joist size

what size wood would i need for a dropped ceiling it has to hold 2 layers of pb with greenglue cheers for any advice

50mm x 50mm would be a sensible minimum. Are you proposing to glue your boards to the timber ? If you are, might I suggest you think again as I cant believe that this would work.

[IronGiant]

teddybass, You put the greenglue between the two sheets of plasterboard, I assume he is going to (preferably) screw the first sheet to the timber.

Dave

no i will screw the pb to the wood the greenglue goes inbetween pb dont no whether 50mm wood will hold that

thanks Regmarch

Excuse my ignorance regarding the greenglue !
The size of the joists depends on the span. I assumed probably incorrectly that you would also be fitting some vertical supports or hangers. if you are, 50 x 50 will be fine. if you are relying on the horizontal strength of the timber alone you may need to go up to 225mm deep x 50mm wide. This would be very expensive.
The usual way, within the building industry, to support plasterboard suspended ceilings is using a MF (metal framed) ceiling system. This uses a lightwell metal frame supported by hangers (metal angle or wire) at regular intervals.

[GasDad]

Originally Posted by anfieldfc

what size wood would i need for a dropped ceiling it has to hold 2 layers of pb with greenglue cheers for any advice

No one has mentioned the span yet - when you say a dropped ceiling is it going to be connected to the existing ceiling or totally issolated.

James

hi its going to be totaly issolated from the other joist the idea is to remove the ceiling the add the new joist inbetween the old joists the span is 12foot

[GasDad]

Originally Posted by anfieldfc

hi its going to be totaly issolated from the other joist the idea is to remove the ceiling the add the new joist inbetween the old joists the span is 12foot

Is the old ceiling also a floor ?

it is

[clockworks]

I've just had a new ceiling fitted in a bedroom. The span is 10 feet.
The builder used 50 x 100 joists, on 16" centres. It's only single layer PB, but the upper surface is sheeted with Weyroc flooring boards.

Takes my weight in the loft OK.

The original ceiling was lath & plaster on 1" "joists" at 18" centres. It was sagging a bit after 200 years...

[GasDad]

If the new joists are also going to support the floor (it isn't clear from your post) - then you need to follow somethign like the table on here http://www.worthing.gov.uk/Planning/...fTimberJoists/

(Note the above link talks about an imposed load of 1.5KN - if that does'nt mean much to you, then realise that a small apple 'weighs' about a newton - so they're talking about the weight of 15 sacks of apples.)

If the new joists are going between (and slightly below) the old joists (which are continuing supporting the floor) - to provide sound isolation perhaps, then you can obviously use smaller timber.

I can't in a brief search find a table for you (must exist somewhere) - just be aware that information from an old book will not be suitable as wood today is grown quickly and is hence less strong than that used prior to WW2.

If I was me I think in terms of 2x4" or 2x5" - better to overengineer in this case.

Just in case you don't know, you need to order sawn timber, not PAR (planned all round) - it will be a lot cheaper.

Also try and lay your new joists out to reduce the number of plaster board cuts you have to make

One final suggestion - if this is for sound isolation, use a bigger size joist and poor dry sand between the joists (after you've done the wiring) ie on top the plasterboard - say an inch deep.


Hope this helps,

James

[woody67]

For a span of this length, the span tables of the approved documents (Building Regs) indicate a minimum timber size of 50 x 150 @ 600 centres. A smaller section of 50 x 125 may suffice, but anything smaller is too small

A smaller section would require support in the centre of the joist to prevent it deflecting. The section size is not just about how much load it will support, but also about it cracking the ceiling plaster as the timber naturally absorbs air moisture and dries through the seasons and slightly sags

[baldrick]

Just a note on the Green Glue, you screw the first layers of boards to the timbers, apply the green glue and then screw the second layer of boards making sure that the joins between the 1st and 2nd layers don't line up....

Regarding the size of timber, I'm no expert but the weight of 2 layers of plaster board is going to require a decent size (2" x 4" min?) timber in order to stop it flexing at the centre point.

We're building a timber framed house and all the joists are at 600mm centers with 3" x 1" baton running perpendicular to the joists at 400mm centers double nailed at each crossing point. If you plan doing something similar I'd recommend hiring a Paslode nail gun!

[IronGiant]

Originally Posted by woody67

For a span of this length, the span tables of the approved documents (Building Regs) indicate a minimum timber size of 50 x 150 @ 600 centres. A smaller section of 50 x 125 may suffice, but anything smaller is too small

A smaller section would require support in the centre of the joist to prevent it deflecting. The section size is not just about how much load it will support, but also about it cracking the ceiling plaster as the timber naturally absorbs air moisture and dries through the seasons and slightly sags

Woody67, 50 x 150 sound like floor joists to me? this is only to hold up the plasterboard ceiling, not support a floor above. Can you confirm?