acoustic quilt which is supposed to sit on top of the joists. Could I
get away with 4x2 if I halve the centres ?

put quilt between on netlon support