New studio in old shop - Ceiling beam size

I am in the process of building a 4.2m * 4.5m (internal dimensions) studio inside a shopfront using 90*45 stud walls (double) with two sheets of Gyprock on each wall. Attached is a plan view of what I am building. I intend to use double plaster on the ceiling and roof also. I am ok with the stud wall layout, but I'm not sure what beams I need for the 4.2m span. A building designer said that I would need 240*45 @0.60m spacings, but that seems like overkill for a ceiling with no supported roof. (At the same time I appreciate that there is a lot of weight in double plaster above and below - 4*7=28 sheets * 25kgs = 700kgs) Can I use three 240*45 beams as per plan with 90*45 inserts? Or can I use 190*45 beams even? The beams here run about $70 per 4.8m length so if I can reduce the number it would be good. Thanks for your time.

Hi Jan, Depends entirely on the beams. I assume we're talking about LVLs (eg HySpan)? I had the same problem with internal rooms - there aren't any "ceiling only" span tables for most materials, because people don't generally build that sort of thing. Most suppliers have software span calculators, and the ones I used still required a wind category for rafters, which of course doesn't apply in our case. That said, I've decided to use 240x45 Hyspan LVLs for similar spans (up to around 5m) in my build. That's based on the supplier's span calculation software here: http://www.chhwoodproducts.com.au/hyspan/ It may be overkill (because it's are based on exterior rafter span tables), the only way to know for sure is to hire an engineer. But the cost of an engineer would be more than just using the 240s. I'm not sure about Vic, but up here the next size down in LVL isn't all that much cheaper anyway.