What ceiling joist size do I need please?

strawberrylane

I am considering having a new false ceiling installed (for soundproofing reasons) in a room which is about 5.1metres (16'8") wide. The joists would be attached to the walls on either end with hangers. The joists would not be supporting a floor above them, only the plasterboard ceiling beneath. The ceiling would comprise two sheets of plasterboard with a total loading of about 23Kg/m2. What size of joist would I need to span 5.1metres, assuming standard C16/C24 structural joists at 400mm centres? Any help appreciated. Thanks.

tomstickland

They could be considerably thinner if you used hangars from the existing ceiling. Screw vertically up through the plaster.

strawberrylane

I am reluctant to attach directly to the existing ceiling as I want to eliminate as much as possible the potential for impact noise being transmitted from the room above. Hence, I have decided to go for a completely independent ceiling but I don't know what size joists to use for the large span. Thanks.

plumb1_2

5"x2", but why 2 sheets of plasterboard? unless you want it for fire regs.

Just use 9mm board(screwed) and use lay 100mm fibreglass above to use as a soundproof.

strawberrylane

I am aiming for a very high level of acoustic insulation, so my plan is to install an independent ceiling with new joists, acoustic insulation fitted snugly between joists, resilient (sound-breaker) bars attached at right-angles to the joists and two sheets of soundbloc plasterboard attached to the reslient bars. I'm lead to believe that the mass of the plasterboard is one of the elements that helps to dampen the impact noise. I guess this sounds like a "rolls royce" solution but I'm keen to make it as good as I can (within reason).

tomstickland

I'd be very interested to know how a stand alone ceiling with double layered soundbloc compares to just adding the soundbloc to the existing ceiling.

I think you are correct - it's mass of material that deadens sound.

strawberrylane

Is anyone able to confirm an earlier poster's view that 2"x5" ceiling joists would be suitable for a span of 5.1m? Thanks.

tomstickland

5.1m is a long way. Looking at the 6m ish walls in this room and making an appropriate joist size with my fingers, I just measured 5" x 2" sort of area, so about that.
There might be some building design guidance on the web somewhere.

rawky

5"x2" sounds good to me

http://www.tameside.gov.uk/tmbc8/note09.pdf

lush_walrus

Speak to an acoustic consultant about this as I am pretty sure that you will get a very good accoustic rating for a normal suspended ceiling, without all of the complications and gumph you are describing doing.

You are pressumably trying to stop airbourne sound rather than impact? or is it both.

I would recommend speaking to a supplier such as British Gypsum who supply suspended ceilings. Then also speak to Celotex (I dont have their number off hand) as they are suppliers of what is considered to be the best and most efficient accoustic insulation on the market. As their material is so dense they are able to acheive very high db reduction in 25mm.

You really would be absolutely insane to consider laying more joists to combat this problem, even nightclubs do not do that!

I forgot to mention that with acoustics and sounds, sound will pass through the point of least resistance so unless everything is able to hold sound to a high degree then you are wasting your time. You will also potentially be making the properties value decrease, as presumably even if you have the floor to ceiling height, then you will be reducing it significantly by having two lots of structure.

Also the most important point is to ensure that all joints are taped and sound, again any gap and it will all go to waste.

Ah and just read you want to reduce impact noise from above, perhaps an option would be to pay for the above floor to be acoustically treated with an acoustic quilt. Impact is always best dealt with at the source.

bcoman

Trada tables -ceiling joists 25kg/m2 loading
47x120 2.56m - 47x195 C24 5.14m