Plasterboarding a ceiling

Evening all,
The room is an extension, single story with a flat roof (yes I know!) approx size is 6.6m * 5.3m (with a corner cut out) the joists run across the short edge (there is a RSJ from the corner of the cut out to the short wall to which the joists 'join'). There was no insulation at all hence it was so hot/cold, I've almost put that right.
The question si as the joists are 15" centres - not a divisor of standard board sizes, which way to run the boards and should I/ do I need to nog the joins/edges of the boards?
My feeling is I need to put quite a bit of wood in there to support board ends/edges to prevent sagging.
As a follow up question 9.5 or 12.5mm board?
TIA
Have a good week-end
Nick
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On 12/11/10 20:43, The Nomad wrote:

The book I'm reading says put 3" by 1" battens perpendicular to the joists, correctly spaced for your boards, and shimmed if there's any unevenness. I havent tried yet myself, I wonder what those who have will suggest.. (the book says the hardest thing is taping over the joins) [g]
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The alternative is to cut the boards short to suit the joist pitch. Using 2400 x 1200 boards you're cutting 120mm off the length of each board which is an easy task with a stanley knife
I'd recommend using 2400 x 1200 boards as it means fewer joints, your 35sqm with cropped 2280 x 1200 boards comes out at about 13 boards whereas using 1800 x 900 mm minis comes out at 22 boards or a lot more joints.
If you want to keep it DIY then use taper edge boards and don't skim. I view 9mm as little better than cardboard so would use 12.5mm but you will need help to fit them.
I wouldn't add extra support for the tapered edges (which are running cross joist) but would leave a 3mm gap, scrim tape and be generous with the board finish jointing mix to fill the gap and add strength.
The end joints aren't tapered and will be (slightly) noticeable but you can save the cost of a plasterer if that is important.
--
fred
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On 12 Nov,

More joints, but heavier boards. I got 1200 boards for my ceiling, and after lifting one, I changed to 900mm, using the 1200 for walls. With assistance 1200 would be ok. 9.5mm at 2400mm board may be too bendy for single handed fitting.
One or two large T squares to support the boards are almost essential, 2x2 with a length of floorboard for the top bit is what I used, a few mm more than room height.
--
B Thumbs
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On Sat, 13 Nov 2010 00:05:59 +0000, me9 wrote:

Guys,
Thank you for that (and the others who have replied) that sounds like a doable job - I've got a favour or two to call in when it comes to the lifting thing.
I am trying to avoid a plasterer for various reasons not least I can't strip the room to make access easy for them.
Thanks again
Nick
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That's quite doable, just leave a 5-7mm gap between the boards so you can squeeze lots of plaster in to reinforce the joint.
Re the boards, you'll need asssitance lifting them, though it is possible, albeit a little awkward, to do 6x3 9mm on your own, using a dead man. I'd suggest 2x 1/2 for it rather than 2x2, makes it easy to bend into position.
NT
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On Sat, 13 Nov 2010 00:05:59 GMT wrote :

Here they use 3600 boards glued and nailed to studs/joists. Walls are done with the boards in landscape mode so that smaller rooms have one horizontal joint only per wall.
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Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
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9mm 6'x3' boards, anything bigger is too heavy & awkward. You dont need thick board for ceilings. Use a dead man to hold them up while you align & screw.
NT
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Gordon Brown any good?
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On Fri, 12 Nov 2010 14:51:31 -0800 (PST), js.b1 wrote:

Nah, that's a zombie[1]
[1] TFBUNDY; a medical term: totally f'd but unfortunately not dead yet. Used by A&E staff.
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Peter.
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On 12 Nov,

You'll not need to nog the joints. Put the length across the joists and cut to length (you'd lose 4" on an 8' board) to put joints in the middle of the joists.
I would use taper edge boards (thickness to suit fire regs, 9.5 would span ok) and just tape and fill the joints.
--
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The Nomad wrote:

Firstly, if you've never plated a ceiling before, you aren't going to be able to get it good enough to just tape the joints, it will need plastering.
15in centres is slightly odd as they are normally 16in, but no, you don't need any extra support or noggins.
You're better off using 6X3 plasterboards and get the thinner ones as they are easier to handle, and they need to be fixed pale side down.
They have to be jointed in the centre of a joist on the shortest edge, that is to say, the edge that is 3ft will be fixed, the long edge won't, and obviously you will need to cut them so that they land on a joist.
If the joists are thin, you might want to get some 2X1 batten to add a bit of extra width to the joists that the boards meet on - you won't need it on all the joists.
Before you start, mark a line on each wall where the joists are, and use screws rather than nails to affix the boards.
--
Phil L
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its not that hard, if youre decent with filler.
NT
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