Flat Roof/Living Roof [Warm Roof] Garage to Living Accomodation Conversion 4.8mtr span

[Yorkshireman]

Morning folks,

 

We're looking to convert a 40ft x 16ft double skin brick garage with no internal pillars into living accomodation via PP. The current flat roof is being replaced with a 150mm PIR warm roof EPDM covered green/living roof and the span is 4.8mtr.  Joist size, we are considering 220 x 75mm spaced at 400mm ctc intervals with a rise of 4inches from back to front for drainage. Would the joists be substantial enough, or would you suggest 2 x 220 x 50mm joists bolted together every 3ft for greater structural integrity? Any help and advice on this self-conversion would be appreciated, thanks folks.

[JohnMo]

Nice use of Imperial and metric units, possibly best to stick with metric throughout.

 

You could easily use posi joust/rafter with that span - we are doing 7m at 600mm centers.

 

But you should be having structural engineer input, not designed by a committee of unknowns.

Edited September 28 by JohnMo