About to embark on some lowered ceilings in a flat.
I will be screwing a wall plate right the way round the rooms, then putting in ceiling joists at 400 centres, 100mm of rockwool aboveresilient bars and then soundboard plasterboard (single layer) which will be skimmed.
All very simple (I said that quickly!)

Anyway, is there a calculator for joist size? they will never be walked on like in a loft, so just supporting the plasterboards (about 50% heavier than std 12.5mm plasterboard)

Spans are 2.1m, 2.4 and the worrying one is 4.2m. Gut feeling is for 45x95 for the smaller spans, and 45x150 for the big one. But that is just gut feeling.....

Any advice/pointers would be great.

TIA
Clive

You could run a 6 x 2 centrally at 90 degrees above the long spans, if youve room, and screw each 4x2 into it.

Building Regs contain an appendix that lists joist sizes, spacing, span etc. The grade of timber is also a factor.
I haven't got it to hand but you should be able to find it with a little googling.....

Ta chaps. After wading through a lot of american sites, and many others that just refer to loadings, I have found what I needed.
Re timber grades,my supplier has a mix of c16 and 24 timber, and having been using it today it seems the biggest difference is the number of knots, which I guess makes sense....

(BTW I underestimated size by a fair bit! but then again I reckon there is a lot of safety margin built in these days)

I would screw battens at 90 degrees to the ceiling through the floor joists then hangers down to your new ceiling joists to reduce the span to around 1.2m, 4x2 would be then be fine.

Ceiling joints in my house are 4x2 about 600mm apart.. builder must have been asking cause I would have them 400 mm apart.
Also I would use 4" celetex with the plasterboard backing.

A hanger in the middle would avoid the need for 6 inch joists. Avoid if you're doing it for sound proofing though.