Correct joist size for false ceiling

7 Jan 2008
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United Kingdom
Hi, I would like some advice on the correct joist size for a false ceiling please.

This is for a first-floor bedroom with existing L&P ceiling which is bowed/sagged in the centre (max sag/drop in the middle is 50mm). House is 90 years old, the existing ceiling has been bowed for at least 50 years but is otherwise in good condition, secure and fully insulated in the loft above. I want to install a new flat ceiling with downlights.

Room is 4100 X 3580, so with 50mm wall battens on both long sides of the room to take hanger brackets, the clear span for joists across the width of the room will be 3500. I want to use single layer 1800 X 900 X 12.5 PB with joist centres at 450 and noggins at all unsupported board joints. Also possible two binders over top of joists.

My question is, will 47 X 75 C16 be suitable for these joists. Your help would be appreciated, plus anything you think I might have overlooked. Thanks.
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To be honest I would much prefer to use Gyproc metal framing on the grounds that it is lighter, quicker to fit and easier to get straight/level. You'd need right angle hanger strip, mains, self drilling wafer head screws, top hats, top hat clips and perimeter (edge) profile. The only specialist piece of kit is a pair if tin snips although a pair of long jaw 150mm mole-type grips would be very handy. Drywall stuff uses Phillip's #2 screw heads almostv exclusively

If you want to use timber then, yes, I think your timbers would be adequate providing you get enough fixings into the original joists to carry the timber framing. The new timbers should run at right angles to the old joists (I.e. counter battened) which helps even out the discrepancies in the old ceiling. The short edge ends of your boards need to be supported on a timber, the long edge joints (which should run at right angles to the timbers) do not need the support providing you use scrim tape on the joints
Thanks J&K, that’s really helpful. Yes I thought about metal framing but to be honest I would prefer to use timber which I have worked with extensively before (Smaller false ceilings and a loft conversion). I only do this stuff for myself so not sure about starting with something new. Also speed of the job is not an issue.

Logic tells me to go with the shortest joist length which is 3.5m instead of down the length of the room at 4.1m, although I get what you’re saying about them being opposite way to the original ceiling above which is also across the width of the room. I had considered some fixings to the existing joists above as belt and braces but my concern here is the old ceiling. Although it’s secure in its present state, I don't want to tempt fate by using it as a support.

It sounds if you are saying that 47 X 75 joists may not be enough on their own, would be grateful if you could confirm that’s you’re thinking. Thanks.
Error. Apologies, I gave incorrect size for the joists I was going to use. Should have been 47 X 97 not 47 X 75. I.e. 4x2 not 3x2 inches.
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Effectively 4 x 2in which will carry a floor over an 8ft (2.4 metre) span. Yes, that will probably be fine, although I would still go for 2 or 3 rows of noggins across the width of the room to help stiffen the structure

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