Another Span question

[jp100]

Hi I wish to put in a false ceiling which will have no load on it other than its own weight and 2 layers of plasterboard.

The span of the room is 4.0m

Having looked at numerours tables, it seems as if i need the joist sizes to be
170x50 .

If I supported the joists at the middle point from the existing ceiling could I use 97 x 50 or 122 x50 instead

Thanks in advance

John

 

[RonnyRaygun]

If I supported the joists at the middle point from the existing ceiling could I use 97 x 50 or 122 x50 instead

That's what I'd do.

 

[^woody^]

If I supported the joists at the middle point from the existing ceiling could I use 97 x 50 or 122 x50 instead

Yes .... if the span tables confirm that a joist that size can span 2m. Otherwise support at 1/3 span

 

[jeds]

If it's only taking its own load your only interested in deflection. Maximum allowable would normally be 0.003L which for 4m span gives you 12mm. If your two layers of board are about 30kN/m/0.6 centres and the timber is 0.1kN/m then joists of 50x125 at 4m span gives you a deflection of 11.5mm. I'd probably go to 50x150 to avoid any vibration.