Timber framed false conservatory ceiling - size of timber??

[glasgowdan]

I would like to build a false ceiling in our conservatory, with 50mm insulation board, then finished with that plastic bathroom ceiling pvc stuff. My question is, with the size being 4.2x5.5m, what grade timber would be required to span the 4.2m length and hold it's shape?

I obviously don't want things to get too heavy.

 

[foxhole]

Span the centre of the 4.2 with a 6x2, then you only need 3x2, or4x2 for the remaining 2.75 span.

 

[glasgowdan]

Don't you think I'd get away with anything lighter? It's just being screwed into upvc so I'm very wary of the extra load around the windows.

 

[foxhole]

Difficult to get any timber of small dimension straight enough to span 4.2m unless you can support the centre off the current roof bars.? (Like suspended ceilings).